Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: 1 The response of peat-rich permafrost soils to human-induced climate change may be especially important in modifying the global C-flux. We examined the Holocene developmental record of a High Arctic peat-forming wetland to investigate its sensitivity to past climate change and aid understanding of the likely effects of future climate warming on high-latitude ecosystems.
2 The microhabitat of mosses was quantified in the present-day polygon-complex at Bylot Island (73° N, 80° W) and used to interpret the radiocarbon-dated macrofossil record of three cores, comprising c. 3500 years of wetland development. Recurrent wet and dry phases in the reconstructed palaeohydrological record indicated pronounced temporal variability. Wet and dry phases were compared between cores and with palaeoclimatic proxy values, measured as percentage melt andδ18 O in nearby ice cores.3 Periodic wet and dry phases appear unrelated to past climate over c. 50% of the combined stratigraphic records, and are attributable instead to geomorphological mechanisms. At other times, association of wet and dry phases with significantly lower and higher values of percentage melt and δ18 O indicate a possible effect of past climate change on polygon hydrology and vegetation, although inconsistencies between cores suggest that local geomorphological processes continued to modify a regional climatic effect. However, during a period incorporating the Little Ice Age (c. 305–530 cal. years bp), reconstructed moisture and vegetation change is pronounced and consistent among all three cores.
4 The results provide strong evidence for the sensitivity of a High Arctic terrestrial ecosystem to past climate change during the Holocene. The estimated magnitude of changes in soil moisture between wet and dry phases is sufficient to imply recurrent shifts in wetland function, periodically impacted upon by pronounced climatic variability, although controlled principally by autogenic processes. The structure and function of such wetlands may therefore be susceptible to predicted, human-induced climate warming.
W. Shotyk, M. Krachler, A. Martinez-Cortizas, A. K. Cheburkin, H. Emons (2002). A peat bog record of natural, pre-anthropogenic enrichments of trace elements in atmospheric aerosols since 12 37014 C yr BP, and their variation with Holocene climate change. Earth and Planetary Science Letters 199 (1-2): 21-37
ABSTRACT: A peat bog in the Jura Mountains, Switzerland, provides a continuous record of peat accumulation since 12 37014 C yr BP. Periods of enhanced soil dust deposition (10 59014 C yr BP, 823014 C yr BP, and after 532014 C yr BP) are characterized by strongly elevated Ti/Sc and Zr/Sc ratios which imply an increase in the abundance (both relative and absolute) of heavy minerals such as ilmenite and zircon. With respect to trace elements such as Cu, Zn, As, Cd, Sb and Au, the M/Sc ratios are at their lowest, and often approach crustal values, during periods of enhanced soil dust deposition. The lowest rates of atmospheric deposition of soil dust date from 8030 to 532014 C yr BP, corresponding to the Holocene climate optimum, but here many trace elements exhibit their greatest natural enrichments: the average enrichment factor (calculated using Sc as the reference element, and normalizing to crustal abundance) was Zn 4.1±1.4, Sb 4.8±1.4, Cu 8.8±3.3, As 14.9±3.2, Au 53.9±25.1, and Cd 357.4±53.8. These enrichments cannot be explained by chemical diagenesis within the deeper sections of the peat profile during or subsequent to peat formation, but rather reflect the chemical composition of airborne material supplied to the surface layers of the bog at the time of deposition. The enrichments of trace metals in ancient peats, relative to crustal abundance, most likely reflects the natural enrichment of these elements in the fine fraction of soils during rock weathering. Periods of enhanced soil dust deposition such as the Younger Dryas cold climate phase (10 59014 C yr BP) are characterized by reduced vegetation cover, greater exposed soil surface, and higher wind strengths; these conditions promote the transport of locally derived soil materials of greater particle size, lower concentration of trace metals, and M/Sc ratios approaching crustal values. During the Holocene climate optimum, vegetation cover was extensive, and with lower wind strengths and a reduction in erodible soil materials, long range transport of soil dust became relatively more important to the soil dust inventory of the bog; soil dust particles in this size class are characterized by strong enrichments of a wide range of trace elements. At the end of the Holocene optimum, dust fluxes increased once again, due mainly to soil erosion resulting from the combined effects of human activities (tillage) and the climatic deterioration at the beginning of the Neoglaciation Period; this promoted the supply of local, more coarse soil particles with M/Sc ratios approaching those of crustal rocks. While biological cycling and volcanic emissions probably also contributed to the atmospheric supply of many of these elements in the pre-anthropogenic past, these contributions appear to be less important than the chemical weathering, physical fractionation, and atmospheric transport of soil dust particles.
G. A. Zielinski, P. A. Mayewski, L. D. Meeker, S. Whitlow, M. S. Twickler, M. Morrison, D. A. Meese, A. J. Gow, R. B. Alley (1994). Record of volcanism since 7000 B.C. from the GISP2 Greenland ice core and implications for the volcano-climate system. Science 264 (5161): 948-952
ABSTRACT: Sulfate concentrations from continuous biyearly sampling of the GISP2 Greenland ice core provide a record of potential climate-forcing volcanism since 7000 B.C. Although 85 percent of the events recorded over the last 2000 years were matched to documented volcanic eruptions, only about 30 percent of the events from 1 to 7000 B.C. were matched to such events. Several historic eruptions may have been greater sulfur producers than previously thought. There are three times as many events from 5000 to 7000 B.C. as over the last two millennia with sulfate deposition equal to or up to five times that of the largest known historical eruptions. This increased volcanism in the early Holocene may have contributed to climatic cooling.
ABSTRACT: Geochemical identification of Mount Mazama ash in the Greenland Ice Sheet Project 2 (GISP2) ice core gives a calendrical age of 7627 ± 150 cal yr B.P. (5677 ± 150 B.C.) for the eruption, thus providing a more accurate early Holocene stratigraphic time line than previously available. The GISP2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 Mt spread over an 6 yr period following the eruption of Mount Mazama. Taking into account the likelihood of some tropospheric aerosol transport to Greenland, realistic estimates of the resulting atmospheric optical depth range from 0.6 to 1.5. These values may have produced a temperature depression of 0.6 to 0.7 °C at mid to high northern latitudes for 1–3 yr after the eruption. These results indicate that the 5677 B.C. eruption of Mount Mazama was one of the most climatically significant volcanic events of the Holocene in the Northern Hemisphere. We also calculate a maximum stratospheric Cl– release of 8.1 Mt by the eruption, which may have led to substantial stratospheric ozone depletion.
ABSTRACT: Direct observations of sunspot numbers are available for the past four centuries but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades.
ABSTRACT: Most scientists have viewed the sun's unvarying brightness as the one constant in the ever-changing climate system. Now, in a paper published online this week by Science (www.sciencexpress.org), paleoceanographers report that the climate of the northern North Atlantic has warmed and cooled nine times in the past 12,000 years in step with the waxing and waning of the sun. Some researchers say the data make solar variability the leading hypothesis to explain the roughly 1500-year oscillation of climate seen since the last ice age, and that the sun could also add to the greenhouse warming of the next few centuries.
Bond, G., B. Kromer, J. Beer, R. Muscheler, M.N. Evans, W. Showers, S. Hoffman, R. Lotti-Bond, I. Hajdas, G. Bonane (2001). Persistent solar influence on North American climate during the Holocene. Science 294: 2130-2136
ABSTRACT: Surface winds and surface ocean hydrography in the subpolar North Atlantic appear to have been influenced by variations in solar output through the entire Holocene. The evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon-14 and beryllium-10 and centennial to millennial time scale changes in proxies of drift ice measured in deep-sea sediment cores. A solar forcing mechanism therefore may underlie at least the Holocene segment of the North Atlantic's "1500-year" cycle. The surface hydrographic changes may have affected production of North Atlantic Deep Water, potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally.
ABSTRACT: The climate of the North Atlantic region underwent a series of abrupt cold/warm oscillations when the ice sheets of the Northern Hemisphere retreated during the last glacial termination (17.7–11.5kyr ago). Evidence for these oscillations, which are recorded in European terrestrial sediments as the Oldest Dryas/Bølling/Older Dryas/Allerød/Younger Dryas vegetational sequence, has been found in Greenland ice cores. The geographical extent of many of these oscillations is not well known, but the last major cold event (the Younger Dryas) seems to have been global in extent. Here we present evidence of four major oscillations in the hydrological balance of the Owens basin, California, that occurred during the last glacial termination. Dry events in western North America occurred at approximately the same time as cold events recorded in Greenland ice, with transitions between climate regimes in the two regions taking place within a few hundred years of each other. Our observations thus support recent climate simulations which indicate that cooling of the North Atlantic Ocean results in cooling of the North Pacific Ocean which, in turn, leads to a drier climate in western North America.
A. L. Sabin, N. G. Pisias (1996). Sea surface temperature changes in the northeastern Pacific Ocean during the past 20,000 years and their relationship to climate change in northwestern North America. Quaternary Research 46 (1): 48-61
ABSTRACT: Modern ocean–atmosphere interactions in the northeastern Pacific Ocean have a significant effect on the climate of the west coast of North America. We present radiolarian microfossil-based temperature reconstructions for the eastern North Pacific spanning the past 20,000 yr to examine possible correlations and linkages between continental climate change and changes in sea surface temperature (SST) in the northeastern Pacific Ocean on millennial time scales. The reconstructions indicate that the regional pattern of ocean circulation off the west coast of North America was further south 15,000 cal yr B.P. than it is today, and reached its present location 13,000 cal yr B.P. The North Pacific Drift and Transition Zone were further south as a result of a more southerly North Pacific high pressure cell prior to 13,000 cal yr B.P. While two continental paleoclimate records from northwestern North America show regional differences, they also can be correlated to the SST changes. A coastal site at 48°N shows similar patterns in summer temperatures, as observed in offshore marine records of SSTs. However, an inland continental record seems to reflect more-regional-scale changes in sea surface conditions showing a thermal maximum centered at 10,000 cal yr B.P which is observed in the marine transect south of 42°N. We conclude, based on the pattern of oceanographic change as reflected in radiolarian assemblages, that changes in the past latitudinal position of the North Pacific Drift played a significant role in controlling continental climate immediately to its east, as it does in the present environment. We also conclude that during the past 20,000 yr much of the evolution of oceanographic change is related to the migration of the atmospheric pressure cells (the North Pacific high and Aleutian low) of the northeastern Pacific.
D. Fisher, A. Dyke, R. Koerner, J. Bourgeois, C. Kinnard, C. Zdanowicz, A. de Vernal, C. Hillaire-Marcel, J. Savelle, A. Rochon (2006). Natural variability of Arctic sea ice over the Holocene. EOS, Transactions of the American Geophysical Union 87 (28): 3 pp.
ABSTRACT: A consortium of Canadian groups is using ocean cores, ice cores, and mammalian and archeological histories to build a Holocene sea ice history; preliminary results are reported in this article.
ABSTRACT: Over vast areas of the world's landmasses, where climate beats out a strong seasonal rhythm, tree growth keeps unerring time. In their rings, trees record many climate melodies, played in different places and different eras. Recent years have seen a consolidation and expansion of tree-ring sample collections across the traditional research areas of North America and Europe, and the start of major developments in many new areas of Eurasia, South America and Australasia. From such collections are produced networks of precisely dated chronologies; records of various aspects of tree growth, registered continuously, year by year across many centuries. Their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensions of time and space. With their extensive coverage, high temporal resolution and rigid dating control, dendroclimatic reconstructions contribute significantly to our knowledge of late Holocene climates, most importantly on timescales ranging from 1 to 100 years. In special areas of the world, where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved, work building chronologies covering many millennia continues apace. Very recently, trees have provided important new information about major modes of general circulation dynamics linked to the El Niño/Southern Oscillation and the North Atlantic Oscillation, and about the effect of large volcanic eruptions. As for assessing the significance of 20th century global warming, the evidence from dendroclimatology in general, supports the notion that the last 100 years have been unusually warm, at least within a context of the last two millennia. However, this evidence should not be considered equivocal. The activities of humans may well be impacting on the ‘natural’ growth of trees in different ways, making the task of isolating a clear climate message subtly difficult.
ABSTRACT: A 17 000 yr fire history from Yellowstone National Park demonstrates a strong link between changes in climate and variations in fire frequency on millennial time scales. The fire history reconstruction is based on a detailed charcoal stratigraphy from Cygnet Lake in the rhyolite plateau region. Macroscopic charcoal particles were tallied from contiguous 1 cm samples of a 6.69-m-long core, and the data were converted to charcoal-accumulation rates at evenly spaced time intervals. Intervals of high charcoal-accumulation rates were interpreted as local fire events on the basis of information obtained from modern charcoal-calibration studies in the Yellowstone region. The record indicates that fire frequency was moderate (4 fires/1000 yr) during the late glacial period, reached highest values in the early Holocene (>10 fires/1000 yr), and decreased after 7000 calendar yr B.P. The present fire regime (2–3 fires/1000 yr) was established in the past 2000 yr. The charcoal stratigraphy correlates well with variations in July insolation through time, which suggests that regional climate changes are responsible for the long-term variations in fire frequency. In the early Holocene, summer insolation was near its maximum, which resulted in warmer, effectively drier conditions throughout the northwestern United States. At this time, the fire frequency near Cygnet Lake was at its highest. After 7000 calendar yr B.P., summer insolation decreased to present values, the regional climate became cooler and wetter, and fires were less frequent. The Cygnet Lake record suggests that long-term fire frequencies have varied continuously with climate change, even when the vegetation has remained constant
FIRST PARAGRAPH: A series of lectures as comprehensive as those reported in this issue leaves very little for a general reviewer to say. The lecturers have included many of the leading specialists in each of the subfields of palaeoclimatology, upon whose work a reviewer must lean. Moreover there has recently been published a fine synthesis of what is known about the overall character of Quaternary and Holocene climates (U.S. GARP committee, 1974) (hereafter referred to as GARP-74). All I can do in this situation is to point out some disturbing and persistent problems that tend to defy solution.
ABSTRACT: The astronomical theory of climatic change provides an alternative to the traditional chronology for Holocene climatic change, which calls for one thermal maximum about 6000 years ago. The theory predicts a series of maxima during the Holocene, one for each season. Because the relation of the perihelion to the spring equinox changes with a 22,000-year period, late summer insolation would have been greatest 5000 years ago, whereas early summer insolation would have been greatest 13,000 years ago. Climatic reconstructions based on the response of ecosystems to late summer climate indicate a later Holocene thermal maximum than paleoclimatic data sensitive to early summer climate. In southern Idaho, three different vegetation types indicate thermal maxima at different times during the Holocene, depending on the climatic variable controlling each type.
ABSTRACT: Records from South America show that modern ENSO (El Nino-Southern Oscillation) did not exist 7000 cal yr B.P. and has developed progressively since then. There has been little information available on oceanic conditions in the eastern equatorial Pacific (EEP) to constrain explanations for ENSO onset. We report quantitative observations on thermocline and mixed-layer conditions in the EEP during ENSO start up. We found important changes in both the thermocline and the mixed layer, indicating increased upwelling of cooler waters since 7000 cal yr B.P. This resulted from change in the source and/or properties of waters supplying the Equatorial Undercurrent, which feeds upwelling along the equator and the Peru margin. Modeling shows that ENSO is sensitive to subsurface conditions in the eastern equatorial Pacific and that the changes in the thermocline we observed were driven by extratropical processes, giving these a role in conditioning the development of ENSO. This is in contrast to models that call for control of equatorial Pacific oceanography by tropical processes only. These infer stronger upwelling and cooler surface waters for the EEP during the mid-Holocene, which is not supported by our results.
ABSTRACT: For more than a century it has been postulated that the Holocene vegetation of western Europe has changed in significant ways. A half-century ago a lively debate revolved on whether there were one or two dry intervals causing bogs to dry out and become forested, or whether instead the climate warmed to a maximum and then cooled. Today none of these climatic schemes is accepted without reservation, because two nonclimatic factors are recognized as significant: the differential immigration rates of dominant tree types (e.g., spruce in the north and beech in the south) brought unexpected changes in forest composition, and Neolithic man cleared the forest for agriculture and thereby disrupted the natural plant associations.
In North America some of the same problems exist. In the hardwood forests of the Northeast, which are richer than but otherwise not unlike those of western Europe, the successive spread of white pine, hemlock, beech, hickory, and chestnut into oak dominated forests provides a pollen sequence that may yield no climatic message. On the other hand, on the ecotone between these hardwood forests and the conifer forests of the Great Lakes-St. Lawrence area, the southward expansion of spruce, fir, and tamarack in the late Holocene implies a climatic cooling of regional importance, although the progressive conversion of lakes to wetlands favored the expansion of wetland forms of these genera.
In the southeastern states the late-Holocene expansion of southern pines has uncertain climatic significance. About all that can be said about the distribution and ecology of the 10 or so species is that some of them favor sandy soils and are adapted to frequent fires. In coastal areas the expansion of pines was accompanied by development of great swamps like Okefenokee and the Everglades—perhaps related to the stabilization of the water table after the early Holocene rise of sea level. The vegetation replaced by the pines in Florida consisted of oak scrub with prairie-like openings, indicating dry early Holocene conditions, which in fact had also prevailed during the time of Wisconsin glaciation.
In the Midwest the vegetation history provides a clearer record of Holocene climatic change, at least along the prairie border in Minnesota. With the withdrawal of the boreal spruce forest soon after ice retreat, pine forest and hardwood forest succeeded rapidly, as in the eastern states. But prairie was not far behind. By 7000 years ago the prairie had advanced into east-central Minnesota, 75 miles east of its present limit. It then withdrew to the west, as hardwoods expanded again, followed by conifers from the north. The sequence easily fits the paleoclimatic concept of gradual warming and drying to a maximum, followed by cooling to the present day. It is supported by independent fossil evidence from lake sediments, showing that lakes were shallow or even intermittently dry during mid-Holocene time.
Here we have a paleoclimatic pattern that is consistent with the record from glaciers in the western mountains—a record that involves a late-Holocene Neoglaciation after a mid-Holocene interval of distant glacial recession. Just as the Neoglaciation is time-transgressive, according to the review of its evidence by Porter and Denton, so also is the mid-Holocene episode of maximum warmth, and they are thus both geologic climate units. The warm episode is commonly termed the Hypsithermal, which, however, was defined by Deevey and Flint as a time-stratigraphic unit that is supposed to have time-parallel rather than time-transgressive boundaries. It was defined on the basis of pollen-zone boundaries in western Europe and the northeastern United States that have a sound biogeographic but questionable paleoclimatic basis. Perhaps it should be redefined as Porter and Denton suggest, as a geologic-climate unit with recognizable time-transgressive boundaries that match the gradual geographic shifts in the general circulation of the atmosphere and the resulting location of storm tracks and weather patterns. Holocene glacial and vegetational progressions provide a good record of climatic change, if one can work out the lag effects related to the glacial economy and the geographic factors controlling tree migration. The terminology for the Holocene, where so much time control is available, should indicate the dynamic character not only of the climate but also of the geologic and biogeographic processes controlled by climate.
Cole, K.L., N. Henderson, D. S. Shafer (1997). Holocene vegetation and historic grazing impacts at Capitol Reef National Park reconstructed using packrat middens. Great Basin Naturalist 57 (4): 315-326
ABSTRACT: Mid- to Late Holocene vegetation change from a remote high desert site was reconstructed using plant macrofossils and pollen from nine packrat middens ranging from 0 to 5400 years in age. Presettlement middens consistently contained abundant macrofossils of plant species palatable to large herbivores that are now absent or reduced, such as winterfat (Ceratoides lanata ) and ricegrass (Stipa hymenoides ). Macrofossils and pollen of pinyon pine (Pinus edulis ) , sagebrush (Artemisia spp.) and roundleaf buffaloberry (Sheperdia rotundifolia ) were also recently reduced to their lowest levels for the 5400 year record. Conversely, species typical of overgrazed range, such as snakeweed (Gutterrezia sarothrae ), viscid rabbitbrush (Chrysothamnus visidiflorus ), and Russian thistle (Salsola sp.) were not recorded prior to the historic introduction of grazing animals. Pollen of Utah juniper (Juniperus osteosperma ) also increased during the last 200 years. These records demonstrate that the most severe vegetation changes of the last 5400 years occurred during the last 200 years. The nature and timing of these changes suggest that they were primarily caused by nineteenth century open-land sheep and cattle ranching. The reduction of pinyon and sagebrush concurrent with the other grazing impacts suggest that the effects of cattle grazing at modern stocking levels may be a poor analog for the effects of intense sheep grazing during drought.
ABSTRACT: The existing warm (Larrea) deserts of the Southwest are Holocene expansions replacing late—Pleistocene, evergreen woodlands of low—statured junipers, pinyon pines, and live oaks; these woodlands have been isolated by complementary contraction to the slopes of higher mountains that rise like islands from the modern desert sea. Because pinyon—juniper woodland is now so widespread on the similar fault—block mountains of the Great Basin, even as far north as southern Idaho, it would seem reasonable to suppose that the modern "cold" (Artemisia ,Atriplex ) deserts were similarly wooded during the last glacial. However, conclusive Neotoma macrofossil evidence (45 14°C—dated assemblages are reported here) documents major latitudinal displacement of vegetation that precludes pinyon—juniper woodland in the northern and central Great Basin at that time. On the other hand, the entire Mohave Desert sector (south of 37°N) served as an extensive Pleistocene refugium for pinyon—juniper woodland, as documented by an additional 48 dated Neotoma deposits. During the Wisconsinan glacial in the southeastern corner of Oregon, a 42°27'N, there was a subarctic landscape of hyperboreal, prostrate shrublet—junipers (Juniperus horizontalis andJ. communis ) and widespread patterned ground, even at the near—basal elevation of 1460 m. The pleniglacial vegetation of the central Great Basin at 39°N in eastern Nevada and western Utah, was dominated by a regional subalpine forest of bristlecone pine (Pinus longaeva ), associated with minor but consistent boreal juniper (J. communis ) down to 1660 m, close to the base level imposed by pluvial Lake Bonneville. Spruce has not been recorded below 1900 m during the last glacial. At a lower range of elevation (1350—1525 m), available south of the southeastern rim of the Bonneville basin at 37°30'N. Pinus longaeva was replaced by limber pine (P. flexilis ), Douglas—fir (Pseudotsuga ), and montane red cedar (J. scopulorum ); existing woodland juniper (J. osteosperma ) was lacking, but the subalpineJ. communis was present at this local base level. Theory of island biogeography, as applied to ecological islands atop the high mountains of the Great Basin, is reexamined in the light of the drastic vegetational displacements documented in the detailed Quaternary macrofossil record. Species/area plots of montane—subalpine conifers presently distributed on 54 Great Basin mountaintops show an overall insular pattern that is especially well developed on the subset of 38 ecological islands east of 116°W; the slope of z = 0.26 is close to the theoretical value for islands in equilibrium. All 11 taxa of montane—subalpine conifers that penetrate the Great Basin deeply have their main distributions in the Rocky Mountains; only three wide—ranging species occur also in the Sierra Nevada. A long sundering trough in the western Great Basin parallels and isolates the Cascade—Sierran uplift with low—elevation barriers that impede migration, but in the eastern Great Basin there are high connecting divides to the western Rockies, especially via an axial route southeast of the Bonneville basin. There is an east—west pattern of declining species richness of11 montane conifers in the Great Basin that correlates with distance from the rocky Mountain pool of 12 coniferous species. The pleniglacial subalpine forests in the lowlands of the central Great Basin had only one to three species of conifers (e.g.,Pinus longaeva ,Picea engelmannii ,Juniperus communis ). During the great late—glacial/Holocene (12 000—8000 yr BP) warming of climate, these shifted upward in elevation and were augmented in the east (but not in the west) by as many as five additional species of montane conifers. Macrofossil evidence indicted that the later Holocene arrivals dispersed across barriers of woodland and desert that by then isolated the shrunken montane islands. Moderately long—range transport of seeds by birds is deduced as follows: a northward latitudinal shift of 500—640 km during the Holocene is documented for several species of relatively thermophilous conifers, including the heavy—seeded, late—maturing pinyon pine. A 640—km migration in 8000 yr (80 m/yr) is indicated for pinyon, but the most generous estimate of its dispersal rate via the wind/gravity mode is a plodding 0.4 m/yr (3.2 km/8000 yr), orders of magnitude too slow. Seed dispersal by Clark's Nutcrackers and Pinyon Jays, however, is both the prevalent mode and amply swift enough to fit the known migrational history. Hence, the islands—in—equilibrium pattern indicated by the typically insular slope (z = 0.26) for montane conifers on the eastern set of mountaintops in the Great Basin is a reflection of Holocene immigration via sweepstakes dispersal being offset by extinction on the smaller islands. Both extinction and immigration of conifers are documented in the late—glacial/early—Holocene Neotoma record from the small Confusion Range in the east—central Great Basin of western Utah.
ABSTRACT: It is commonly thought that the climate conditions that supported lakes over a period of years in the Mojave Desert in southern California, only existed before 8,000 yr BP and that the environment has been arid since1,2 . Here we look at a drill core in the Silver Lake playa at the terminus of the Mojave River and find Holocene lake deposits which indicate that shallow lakes existed for at least a few decades. These deposits were radiocarbon dated at 3620 ±70 and 390 ± 90 yr BP, corresponding to the early Neo-glacial and the 'little ice age' respectively3 . To identify the conditions necessary to produce these Holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed. Available data indicate that there is a link between anomalous winter atmospheric conditions over the North Pacific and Mojave River floods that produced ephemeral lakes in the Silver Lake playa and that the Mojave River filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions. We suggest that the late Holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods.
ABSTRACT: While long recognized as important sources of information on sub-millenial-to millenial-scale changes in climate, hydrographically closed lakes have seldom been used to illuminate past climatic vicissitudes in the sub-century-to centuries time frame. Mono Lake, an artificially depressed, hydrographically closed water body that abuts the eastern escarpment of California's Sierra Nevada, is well-suited for this type of detailed, high-resolution paleoclimatic reconstruction. Streamcuts through the deltas of the main feeder streams reveal lake-transgressive and -regressive sedimentary sequences with intervening soils, as well as an abundance of datable materials and tephra layers. The expansiveness and clarity of these exposures make it possible to trace individual transgressive and regressive units landward from the lake for as much as 1000 m, enabling the elevations of past high stands and low stands to be defined with a high degree of precision. Twenty-five radiocarbon dates, many of them on the remains of vegetation killed during the lake transgressions, together with 4 tephra units of known age, provide chronometric control for the fluctuation curve.
The deltaic sequences, in combination with sedimentary, geomorphic, biotic, tephrostatigraphic, and historic evidence, indicate that during the past 3800 years Mono Lake has fluctuated over a vertical range of 40 m in response to changes in inflow and evaporation. Approximately 3770 cal B.P. the lake reached the Dechambeau Ranch High Stand at ~1980.8 m, a level that it had not occupied for perhaps 7000 years, and that it has not attained since. By ~1807 cal B.P. the lake had declined to an extreme low stand (the Marina Low Stand) at 1940.9 m. Over the ensuing six centuries it fluctuated little, remaining within an elevation interval (1945–1952 m) which is low by historic standards. The past 1200 years has been a period of rapid and large-scale fluctuations. During this interval Mono Lake has alternated between high stands (the Post Office, Rush Delta, Danberg Beach, Clover Ranch, and Historic high stands) of up to 1967.7 m, and low stabds (the Lee Vining Delta, Simis Ranch, Navy Beach, Rush Delta, and Pre-Historic low stands) as low as 1941 m.
The water balance model for the Mono Basin developed by Vorster (1985) was used to assess the changes in hydroclimatic conditions necessary to account for the reconstructed lake behavior. The model indicates that, on the sub-century to centuries time scale, effective inflow to the Mono groundwater basin over the past 3800 years has varied from greater than 134% to less than 68% of the modern (1937–1979) mean value. For the past 2 millennia the lake fluctuations seem to correspond in time to de Vries-type variations in solar activity.
L. V. Benson, D.R. Currey, R. I. Dorn, K. R. Lajoie, C.G. Oviatt, S. W. Robinson, G. I. Smith, S. Stine (1990). Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years. Paleogeography, Paleoclimatology, Paleoecology 78 (3-4): 241-286
ABSTRACT: During the past 35,000 years, Lake Bonneville, Lake Russell, and Lake Searles underwent a major period of lake-level change. The lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15,000 and 13,500 yr B.P. The rise of Lake Lahontan was gradual but not continuous, in part because of topographic constraints (intrabasin spill). Lake Lahontan also had an oscillation in lake level at 15,500 yr B.P. Radiocarbon-age estimations for materials that were deposited in the lake basins indicate that Lake Bonneville rose more or less gradually from 32,000 yr B.P., and had major oscillations in level between 23,000 and 21,000 yr B.P. and between 15,250 and 14,500 yr B.P. Lake Russell and Lake Searles had several major oscillations in lake level between 35,000 and 14,000 yr B.P. The timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states. All four lakes may have had nearly synchronous recessions between about 14,000 and 13,500 yr B.P. After the recessions, the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11,500 and 10,000 yr B.P. These data provide circumstantial evidence that the Younger Dryas Event affected climate on at least a hemispheric scale. During the Holocene, the four lakes remained at low levels, and small oscillations in lake level occurred. An important aspect of the lake-level data is the accompanying expansion of lake-surface area at the time of the last highstand. Lake Bonneville and Lake Lahontan had surface areas about 10 times larger than their mean-historical reconstructed areas whereas Lake Russell and Lake Searles had surface areas about 5 times larger than their mean-historical reconstructed areas. Differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream, or they may have resulted from lake/atmosphere feedback processes.
Benson, L.B., M. Kashgarian, R. Rye, S. Lund, F. Paillet, J. Smoot, C. Kester, S. Mensing, D. Meko, S. Lindström (2002). Holocene multidecadal and multicentennial droughts affecting northern California and Nevada. Quaternary Science Reviews 21 (4-6): 659-682
ABSTRACT: Continuous, high-resolutiond18 O records from cored sediments of Pyramid Lake, Nevada, indicate that oscillations in the hydrologic balance occurred, on average, about every 150 years (yr) during the past 7630 calendar years (cal yr). The records are not stationary; during the past 2740 yr, drought durations ranged from 20 to 100 yr and intervals between droughts ranged from 80 to 230 yr. Comparison of tree-ring-based reconstructions of climate change for the past 1200 yr from the Sierra Nevada and the El Malpais region of northwest New Mexico indicates that severe droughts associated with Anasazi withdrawal from Chaco Canyon at 820 cal yr BP (calendar years before present) and final abandonment of Chaco Canyon, Mesa Verde, and the Kayenta area at 650 cal yr BP may have impacted much of the western United States. During the middle Holocene (informally defined in this paper as extending from 8000 to 3000 cal yr BP), magnetic susceptibility values of sediments deposited in Pyramid Lake's deep basin were much larger than late–Holocene (3000–0 cal yr BP) values, indicating the presence of a shallow lake. In addition, the meand18 O value of CaCO3 precipitated between 6500 and 3430 cal yr BP was 1.6‰ less than the mean value of CaCO3 precipitated after 2740 cal yr BP. Numerical calculations indicate that the shift in thed18 O baseline probably resulted from a transition to a wetter (>30%) and cooler (3–5°C) climate. The existence of a relatively dry and warm middle-Holocene climate in the Truckee River–Pyramid Lake system is generally consistent with archeological, sedimentological, chemical, physical, and biological records from various sites within the Great Basin of the western United States. Two high-resolution Holocene-climate records are now available from the Pyramid and Owens lake basins which suggest that the Holocene was characterized by five climatic intervals. TIC andd18 O records from Owens Lake indicate that the first interval in the early Holocene (11,600–10,000 cal yr BP) was characterized by a drying trend that was interrupted by a brief (200 yr) wet oscillation centered at 10,300 cal yr BP. This was followed by a second early-Holocene interval (10,000–8000 cal yr BP) during which relatively wet conditions prevailed. During the early part of the middle Holocene (8000–6500 cal yr BP), high-amplitude oscillations in TIC in Owens Lake andd18 O in Pyramid Lake indicate the presence of shallow lakes in both basins. During the latter part of the middle Holocene (6500–3800 cal yr BP), drought conditions dominated, Owens Lake desiccated, and Lake Tahoe ceased spilling to the Truckee River, causing Pyramid Lake to decline. At the beginning of the late Holocene (~3000 cal yr BP), Lake Tahoe rose to its sill level and Pyramid Lake increased in volume.
O'Connor, J. E., L. L. Ely, E. E. Wohl, L. E. Stevens, T. S. Melis, V. S. Kale, V. R. Baker (1994). A 4500-year record of large floods on the Colorado River in Grand Canyon, Arizona. Journal of Geology 102 (1): 1-9
ABSTRACT: A sequence of flood deposits left by the Colorado River in the Grand Canyon, Arizona, provides evidence of at least 15 floods with peak discharges greater than 5500 m3 sec-1 over the last 4500 yr. Ten floods during the last 2000-2300 yr had discharges greater than 6800 m3 sec-1 . One flood, 1600-1200 yr ago, had a discharge exceeding 14,000 m3 sec-1 , a flow rate more than twice the largest gaged flood. This record of flooding is one of the longest for a major U.S. river, and, combined with the gaged record of twentieth century floods, allows determination of the frequency and history of large floods that have affected key aspects of Colorado River geomorphology.
G.A. Meyer, S. G. Wells, A. J. T. Jull (1995). Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes. Geological Society of America Bulletin 107 (10): 1211-1230
ABSTRACT: We employed a systemwide approach, a large and robust set of radiocarbon ages, and modern process analogs to interpret the Holocene history of forest fire–related sedimentation and overall alluvial activity in northeastern Yellowstone National Park. Debris-flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire-related sedimentation within valley-side alluvial fans of Soda Butte Creek. Fire-related deposits make up approximately 30% of the late Holocene fan alluvium. Fifty14 C ages on fire-related events cluster within the intervals of 3300–2900, 2600–2400, 2200–1800, and 1400–800 yr B.P. and suggest earlier episodes of large fires and fan aggradation around 7500, 5500, and 4600–4000 yr B.P. A major pulse of fire-related debris-flow activity between 950 and 800 yr B.P. coincided with the height of the widely recognized Medieval Warm Period (ca. a.d. 1050–1200). Instrumental climate records over the last 100 yr in Yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods, enhancing the potential for severe short-term drought, major forest fires, and storm-generated fan deposition.
Along lower Soda Butte Creek, fill-cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca. 8000 yr B.P. (terrace level T1a), 7000–5600 (T1b), 3100–2600 (T2), 2000–1300 (T3), and post–800 yr B.P. (T4). These periods coincide with overbank sedimentation on Slough Creek and the Lamar River but alternate with intervals of fire-related fan deposition, implying a strong climatic control. Local paleoclimatic data suggest cooler, effectively wetter conditions during terrace tread formation. In warmer, drier intervals, reduced average runoff in axial streams results in meander-belt narrowing; concurrent channel incision may be caused by infrequent large floods. Greater resistance to downcutting, however, allowed fewer terraces to be formed along Slough Creek and the Lamar River. Alluvial systems in northeastern Yellowstone show a clear response to millennial-scale climatic cycles, wherein alluvial fans aggrade and prograde over flood plains in drier periods. Axial streams widen their flood plains and trim back the fans during wetter periods. “Small-scale” climatic fluctuations of the Holocene thus had substantial impact on postglacial landscapes in northeastern Yellowstone.
ABSTRACT: The Stanley River in western Tasmania, Australia, contains sub-fossil rainforest logs within the channel and floodplain. Of the more than 85 radiocarbon dates obtained, all but 3 date from 17 ka to the present and permit an interpretation of fluvial and related environmental changes over this period. Particular attention is focused on the interactive relationship between the river and its riparian rainforest. Following the Last Glacial Maximum, the Stanley River was a laterally active gravel-load system reworking most of its valley floor in the upstream reaches. With ameliorating conditions at the end of the Pleistocene, climate became less seasonal and flow regimes less energetic. Huon pines already present in the catchment, re-asserted themselves in the form of dense tree cover along the river banks and floodplains with basal floodplain deposition shifting from gravels to coarse sands and granules. By about 3.5 ka, a further change in climate reduced stream discharges substantially. As a result the channel reduced in size, transported finer sediment, became laterally stable, and the floodplain accreted with overbank deposits of sand and silt. Huon pines falling into the channel formed obstructions of woody debris, some surviving for 2 ka. These have reduced stream power and boundary shear stress, further contributing to channel stability. Generational sequences of Huon pines on the river banks, some extending back 1–2 ka, are additional evidence of this stability. Since the Pleistocene, changing climate and the re-establishment of dense riparian rainforest appear to have stabilised the river channels and floodplains of western Tasmania.
ABSTRACT: Evidence from paleoclimatological research indicates that major climatic changes, such as the rapid increase in temperatures at the end of the Younger Dryas event ~11000 years ago, can occur over the span of a few decades. Vegetation response to climatic variation and change, is, by contrast, often assumed to occur gradually over much longer timescales. Two recent papers confirm earlier, theoretical predictions that changes in species composition of plant communities following climatic shifts can, however, occur with striking rapidity.
ABSTRACT: Sediments from Rapid Lake document glacial and vegetation history in the Temple Lake valley of the Wind River Range, Wyoming over the past 11,000 to 12,000 yr. Radiocarbon age determinations on basal detrital organic matter from Rapid Lake (11,770 ± 710 yr B.P.) and Temple Lake (11,400 ± 630 yr B.P.) bracket the age of the Temple Lake moraine, suggesting that the moraine formed in the late Pleistocene. This terminal Pleistocene readvance may be represented at lower elevations by the expansion of forest into intermontane basins 12,000 to 10,000 yr B.P. Vegetation in the Wind River Range responded to changing environmental conditions at the end of the Pleistocene. Following deglaciation, alpine tundra in the Temple Lake valley was replaced by aPinus albicaulis parkland by about 11,30014 C yr B.P.Picea andAbies , established by 10,60014 C yr B.P., grew withPinus albicaulis in a mixed conifer forest at and up to 100 m above Rapid Lake for most of the Holocene. Middle Holocene summer temperatures were about 1.5°C warmer than today. By about 540014 C yr B.P.Pinus albicaulis andAbies became less prominent at upper treeline because of decreased winter snowpack and higher maximum summer temperatures. The position of the modern treeline was established by 300014 C yr B.P. when Picea retreated downslope in response to Neoglacial cooling.
FIRST PARAGRAPH: With the prospect of global warming, it is interesting to look back at past climate change and its effects on vegetation. Although the greatest change in climate probably occurred during deglaciation, 12,500 to 11,000 years ago, significant climate changes have occurred more recently in the intermountain region of the western United States, the vast area in the West lying between the Sierra Nevadas and Cascades and the Rocky Mountains (Antevs 1938, Davis 1982, Mehringer and Wigand 1990). Climate changes during this period caused major shifts in plant distribution and composition throughout the region. However, rates of vegetation change during the past 120 years, primarily due to anthropogenic factors, have been unprecedented in the intermountain region (Miller et al. 1994).
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ABSTRACT: Pollen and algae microfossils preserved in sediments from Pyramid Lake, Nevada, provide evidence for periods of persistent drought during the Holocene age. We analyzed one hundred nineteen 1-cm-thick samples for pollen and algae from a set of cores that span the past 7630 years. The early middle Holocene, 7600 to 6300 cal yr B.P., was found to be the driest period, although it included one short but intense wet phase. We suggest that Lake Tahoe was below its rim for most of this period, greatly reducing the volume and depth of Pyramid Lake. Middle Holocene aridity eased between 5000 and 3500 cal yr B.P. and climate became variable with distinct wet and dry phases. Lake Tahoe probably spilled intermittently during this time. No core was recovered that represented the period between 3500 and 2600 cal yr B.P. The past 2500 years appear to have had recurrent persistent droughts. The timing and magnitude of droughts identified in the pollen record compares favorably with previously published18 O data from Pyramid Lake. The timing of these droughts also agrees with the ages of submerged rooted stumps in the Eastern Sierra Nevada and woodrat midden data from central Nevada. Prolonged drought episodes appear to correspond with the timing of ice drift minima (solar maxima) identified from North Atlantic marine sediments, suggesting that changes in solar irradiance may be a possible mechanism influencing century-scale drought in the western Great Basin.
ABSTRACT: Stratigraphic, sedimentologic, and pedologic studies of beach ridge and lacustrine deposits indicate that up to five times during the Holocene, shallow lakes covered Silver Lake playa in southeastern California for periods of years to decades. The two youngest lacustrine events (at about 390 ± 90 yr B. P. and 3620 ± 70 yr B. P.) coincide with the early and late Neoglacial episodes of North America. Increasing evidence in recent years from other nonglaciated areas leads us to conclude that the effects of these climatic episodes were much more widespread than previously thought. The climate during these episodes was characterized by an increased frequency of winter storms in the southwestern United States, causing wetter conditions that affected diverse, hyperarid environments in the Mojave Desert and adjacent regions. We propose that this wide areal coverage was caused by large-scale, winter atmospheric circulation patterns, which are probably related to changes in sea-surface temperatures and oceanic circulation in the eastern North Pacific Ocean.
ABSTRACT: New dating in the Carson Sink at the termini of the Humboldt and Carson rivers in the Great Basin of the western United States indicates that lakes reached elevations of 1204 and 1198 m between 915 and 652 and between 1519 and 1308 cal yr B.P., respectively. These dates confirm Morrison's original interpretation (Lake Lahontan: Geology of the Southern Carson Desert, Professional Paper 40, U.S. Geol. Survey, 1964) that these shorelines are late Holocene features, rather than late Pleistocene as interpreted by later researchers. Paleohydrologic modeling suggests that discharge into the Carson Sink must have been increased by a factor of about four, and maintained for decades, to account for the 1204-m lake stand. The hydrologic effects of diversions of the Walker River to the Carson Sink were probably not sufficient, by themselves, to account for the late Holocene lake-level rises. The decadal-long period of increased runoff represented by the 1204-m lake is also reflected in other lake records and in tree ring records from the western United States.
ABSTRACT: Palynological data from sediment cores from the Ruby Marshes provide a record of environmental and climatic changes over the last 40,000 yr. The modern marsh waters are fresh, but no deeper than ~3 m. A shallow saline lake occupied this basin during the middle Wisconsin, followed by fresh and perhaps deep waters by 18,000 to 15,000 yr B.P. No sediments were recovered for the period between 15,000 and 11,000 yr B.P., possibly due to lake desiccation. By 10,800 yr B.P. a fresh-water lake was again present, and deeper-than-modern conditions lasted until 6800 yr B.P. The middle Holocene was characterized by very shallow water, and perhaps complete desiccation. The marsh system deepened after 4700 yr B.P., and fresh-water conditions persisted until modern times. Vegetation changes in Ruby Valley were more gradual than those seen in the paleolimnological record. Sagebrush steppe was more widespread than at present through the late Pleistocene and early Holocene, giving way somewhat to expanded shadscale vegetation between 8500 and 6800 yr B.P. Shadscale steppe contracted by 4000 yr B.P., but had greater than modern coverage until 1000 to 500 yr ago. Pinyon-juniper woodland was established in the southern Ruby Mountains by 4700 yr B.P.
ABSTRACT: Chronological correlations established at different time scales among the lake-level fluctuations in the Jura and French Subalpine ranges, glacier movements in the Swiss and Austrian Alps, and the atmospheric14 C record during the last 7 millennia show coincidences between lake-level rises, glacier advances, and high14 C production and vice versa. These correspondences suggest that the short-term14 C variations may be an empirical indicator of Holocene palaeoclimates and argue for possible origins of Holocene climatic oscillations: (1) The varying solar activity refers to secular climatic oscillations and to major climatic deteriorations showing a ca. 2300-yr periodicity. (2) A question is raised about a relationship between the earth's magnetic field and climate. First, the weak-strength periods of the earth's dipole magnetic field (between 3800 and ca. 2500 B.C.) coincide with higher climate variability, and vice versa. Second, the ca. 2300-yr periods revealed by the14 C record and also by the major climatic deteriorations re. corded in Jurassian lakes (ca. 1500 A.D., ca. 800 B.C., and ca. 3500 B.C.) coincide with the ca. 2300-yr periods revealed by the earth's nondipole geomagnetic field. The present warming induced by anthropogenic factors should be intensified during the next few centuries by natural factors of climate evolution.
Baker, R.G., G. G. Fredlund, R. D. Mandel, E. A. BettisIII (2000). Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska. Quaternary International 67 (1): 75-88
ABSTRACT: Pollen, plant macrofossils, phytoliths, carbon isotopes, and alluvial history from sediments exposed along the South Fork of the Big Nemaha River, southeastern Nebraska, USA, provide an integrated reconstruction of changes in Holocene vegetation, climate, and fluvial activity. From 9000 to 8500 uncalibrated14 C yr BP, climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly, upland deciduous forest declined, and prairie attained its Holocene dominance. From 8500 to 5800 yr BP. upland forest elements disappeared, and even riparian trees were sparse under dry climatic conditions. Alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr BP. From 5800 to 3100 yr BP, riparian forests returned to prominence, and droughts were intermittent. Alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley. Aggradation on alluvial fans slowed and finally ceased near the end of this period. During a short dry interval from 3100 to 2700 yr BP riparian trees (except elm) disappeared, and prairie and weedy species became more abundant. This interval is represented by the organic Roberts Creek Member, and the alluvial setting was a slightly incised meandering channel belt. Habitats became similar to presettlement conditions during the last 2700 yr BP. Weedy taxa dominate modern samples, reflecting widespread disturbance. Alluvial fans and terrace surfaces were stable during the last 2500 years, but episodes of floodplain aggradation were punctuated by incision of the main channel.
ABSTRACT: Measured18 O/16 O ratios from the Greenland Ice Sheet Project 2 (GISP2) ice core extending back to 16,500 cal yr B.P. provide a continuous record of climate change since the last glaciation. High-resolution annual18 O/16 O results were obtained for most of the current millennium (A.D. 818-1985) and record the Medieval Warm Period, the Little Ice Age, and a distinct 11-yr18 O/16 O cycle. Volcanic aerosols depress central Greenland annual temperature (~1.5°C maximally) and annual18 O/16 O for about 4 yr after each major eruptive event. On a bidecadal to millennial time scale, the contribution of solar variability to Holocene Greenlandic temperature change is ~0.4°C. The role of thermohaline circulation change on climate, problematic during the Holocene, is more distinct for the 16,500-10,000 cal yr B.P. interval. (Analogous to14 C age calibration terminology, we express time in calibrated (cal) yr B.P. (A.D. 1950 = 0 cal yr B.P.)). The Oldest Dryas/Bølling/Older Dryas/Allerød/Younger Dryas sequence appears in great detail. Bidecadal variance in18 O/16 O, but not necessarily in temperature, is enhanced during the last phase of lateglacial time and the Younger Dryas interval, suggesting switches of air mass transport between jet stream branches. The branched system is nearly instantaneously replaced at the beginning of the Bølling and Holocene (at ~14,670 and ~11,650 cal yr B.P., respectively) by an atmospheric circulation system in which18 O/16 O and annual accumulation initially track each other closely. Thermodynamic considerations of the accumulation rate-temperature relationship can be used to evaluate the18 O/16 O-temperature relationship. The GISP2 ice-layer-count years of major GISP2 climate transitions also support the use of coral14 C ages for age calibration.
ABSTRACT: A coupled ocean-atmosphere general circulation model is used to investigate climatic shift of El Nino in the Holocene. The model simulates a reduced ENSO intensity in the early and mid-Holocene, in agreement with paleoclimate record. The ENSO reduction is proposed to be caused by both an intensified Asian summer monsoon and a warm water subduction from the South Pacific into the equatorial thermocline.
ABSTRACT: An understanding of the response of a fluvial system to past climatic changes is useful for predicting its response to future shifts in temperature and precipitation. To determine the response of the Columbia River system to previous climatic conditions and transitions, a well-dated sequence of floodplain development in the Wells Reservoir region was compared with the paleoenvironmental history of the Columbia River Basin. Results of this comparison indicate that aggradation episodes, occurring approximately 9000-8000, 7000-6500, 4400-3900, and 2400-1800 yr B.P., coincided with climatic transitions that share certain characteristics. The inferred climates associated with aggradation had at least moderate rates of precipitation that occurred mainly in winter coupled with moderate winter temperatures. Such conditions would have resulted in the buildup of snowpacks and a high frequency of rain-on-snow events. The warming and precipitation increases predicted for the Pacific Northwest under most CO2 -doubling scenarios are likely to repeat these conditions, which could increase the frequency of severe, sediment-laden floods in the Columbia River Basin.
ABSTRACT: Recent examinations of the possible hydrological response to global warming have emphasized changes in average conditions, rather than individual flooding events. Historical accounts suggest, however, that such events may have had a considerable regional impact even in the face of any relatively modest climate change. Here I present a 7,000-year geological record of overbank floods for upper Mississippi river tributaries in mid-continent North America, which provides concrete evidence for a high sensitivity of flood occurrence to changing climate. During a warmer, drier period between about 3,300 and 5,000 years ago, the largest, extremely rare floods were relatively small—the size of floods that now occur about once every fifty years. After ~3,300 years ago, when the climate became cooler and wetter, an abrupt shift in flood behaviour occurred, with frequent floods of a size that now recurs only once every 500 years or more. Still larger floods occurred between about AD 1250 and 1450, during the transition from the medieval warm interval to the cooler Little Ice Age. All of these changes were apparently associated with changes in mean annual temperature of only about 1–2 °C and changes in mean annual precipitation of 10–20%.
Enzel, Y., S. G. Wells (1997). Extracting Holocene paleohydrology and paleoclimatology information from modern extreme flood events: An example from southern California. Geomorphology 19 (3-4): 203-226
ABSTRACT: The extraction of paleohydrological and paleoclimatological information from a modern hydrological system, shown to represent unique and extreme hydroclimatological conditions, is illustrated by an example from the Mojave River drainage basin in southern California. The Mojave River allows only the most extreme floods to reach its terminal basin in the Silver Lake playa and to form ephemeral lakes. All the other floods are lost by transmission into the alluvial aquifer along its 200 km channel. This filtering out of regular floods by the river provides an essential tool in establishing a physical link between atmospheric and hydrologic conditions. We demonstrate such a link between anomalous, present-day atmospheric circulation patterns over the North Pacific Ocean, extreme storms in southern California that produced the heaviest precipitation on record, the largest floods of record in the Mojave River watershed, and ephemeral lakes in its terminal playa. This physical link determines the possible cause of the formations of perennial, short-duration, shallow lakes in Silver Lake playa during the late Holocene and characterizes the hydroclimatic conditions that prevailed during these lacustrine episodes. Hydrological simulations of this river and its filtering character demonstrate that these lakes could have formed only if the most extreme modern storms and floods were more frequent in at least an order of magnitude during specific time episodes. We conclude that such extreme hydroclimatic conditions occurred more frequent in past episodes during which the Holocene lakes formed. In turn, this conclusion indicates that the cause of these storms and floods, i.e. the anomalous atmospheric circulation pattern, must have been more frequent. This research outlines a way to extract information on Holocene climates in hydrologic settings that demonstrate a unique cause and effect relationship.
ABSTRACT: We reconstructed a 3500-yr history of fluctuations in treeline elevation and tree abundance in the southern Sierra Nevada. Treeline elevation was higher than at present throughout most of the last 3500 yr. Declines in the abundance of live trees and treeline elevation occurred twice during the last 1000 yr: from 950 to 550 yr BP and from 450 to 50 yr BP. The earlier decline coincided with a period of warm temperatures (relative to present) in which at least two severe, multidecadal droughts occurred. This decline was apparently triggered by an increase in the rate of adult mortality in treeline forests. The more recent decline occurred during a period of low temperatures lasting for up to 400 yr and was apparently caused by a sustained failure of regeneration in combination with an increased rate of adult mortality. The apparent past importance of precipitation in controlling the position and structure of the treeline ecotone suggests that climatic controls over treeline may be more complex than previously thought. In the Sierra Nevada, responses of high-elevation forests to future warming may depend strongly on water supply.
ABSTRACT: Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene. By tracking climatically sensitive forest boundaries, the moisture-controlled lower timberline and the temperature-controlled upper timberline, paleoclimatic estimates can be derived from modern temperature and precipitation lapse rates. Pollen data suggest that prior to 11 000 yr B.P., a subalpine forest dominated byPicea (spruce) andPinus (pine) grew 300–700 m below its modern limit. The inferred climate was 2–5 °C cooler and had 7–16 cm greater precipitation than today.Abies (fir) increased in abundance in the subalpine forest around 11 000 yr B.P., probably in response to cooler conditions with increased winter snow. Pollen and plant macrofossil data demonstrate that from 9000 to 4000 yr B.P. the subalpine forest occupied a greater elevational range than it does today. Upper timberline was 270 m above its modern limit, suggesting that mean annual and mean July temperatures were 1–2 °C warmer than today. Intensification of the summer monsoon, coupled with increased summer radiation between 9000 and 6000 yr B.P., raised mean annual precipitation by 8–11 cm and allowed the lower limit of the subalpine and montane forests to descend to lower elevations. The lower forest border began to retreat upslope between 6000 and 4000 yr B.P. in response to drier conditions, and the upper timberline descended after 4000 yr B.P., when temperatures cooled to about 1 °C warmer than today. The modern climatic regime was established about 2000 yr B.P., when the summer precipitation maxima of the early and middle Holocene were balanced by increased winter precipitation.
ABSTRACT: Field investigations at Dugway Proving Ground in western Utah have produced new data on the chronology and human occupation of late Pleistocene and early Holocene lakes, rivers, and wetlands in the Lake Bonneville basin. We have classified paleo-river channels of these ages as "gravel channels" and "sand channels." Gravel channels are straight to curved, digitate, and have abrupt bulbous ends. They are composed of fine gravel and coarse sand, and are topographically inverted (i.e., they stand higher than the surrounding mudflats). Sand channels are younger and sand filled, with well-developed meander-scroll morphology that is truncated by deflated mudflat surfaces. Gravel channels were formed by a river that originated as overflow from the Sevier basin along the Old River Bed during the late regressive phases of Lake Bonneville (after 12,500 and prior to 11,00014 C yr B.P.). Dated samples from sand channels and associated fluvial overbank and wetland deposits range in age from 11,000 to 880014 C yr B.P., and are probably related to continued Sevier-basin overflow and to groundwater discharge. Paleoarchaic foragers occupied numerous sites on gravel-channel landforms and adjacent to sand channels in the extensive early Holocene wetland habitats. Reworking of tools and limited toolstone diversity is consistent with theoretical models suggesting Paleoarchaic foragers in the Old River Bed delta were less mobile than elsewhere in the Great Basin.
ABSTRACT: Holocene fire–climate–vegetation linkages are mostly understood at individual sites by comparing charcoal and pollen records with other paleoenvironmental proxy and model simulations. This scale of reconstruction often obscures detection of large-scale patterns in past fire activity that are related to changes in regional climate and vegetation. A network of 31 charcoal records from southern South America was examined to assess fire history along a transect from subtropic to subantarctic biomes. The charcoal data indicate that fire activity was greater than present at ca. 12,000 cal yr BP and increased further and was widespread at 9500 cal yr BP. Fire activity decreased and became more spatially variable by 6000 cal yr BP, and this trend continued to present. Atmospheric circulation anomalies during recent high-fire years show a southward shift in westerlies, and paleoclimate model simulations and data syntheses suggest that such conditions may have prevailed for millennia in the early Holocene when the pole-to-equator temperature gradients were weaker and annual temperatures were higher than present, in response to orbital-time-scale insolation changes.
ABSTRACT: Fire history data provide information on the role of fire as a long-term ecological process and the changes in fire activity that have occurred in response to variations in climate, vegetation and human activity. Fire reconstructions are derived from dendrochronological records that span the last few centuries and charcoal records obtained from the sediments of lakes and wetlands that span millennia. Several advances in the study of charcoal records have improved their usefulness for palaeoecological research. For example, calibration studies of charcoal inputs during and following recent fires indicate that macroscopic charcoal particles are not transported far from the source area. In addition, high-resolution charcoal time series are now interpreted in terms of two components—a peaks component that registers past fire events and a background component that represents long-term variations in charcoal production and transport. This paper presents an examination of charcoal and pollen records from the western United States that illustrates the potential of this type of research for reconstructing past environments.
ABSTRACT: The environmental history of the Northern Rocky Mountains was reconstructed using lake sediments from Burnt Knob Lake, Idaho, and comparing the results with those from other previously published sites in the region to understand how vegetation and fire regimes responded to large-scale climate changes during the Holocene. Vegetation reconstructions indicate parkland or alpine meadow at the end of the glacial period indicating cold-dry conditions. From 14,000 to 12,000 cal yr B.P., abundant Pinus pollen suggests warmer, moister conditions than the previous period. Most sites record the development of a forest with Pseudotsuga ca. 9500 cal yr B.P. indicating warm dry climate coincident with the summer insolation maximum. As the amplification of the seasonal cycle of insolation waned during the middle Holocene, Pseudotsuga was replaced by Pinus and Abies suggesting cool, moist conditions. The fire reconstructions show less synchroneity. In general, the sites west of the continental divide display a fire-frequency maximum around 12,000–8000 cal yr B.P., which coincides with the interval of high summer insolation and stronger-than-present subtropical high. The sites on the east side of the continental divide have the highest fire frequency ca. 6000–3500 cal yr B.P. and may be responding to a decrease in summer precipitation as monsoonal circulation weakened in the middle and late Holocene. This study demonstrated that the fire frequency of the last two decades does not exceed the historical range of variability in that periods of even higher-than-present fire frequency occurred in the past.
ABSTRACT: A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The flood chronologies were constructed using fine-grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events. High-magnitude floods were frequent on rivers throughout the region from 5000 to 360014 C yrs BP (dendrocalibrated age = 3800-2200 BC) and increased again after 2200 BP (400 BC), with particularly prominent peaks in magnitude and frequency around 1100-900 BP (AD 900-1100) and after 500 yrs BP (AD 1400). In contrast, the periods 3600-2200 BP (2200-400 BC) and 800-600 yrs BP (1200-1400 AD) are marked by sharp decreases in the occurrence of large floods on these rivers.
In the modern record, storms that generate large floods ( 10-year) in the region fall into three categories: (1) winter North Pacific frontal storms; (2) late-summer and fall storms that draw in moisture from recurved Pacific tropical cyclones; and (3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study, and are the most probable causes of the paleofloods over the last 5000 years. Floods from both winter storms and tropical cyclones occur when deep mid-latitude troughs steer storm systems into the region. Composite anomaly maps of daily 700-mbar heights indicate that these floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. A strong connection exists between the negative phase of the Southern Oscillation Index (often associated with El Niño conditions) and the large floods associated with winter storms and tropical cyclones.
The paleoflood records confirm the existence of centennial-scale variations in the conditions conducive to the occurrence of extreme floods and flood-generating storms in this region. The episodes with an increased frequency of high-magnitude floods coincide with periods of cool, wet climate in the western U.S., whereas warm intervals, such as the Medieval Warm Period, are times of dramatic decreases in the number of large floods. A positive relationship between the paleofloods and long-term variations in the frequency of El Niño events is evident over the last 1000 years. This relationship continues over at least the last 3000 years with warm coastal sea-surface temperatures indicative of El Niño-like conditions.
Pederson, J., Smith, G., Pazzaglia, F. (2001). Comparing the modern, Quaternary, and Neogene records of climate-controlled hillslope sedimentation in southeast Nevada. Geological Society of America Bulletin 113 (3): 305-319
ABSTRACT: The vast majority of all sediment is derived from hillslopes. Attempts to understand what controls variability in the sedimentary record should therefore consider the primary variability of hillslope sediment yield to depositional basins. But our understanding of controls on sedimentation, particularly climatic controls, is limited by poor understanding of the links between hillslopes and depositional systems. This is partly because the applications of geomorphic research to sedimentology are not fully realized. The long-term hillslope stratigraphic records of this study provide a crucial physical link between hillslope sediment sources and depositional basins, and between geomorphology and sedimentology. We compare rare Neogene colluvium and buried hillslopes preserved in superproximal basin-fill exposures to their Quaternary and modern equivalents in two tectonically quiescent basins in southeastern Nevada. Field and laboratory geomorphic and sedimentologic methods are employed to document the provenance of proximal basin sediment, the character and relative amount of sediment produced on local hillslopes at different times, the hillslope weathering and transport processes occurring through time, and the role that rock-type differences have played. Physical weathering processes have dominated the production of angular, pebbly colluvium on both ancient and modern slopes, and overland flow has been the main process transporting detritus off slopes. Although hillslope processes and products in the study area remained the same in upper Miocene, Pliocene, Pleistocene, and modern records, process rates have varied greatly, indicating that orbital-scale climatic cyclicity can be, but is not always, well expressed in the stratigraphy of continental basins. The vast majority of basin sediment in the study area is derived from hillslopes underlain by volcanic rather than carbonate bedrock, and rock type is the dominant control on sediment yield and landscape development in this tectonically inactive, dry setting.
Forman, S.L., Oglesby, R., Webb, R.S. (2001). Temporal and spatial patterns of Holocene dune activity on the Great Plains of North America: megadroughts and climate links. Global and Planetary Change 29: 1-29
ABSTRACT: The Holocene record of eolian sand and loess deposition is reviewed for numerous presently stabilized dune fields on the Great Plains of North America. Dune field activity reflects decade-to-century-scale dominance of drought that exceeded historic conditions, with a growing season deficit of precipitation >25%. The largest dune fields, the Nebraska Sand Hills and ergs in eastern Colorado, Kansas and the Southern High Plains showed peak activity sometime between ca. 7 and 5 cal. ka. Loess deposition between ca. 10 and 4 cal. ka also signifies widespread aridity. Most dune fields exhibit evidence for one or more reactivation events sometime in the past 2 cal. ka; a number of localities register two events post 1 cal. ka, the latest potentially after 1400 AD. However, there is not a clear association of the latest dune remobilization events with up to 13 droughts in the past 2 cal. ka identified in dendroclimatic and lacustrine records. Periods of persistent drought are associated with a La Niña-dominated climate state, with cooling of sea surface temperatures in the tropical Pacific Ocean and later of the tropical Atlantic Ocean and the Gulf of Mexico that significantly weakens cyclogenesis over central North America. As drought proceeds, reduced soil moisture and vegetation cover would lessen evaporative cooling and increase surface temperatures. These surface changes strengthen the eastward expansion of a high-pressure ridge aloft and shift the jet stream northward, further enhancing continent-wide drought. Uncertainty persists if dune fields will reactivate in the future at a scale similar to the Holocene because of widespread irrigation, the lack of migratory bison herds, and the suppression of prairie fires, all of which enhance stabilization of dune fields in the Great Plains.
ABSTRACT.—Plant macrofossil analyses of 16 radiocarbon-dated woodrat middens spanning the past 4000 years from the Wind River Canyon region in central Wyoming provide information concerning late Holocene development of juniper woodlands. The study sites are currently dominated byJuniperus osteosperma , withJ. scopulorum present locally. Woodlands in the region were dominated byJ. scopulorum from ca 4000 yr BP until at least 2800 yr BP.Juniperus osteosperma invaded and expanded before 2000 yr BP. This expansion fits a regional pattern ofJ. osteosperma colonization and expansion in north central Wyoming during a relatively dry period between 2800 and 1000 yr BP. At the time the Wind River Canyon region was colonized byJ. osteosperma , the species had populations 50–100 km to both the north and south. Long-distance seed dispersal was required for establishment in the study area. Genetic studies are necessary to identify source populations and regions.
ABSTRACT: The forests of the Siskiyou Mountains are among the most diverse in North America, yet the long-term relationship among climate, diversity, and natural disturbance is not well known. Pollen, plant macrofossils, and high-resolution charcoal data from Bolan Lake, Oregon, were analyzed to reconstruct a 17,000-yr-long environmental history of high-elevation forests in the region. In the late-glacial period, the presence of a subalpine parkland ofArtemisia , Poaceae,Pinus , andTsuga with infrequent fires suggests cool dry conditions. After 14,500 cal yr B.P., a closed forest ofAbies ,Pseudotsuga ,Tsuga , andAlnus rubra with more frequent fires developed which indicates more mesic conditions than before. An open woodland ofPinus ,Quercus , andCupressaceae , with higher fire activity than before, characterized the early Holocene and implies warmer and drier conditions than at present. In the late Holocene,Abies andPicea were more prevalent in the forest, suggesting a return to cool wet conditions, although fire-episode frequency remained relatively high. The modern forest ofAbies andPseudotsuga and the present-day fire regime developed ca. 2100 cal yr B.P. and indicates that conditions had become slightly drier than before. Sub-millennial-scale fluctuations in vegetation and fire activity suggest climatic variations during the Younger Dryas interval and within the early Holocene period. The timing of vegetation changes in the Bolan Lake record is similar to that of other sites in the Pacific Northwest and Klamath region, and indicates that local vegetation communities were responding to regional-scale climate changes. The record implies that climate-driven millennial- to centennial-scale vegetation and fire change should be considered when explaining the high floristic diversity observed at present in the Siskiyou Mountains.
ABSTRACT: A large assemblage of salmon bones excavated 50 yr ago from an ~10,000-yr-old archaeological site near The Dalles, Oregon, USA, has been the primary evidence that early native people along the Columbia River subsisted on salmon. Recent debate about the human role in creating the deposit prompted excavation of additional deposits and analysis of archaeologic, geologic, and hydrologic conditions at the site. Results indicate an anthropogenic source for most of the salmonid remains, which have associated radiocarbon dates indicating that the site was occupied as long ago as 9300 cal yr B.P. The abundance of salmon bone indicates that salmon was a major food item and suggests that migratory salmonids had well-established spawning populations in some parts of the Columbia Basin by 9300–8200 yr ago.
ABSTRACT: Modern complex societies exhibit marked resilience to interannual-to- decadal droughts, but cultural responses to multidecadal-to-multicentury droughts can only be addressed by integrating detailed archaeological and paleoclimatic records. Four case studies drawn from New and Old World civilizations document societal responses to prolonged drought, including population dislocations, urban abandonment, and state collapse. Further study of past cultural adaptations to persistent climate change may provide valuable perspective on possible responses of modern societies to future climate change.
E. C. Carson, J. C. Knox, D. M. Mickelson (2007). Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah. Geological Society of America Bulletin 119 (9): 1066-1078
ABSTRACT: Long-term variations in Holocene flood magnitude were quantified from the bankfull dimensions of abandoned channels preserved on floodplain surfaces in the northern Uinta Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were reconstructed, and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross-section areas. The results indicate systematic (nonrandom) variations of bankfull floods in the northern Uinta Mountains. Large floods, as much as 10%–15% greater than modern, dominated from 8500 to 5000 calendar yr B.P., and again from 2800 to 1000 cal yr B.P. Small floods, as much as 15%–20% less than modern, characterize the periods from 5000 to 2800 cal yr B.P., and from 1000 cal yr B.P. to near present.
The middle and late Holocene record of bankfull flood magnitude compares well with independent evidence for climatic variation in the area. The early Holocene record indicates that larger than modern bankfull floods coincide with warmer than modern mean annual temperature. We hypothesize that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid melting of deep snowpacks in the high Uinta Mountains, thus producing large floods despite warmer mean annual temperatures. The episode of smaller than modern bank-full floods between 5000 and 2800 cal yr B.P. coincides with records of increased forest fire frequency in the northern Uintas. Larger than modern floods from 2800 to 1000 cal yr B.P. coincide with a local decrease in forest fire frequency and evidence for minor local glacial readvances. The decrease in flood magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly.
ABSTRACT: The vegetation of the montane and subalpine zones of the Rocky Mountains is a mosaic of conifer forests and large (1 ha to several square kilometers) treeless “parks” dominated by sagebrush (Artemisia spp.), grasses, and forbs. Three hypotheses for the origin of parks are proposed. The “permanent site hypothesis” states that the park–forest vegetation mosaic is a result of differences in physical characteristics of sites. In the “remnant hypothesis” parks are thought to be remnants of vegetation that was widespread under previous climate conditions. The “replacement hypothesis” states that parks replace forest vegetation in response to disturbance, climate change, or a combination of these two factors. Patterns in the past distribution of park and forest vegetation in the vicinity of Fish Creek Park (elevation 2750 m) were used to test these hypotheses.
Fossil pollen extracted from the sediments of five small ponds in and around Fish Creek Park was used to reconstruct Holocene vegetation changes. Changes in vegetation were reconstructed through the use of multivariate analyses and pollen ratios derived from modern surface samples and by comparison with pollen data from other studies. The pollen record indicates that shortly after deglaciation (11000 yr BP) the area supported alpine tundra, followed by whitebark pine–spruce–fir parkland at 9500 yr BP. From 8500 to 6000 yr BP, a pine parkland occupied the area, perhaps in response to climate conditions warmer than today. By 5000 yr BP a mixed pine–spruce–fir forest resembling the modern subalpine forest near Fish Creek Park probably replaced the pine parkland at all five sites. The modern park vegetation originated only within the last 2500 yr.
The conversion to park vegetation may not have been synchronous at all three sites, and the replacement of forest by park did not always result in a long-term conversion to park vegetation. The timing and pattern of changes in the vegetation mosaic eliminate the permanent site and remnant hypotheses and suggest instead that climatic cooling over the last several thousand years, possibly combined with removal of forest cover by fire or some other disturbance, could explain the origin of Fish Creek Park.
Whitlock, C., S. L. Shafer, J. Marlon (2003). The role of climate and vegetation change in shaping past and future fire regimes in the northwestern US and the implications for ecosystem management. Forest Ecology and Management 178 (1-2): 5-21
ABSTRACT: Fire is an important part of the disturbance regimes of northwestern US forests and its role in maintaining and altering forest vegetation is evident in the paleoecological record of the region. Long-term reconstructions of Holocene fire regimes, provided by the analysis of charcoal, pollen, and other fire proxies in a network of lake records, indicate that the Pacific Northwest and summer-dry regions of the northern Rocky Mountains experienced their highest fire activity in the early Holocene (11,000–7000 years ago) and during the Medieval Warm Period (ca. 1000 years ago) when drought conditions were more severe than today. In contrast, in summer-wet areas of the northern Rocky Mountains, the period of highest fire activity was registered in the last 7000 years when dry woodland vegetation developed. When synthesized across the entire northwestern US, the paleoecological record reveals that past and present fire regimes are strongly controlled by climate changes occurring on multiple time scales. The scarcity of fires in the 20th century in some northwestern US ecosystems may be the result of successful fire suppression policies, but in wetter forests this absence is consistent with long-term fire regime patterns. In addition, simulations of potential future climate and vegetation indicate that future fire conditions in some parts of the northwestern US could be more severe than they are today. The Holocene record of periods of intensified summer drought is used to assess the nature of future fire–climate–vegetation linkages in the region.
ABSTRACT: Fifteen alluvial sequences in Texas and Oklahoma exhibit the same late Holocene record of channel trenching at 1 ka. The erosion was preceded by slow alluvial sedimentation in most stream valleys, resulting in the formation of a cumulic, organic-rich flood-plain soil previously named the Copan Soil. The soil formed during a period of regionally moister climate, as indicated by pollen spectra, molluscan faunas, vertebrate faunas, sedimentary structures, and high alluvial water tables. At 1 ka, the regional climate changed from moist to dry, coinciding with an episode of channel incision of valley floors throughout the southern Great Plains. Channel trenching occurred simultaneously in both small and large streams in drainage basins of the Arkansas, Red, Trinity, Brazos, and Colorado rivers; the sequences are the first documented example of widespread Holocene incision accompanied by firm evidence for a synchronous change in regional climate.
Macklin, M. G., B. T. Rumsby, T. R. Heap (1992). Flood alluviation and entrenchment: Holocene valley floor development and transformation in British uplands. Geological Society of America Bulletin 104 (6): 631-643
ABSTRACT: The morphology, sedimentary properties, and sequence of recent coarse-grained flood deposits and earlier Holocene alluvial fills were investigated in Thinhope Burn, a small (12-km2 ) catchment in the Northern Pennine uplands, northern England. Twenty-one large flood events are recorded by distinctive cobble-boulder bars, sheets and splays, and boulder berms and lobes. Lichenometric analysis showed that all but one of these floods dated from the mid-eighteenth century. The timing of large floods between 1766 and 1960 corresponds with major hydroclimatic trends evident in northern Britain and northwest Europe over this period. Discharge estimates suggest that flood magnitudes have decreased since the mid-eighteenth century. Channel and flood-plain metamorphosis in late Roman times and in the eighteenth century, following major valley-floor entrenchment (locally as much as 8 m), would appear to have been caused by increased runoff and flood magnitude. This was linked to a shift to a wetter climate with flow augmented by Iron Age and Roman woodland clearance, and drainage of the catchment in more recent times. Results from this study suggest that current models of longer-term Holocene and Pleistocene valley-floor development in the British uplands may need to be re-evaluated.
ABSTRACT: Flood-plain aggradation within the Loup River Basin of central Nebraska was episodic and alternated with incision throughout much of the Holocene. A widespread episode of flood-plain stability, however, occurred about 5700–5100 cal. year BP. The purpose of this paper is to describe the properties of this buried flood-plain at six sites in the basin, to consider why the properties of the buried flood-plain vary from site to site, and to evaluate possible reasons why the Loup River flood-plains stabilized 5500 years ago. Episodic valley-bottom aggradation was common during flood-plain formation at five of the six sites. The radiocarbon ages, particle-size data, and organic-carbon data for the buried flood-plain reveal that valley-bottom aggradation generally slowed between about 5700 and 5100 cal. year BP. Erratic down-profile changes in percentages of sand, clay, and organic matter indicate flood-plain sedimentation and soil formation were often episodic. Sand and clay rarely show a steady fining-upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. At two localities, the buried flood-plain is evident as a clay-rich stratum that must have been deposited in a paleochannel. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the effects of external climate forcing on vegetation and hydrologic changes. Flood-plains of other rivers in the central Great Plains also stabilized at this time, further supporting a climatic explanation for slowing of valley aggradation and formation of a flood-plain at this time. Recognition of buried flood-plains is important to both soil mapping in valleys and to the discovery of cultural resources in valleys.