Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: Air trapped in bubbles in polar ice cores constitutes an archive for the reconstruction of the global carbon cycle and the relation between greenhouse gases and climate in the past. High-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 ± 400 years after the warming of the last three deglaciations. Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere.
J. Imbrie, E. A. Boyle, S. C. Clemens, A. Duffy, W. R. Howard, G. Kukla, J. Kutzbach, D. G. Martinson, A. McIntyre, A. C. Mix, B. Molfino, J. J. Morley, L. C. Peterson, N. G. Pisias, W. L. Prell, M. E. Raymo, N. J. Shackleton, J. R. Toggweiler (1992). On the structure and origin of major glaciation cycles, 1, Linear responses to Milankovitch forcing. Paleoceanography 7 (6): 701-738
ABSTRACT: Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind-driven and density-driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000- and 41,000-year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean.
PUBLISHER'S DESCRIPTION: This book tells the exciting story of the ice ages--what they were like, why they occurred, and when the next one is due. The solution to the ice age mystery originated when the National Science Foundation organized the CLIMAP project to study changes in the earth's climate over the past 700,000 years. One of the goals was to produce a map of the earth during the last ice age. Scientists examined cores of sediment from the Indian Ocean bed and deciphered a continuous history for the past 500,000 years. Their work ultimately confirmed the theory that the earth's irregular orbital motions account for the bizarre climatic changes which bring on ice ages.
This is a tale of scientific discovery and the colorful people who participated: Louis Agassiz, the young Swiss naturalist whose geological studies first convinced scientists that the earth has recently passed through an ice age; the Reverend William Buckland, an eccentric but respected Oxford professor who fought so hard against the ice-age theory before accepting it; James Croll, a Scots mechanic who educated himself as a scientist and first formulated the astronomic theory of ice ages; Milutin Milankovitch, the Serbian mathematician who gave the astronomic theory its firm quantitative foundation; and the many other astronomers, geochemists, geologists, paleontologists, and geophysicists who have been engaged for nearly a century and a half in the pressing search for a solution to the ice-age mystery.
ABSTRACT: Climate-proxy records of the past 100,000 years show that the Earth's climate has varied significantly and continuously on timescales as short as a few thousand years (refs 1–7). Similar variability has also recently been observed for the interval 340–500 thousand years ago. These dramatic climate shifts, expressed most strongly in the North Atlantic region, may be linked to — and possibly amplified by — alterations in the mode of ocean thermohaline circulation. Here we use sediment records of past iceberg discharge and deep-water chemistry to show that such millennial-scale oscillations in climate occurred over one million years ago. This was a time of significantly different climate boundary conditions; not only was the early Pleistocene epoch generally warmer, but global climate variations were governed largely by changes in Earth's orbital obliguity. Our results suggest that such millennial-scale climate instability may be a pervasive and long-term characteristic of Earth's climate, rather than just a feature of the strong glacial–interglacial cycles of the past 800,000 years.
ABSTRACT: Most paleoclimate researchers would probably agree that variations in Earth's axial tilt and precession parameters have influenced past climate change. However, claims of connections between orbital eccentricity and ice age climate are more difficult to demonstrate or accept, especially since the amplitude of the strongest component of eccentricity-induced insolation, the 413-ky signal, is conspicuously small or absent from the power spectra of the last million years of paleoclimate data, and climate models without external forcing can easily reproduce the main ~100-ky cycles of the late Pleistocene and Holocene. Here I show that it is possible to tease out the 413-ky component of eccentricity directly from orbitally untuned deep-sead18 O time series, and that the signal is strong, albeit buried deep in thed18 O time series, concealed by frequency modulation (FM). To extract the 413-ky signal, the data is frequency and phase demodulated numerically, while synthetic surrogate time series with properties believed similar to the actual data are used to test the nature of the modulator and the accuracy of each step in the inversion.
ABSTRACT: For most of the Northern Hemisphere Ice Ages, from ~3.0 to 0.8 m.y., global ice volume varied predominantly at the 41,000 year period of Earth's orbital obliquity. However, summer (or summer caloric half year) insolation at high latitudes, which is widely believed to be the major influence on high-latitude climate and ice volume, is dominated by the 23,000 year precessional period. Thus the geologic record poses a challenge to our understanding of climate dynamics. Here we propose that variations in the insolation gradient between high and low latitudes control high-latitude climate and ice volume during the late Pliocene and early Pleistocene. The differential heating between high and low latitudes, driven by obliquity, controls the atmospheric meridional flux of heat, moisture, and latent energy, which may exert the dominant control on high-latitude climate on Milankovitch timescales. In the two-dimensional zonal energy balance models typically used to study the long-term evolution of climate, the meridional atmospheric moisture flux is usually kept fixed. The hypothesis that insolation gradients control the poleward energy fluxes, precipitation, and ice volume at high latitudes has never been directly examined within the context of an ice sheet model. In light of what we know about modern energy fluxes and their relative influence on high-latitude climate, this possibility should be examined.
ABSTRACT: The cause of the 100 kyr glacial–interglacial cycles during the past 800 kyr is one of the fundamental puzzles in paleoclimatology. The widely accepted Milankovitch theory, relating earth's climate cycles to variations in insolation caused by periodic changes in orbital parameters, has difficulties to explain the predominant 100 kyr rhythm. Although earth's eccentricity varies with a period of 100 kyr, the resulting change in insolation is too small to produce the corresponding climate cycle by direct forcing (Imbrie et al., Paleoceanography 8 (1993) 699). In order to solve the 100 kyr problem', Muller and MacDonald (Nature 377 (1995) 107; Science 277 (1997a) 215; Proc. Nat. Acad. Sci. USA 94 (1997b) 8329) proposed an alternative orbital but non-Milankovitch mechanism attributing the glacial cycles to regular variations in the accretion of interplanetary dust particles (IDP) caused by 100 kyr cycles in the orbital inclination of the earth. To test this controversial hypothesis, we study the IDP accumulation in deep-sea sediments from a period in the early Pleistocene. We find apparent 41 kyr cycles but no 100 kyr periodicity in the IDP accumulation rate. As there is no known mechanism to produce 41 kyr cycles in IDP supply from space, we conclude that the 41 kyr cycles are caused by the dynamics of sediment accumulation, and that changes in the IDP flux do not drive the Pleistocene glacial cycles.
ABSTRACT: Oceans cover more than two-thirds of our blue planet. The waters move in a global circulation system, driven by subtle density differences and transporting huge amounts of heat. Ocean circulation is thus an active and highly nonlinear player in the global climate game. Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5–10 °C and massive surges of icebergs into the North Atlantic Ocean — events that have occurred repeatedly during the last glacial cycle.
ABSTRACT: The oscillations between glacial and interglacial climate conditions over the past three million years have been characterized by a transfer of immense amounts of water between two of its largest reservoirs on Earth — the ice sheets and the oceans. Since the latest of these oscillations, the Last Glacial Maximum (between about 30,000 and 19,000 years ago), 50 million cubic kilometres of ice has melted from the land-based ice sheets, raising global sea level by 130 metres. Such rapid changes in sea level are part of a complex pattern of interactions between the atmosphere, oceans, ice sheets and solid earth, all of which have different response timescales. The trigger for the sea-level fluctuations most probably lies with changes in insolation, caused by astronomical forcing, but internal feedback cycles complicate the simple model of causes and effects.
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.
ABSTRACT: Characteristic global climatic events are employed to interpret the regional, land-based European stratigraphy. The Chinese loess record shows well expressed, continuous and reliably dated climatic signals of worldwide significance, which are well correlated with the marine oxygen isotope stratigraphy. The sequence in the surroundings of Ferdinandow in Poland plays a key role in the correlation of the European stratigraphy with the loess record and the oxygen isotope record. In particular, the complex but well-expressed Ferdinandowian Interglacial is a key marker because it shows striking similarities with the most pronounced soil complex of the last 700 ka years of the Chinese loess sequence (S5) and, by correlation, Oxygen isotope stages (OIS) 13–15. It follows that the Holsteinian Interglacial corresponds with the S4-soil and OIS 11. This appears to be confirmed by sequences deposited in the craters of the Central Massif, France. Problems of regional stratigraphical correlation in Europe persist, but is suggested that a general framework is now established.
G. D. Thackray (2001). Extensive early and middle Wisconsin glaciation on the western Olympic Peninsula, Washington, and the variability of Pacific moisture delivery to the Northwestern United States. Quaternary Research 55 (3): 257-270
ABSTRACT: Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,00014 C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,00014 C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,30014 C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,30014 C yr B.P.; the Twin Creeks 1 advance, 19,100–18,30014 C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.
ABSTRACT: Pollen records from two sites in western Oregon provide information on late-glacial variations in vegetation and climate and on the extent and character of Younger Dryas cooling in the Pacific Northwest. A subalpine forest was present at Little Lake, central Coast Range, between 15,700 and 14,850 cal yr B.P. A warm period between 14,850 and 14,500 cal yr B.P. is suggested by an increase inPseudotsuga pollen and charcoal. The recurrence of subalpine forest at 14,500 cal yr B.P. implies a return to cool conditions. Another warming trend is evidenced by the reestablishment ofPseudotsuga forest at 14,250 cal yr B.P. Increased haploxylonPinus pollen between 12,400 and 11,000 cal yr B.P. indicates cooler winters than before. After 11,000 cal yr B.P. warm dry conditions are implied by the expansion ofPseudotsuga . A subalpine parkland occupied Gordon Lake, western Cascade Range, until 14,500 cal yr B.P., when it was replaced during a warming trend by a montane forest. A rise inPinus pollen from 12,800 to 11,000 cal yr B.P. suggests increased summer aridity.Pseudotsuga dominated the vegetation after 11,000 cal yr B.P. Other records from the Pacific Northwest show an expansion ofPinus from ca. 13,000 to 11,000 cal yr B.P. This expansion may be a response either to submillennial climate changes of Younger Dryas age or to millennial-scale climatic variations.
D.R. Currey (1990). Quaternary palaeolakes in the evolution of semidesert basins, with special emphasis on Lake Bonneville and the Great Basin, U.S.A.. Paleogeography, Paleoclimatology, Palaeoecology 76 (3-4): 189-214
ABSTRACT: This paper reviews attributes of Quaternary lakes and lake basins which are often important in the environmental prehistory of semideserts. Basin-floor and basin-closure morphometry have set limits on palaeolake sizes; lake morphometry and basin drainage patterns have influenced lacustrine processes; and water and sediment loads have influenced basin neotectonics. Information regarding inundated, runoff-producing, and extra-basin spatial domains is acquired directly from the palaeolake record, including the littoral morphostratigraphic record, and indirectly by reconstruction. Increasingly detailed hypotheses regarding Lake Bonneville, the largest late Pleistocene palaeolake in the Great Basin, are subjects for further testing and refinement. Oscillating transgression of Lake Bonneville began about 28 ka, the highest stage occurred about 15 ka, and termination occurred abruptly about 13 ka. A final resurgence of perennial lakes probably occurred in many subbasins of the Great Basin between 11 and 10 ka, when the highest stage of Great Salt Lake (successor to Lake Bonneville) developed the Gilbert shoreline — and when the Russell shoreline formed 500–600 km away, in the Carson Desert of the Lake Lahontan basin. The highest post-Gilbert stage of Great Salt Lake, which has been one of the few permanent lakes in the Great Basin during Holocene time, probably occurred between 3 and 2 ka. A set of eighteen general observations regarding geologic aspects of the palaeolake record in semidesert basins is helping to guide Quaternary studies in the Great Basin.
Madsen, D.B., D. Rhode, D.K. Grayson, J.M. Broughton, S.D. Livingston, J.Hunt, J. Quade, D.N. Schmitt, M.W. Shaver, III (2001). Late Quaternary environmental change in the Bonneville Basin, western U.S.A.. Paleogeography, Paleoclimatology, Paleoecology 167 (3-4): 243-271
ABSTRACT: Excavation and analyses of small animal remains from stratified raptor deposits spanning the last 11.5 ka, together with collection and analysis of over 60 dated fossil woodrat midden samples spanning the last 50 ka, provide a detailed record of changing climate in the eastern Great Basin during the late Pleistocene and Holocene. Sagebrush steppe dominated the northern Bonneville basin during the Full Glacial, suggesting that conditions were cold and relatively dry, in contrast to the southern basin, which was also cold but moister. Limber pine woodlands dominated ~13–11.5 ka, indicating increased dryness and summer temperatures ~6–7°C cooler than present. This drying trend accelerated after ~11.5 ka causing Lake Bonneville to drop rapidly, eliminating 11 species of fish from the lake. From ~11.5–8.2 ka xerophytic sagebrush and shadscale scrub replaced more mesophilic shrubs in a step-wise fashion. A variety of small mammals and plants indicate the early Holocene was ~3°C cooler and moister than at present, not warmer as suggested by a number of climatic models. The diversity of plants and animals changed dramatically after 8.2 ka as many species disappeared from the record. Some of the upland species returned after ~4 ka and Great Salt Lake became fresh enough at ~3.4 and ~1.2 ka to support populations of Utah chub.
R. Negrini, D. Erbes, K. Faber, A. Herrera, A. Roberts, A. Cohen, P. Wigand, F. Foit (2000). A paleoclimate record for the past 250,000 years from Summer Lake, Oregon, USA. 1. chronology and magnetic proxies for lake level. Journal of Paleolimnology 24 (2): 125-149
ABSTRACT: This study presents the age control and environmental magnetism components of a new, late Pleistocene paleoclimate record for the Great Basin of western North America. Two new cores from the Summer Lake sub-basin of pluvial Lake Chewaucan, Oregon, USA are correlated to basin margin outcrops on the basis of tephrochronology, lithostratigraphy, sediment magnetism and paleomagnetic secular variation. Eleven tephra layers were found in the cores that correlate to tephra identified previously in the outcrop. The Olema ash was also found in one of the cores; its stratigraphic position, relative to 3 dated tephra layers, indicates that its age is 50-55 ka, somewhat younger than has been previously reported. The Summer Lake sediments are divided into deep and shallow lake lithosomes based on sedimentary features. The stratigraphic position of these lithosomes support the tephra-based correlations between the outcrop and the cores. These sediments contain a well resolved record of the Mono Lake Excursion (MLE) and an earlier paleomagnetic excursion as well as a high quality replication of the paleosecular variation immediately above the MLE.
Relative sedimentation rates increased dramatically toward the depocenter during intervals of low-lake level. In contrast, during intervals of high-lake level, relative sedimentation rates were comparable along the basin axis from the basin margin to the depocenter. The magnetic mineralogy of the Summer Lake sediments is dominated by pseudo-single domain (titano)magnetite and intervals of high/low magnetite concentration coincide with lithosomes that indicate high/low lake levels. Magnetic grain size also varies in accord with bulk sediment grain size as indicated by the silt/clay ratio. To a first order, variations in magnetic parameters, especially those attributable to the concentration of magnetic minerals, correlate well with global glacial/interglacial oscillations as indicated by marine oxygen isotope stages. This relationship can be explained by increased dissolution of (titano)magnetite minerals as lake level dropped and the lake became more productive biologically. This inference is supported by a correspondence between lower concentrations of magnetite with higher levels of total organic carbon and vice-versa.
D. P. Balch, A. S. Cohen, D. W. Schnurrenberger, B. J. Haskell, B. L. Valero-Garces, J. W. Beck, H. Cheng, R.L. Edwards (2005). Ecosystem and paleohydrological response to Quaternary climate change in the Bonneville Basin, Utah. Paleogeography, Paleoclimatology, Paleoecology 221 (1-2): 99-122
ABSTRACT: We report the results of a detailed paleoecological study of the Bonneville basin covering the last ~280,000 yr. Our study used fossil ostracodes and a sedimentological record obtained from the August 2000 GLAD800 drilling operation at Great Salt Lake. We analyzed 125 samples, taken at ~1 m intervals from Site 4 (GSL00-4), for ostracodes and other paleoecologic and sedimentologic indicators of environmental change. Multivariate analyses applied to the ostracode data and qualitative analyses of fossil and sedimentological data indicate an alternation between three major environments at the core site over the cored interval: (1) shallow saline or hypersaline lakes; (2) salt or freshwater marshes; and (3) occasional deep freshwater lakes. These environmental changes are consistent with shoreline studies of regional lake level fluctuations, but provide considerable new detail on both the timing and environmental conditions associated with the various lake phases. Our age model (using14 C, U-series, tephra and biostratigraphic chronologies) allowed us to associate the core's record of regional paleohydrology with the marine oxygen isotope stages of global climate change. The core contains continuous records for the last four glacial/interglacial sequences. Salt/freshwater marshes were common during the interglacials and deep freshwater conditions correspond with maximum global ice volume in OIS 2, and before a maximum in global ice during OIS 6. Immediately following deep lake phases, crashes in lake level from rapid desiccation resulted in the deposition of thick evaporite units. Our study suggests that the climate of the Great Salt Lake catchment appears to have been drier during OIS 6 than during OIS 2.We compare our record of environmental change during OIS 6 glaciation with other records from the western United States and find that the overall pattern of climate was similar throughout the West, but differences in the timing of climate change (i.e. when a region became drier or moister) are common.
ABSTRACT: Lake Bonneville, the largest late Pleistocene closed-basin lake in the North American Great Basin, fluctuated widely in response to changes in climate. The geochemistry and mineralogy of endogenic calcium carbonate deposited in deep water, and stratigraphic studies of shore-zone deposits, provide evidence of millennial-scale lake-level fluctuations that had amplitudes of about 50 m between 30 and 10 ka. Falling-lake events occurred at 21, 18.5–19, 17.5, 16–15.5, 14–13, and 10 ka (radiocarbon years) synchronously with the terminations of Heinrich events H1 and H2 and other smaller scale iceberg-rafting events (a, b, c, and Younger Dryas) in the North Atlantic Ocean. The Lake Bonneville results thus support other climate records that suggest that late Pleistocene millennial-scale climate change was global in extent. The size and shape of the Northern Hemisphere ice sheets, which determined the mean positions of storm tracks, may have been the primary control on late Pleistocene water budgets of Great Basin lakes.
ABSTRACT: From ca. 50 to 20 ka, Summer Lake, Oregon, rose and fell in tune with North Atlantic interstadial and stadial climatic oscillations, respectively. This record exhibits the complete morphology of the North Atlantic millennial-scale climate-change signal including Dansgaard-Oeschger oscillations, Heinrich events, and Bond cycles. The phase relationship of these climate change records (high Summer Lake during warm North Atlantic; low during cold) is demonstrated at millennial-scale resolution by the relative positions of the Mono Lake and Laschamp paleomagnetic excursions in these records. These results, in conjunction with comparisons of historical climate records, also presented here, imply a direct temporal connection at the subcentury scale between the North Atlantic and the northwestern Great Basin via an atmospheric teleconnection.
H. S. Godsey, D.R. Currey, M. A. Chan (2005). New evidence for an extended occupation of the Provo shoreline and implications for regional climate change, Pleistocene Lake Bonneville, Utah, USA. Quaternary Research 63 (2): 212-223
ABSTRACT: Lake Bonneville was a climatically sensitive, closed-basin lake that occupied the eastern Great Basin during the late Pleistocene. Ongoing efforts to refine the record of lake level history are important for deciphering climate conditions in the Bonneville basin and for facilitating correlations with regional and global records of climate change. Radiocarbon data from this and other studies suggest that the lake oscillated at or near the Provo level much longer than depicted by current models of lake level change. Radiocarbon data also suggest that the lake dropped from threshold control much more rapidly than previously supposed. These revisions to the Lake Bonneville hydrograph, coupled with independent evidence of climate change from vegetation and glacial records, have important implications for conditions in the Bonneville basin and during the Pleistocene to Holocene transition.
A. Cohen, M. Palacios-Fest, R. Negrini, P. Wigand, D. Erbes (2000). A paleoclimate record for the past 250,000 years from Summer Lake, Oregon, USA: II. Sedimentology, paleontology and geochemistry. Journal of Paleolimnology 24 (2): 151-182
ABSTRACT: We have obtained a detailed paleoenvironmental record in the Summer Lake Basin, Oregon (northwestern Great Basin, US) spanning from 250 ka-5 ka. This record is derived from core and outcrop sites extending from a proximal deltaic setting to near the modern depocenter. Lithostratigraphic, paleontologic (ostracodes and pollen) and geochemical indicators all provide evidence for hydroclimate and climate change over the study interval. Lithostratigraphic analysis of the Summer Lake deposits allows subdivision into a series of unconformity - or paraconformity-bound lithosomes. The unconformity and facies histories indicate that the lake underwent several major lake-level excursions through the Middle and Late Pleistocene. High stands occurred between ~200 and ~165 ka, between ~89 and 50 ka and between ~25 and 13 ka. Uppermost Pleistocene and Holocene sediments have been removed by deflation of the basin, with the exception of a thin veneer of late Holocene sediment. These high stands correspond closely with Marine Oxygen Isotope Stages 6, 4 and 2, within the margin of error associated with the Summer Lake age model. A major unconformity from ~158 ka until ~102 ka (duration varies between sites) interrupts the record at both core and outcrop sites. Lake level fluctuations, in turn are closely linked with TOC and salinity fluctuations, such that periods of lake high stands correlate with periods of relatively low productivity, fresher water and increased water inflow/evaporation ratios. Paleotemperature estimates based on palynology and geochemistry (Mg/Ca ratios in ostracodes) indicate an overall decrease in temperature from ~236 ka-165 ka, with a brief interlude of warming and drying immediately after this (prior to the major unconformity). This temperature decrease was superimposed on higher frequency variations in temperature that are not evident in the sediments deposited during the past 100 ka. Indicators disagree about temperatures immediately following the unconformity (~102-95 ka), but most suggest warmer temperatures between ~100-89 ka, followed by a rapid and dramatic cooling event. Cooler conditions persisted throughout most of the remainder of the Pleistocene at Summer Lake, with the possible exception of brief warm intervals about 27-23 ka. Paleotemperature estimates for the proximal deltaic site are more erratic than for more distal sites, indicative of short term air temperature excursions that are buffered in deeper water. Estimates of paleotemperature from Mg/Ca ratios are generally in good agreement with evidence from upland palynology. However, there is a significant discordance between the upland pollen record and lake indicators with respect to paleoprecipitation for some parts of the record. Several possibilities may explain this discordance. We favor a direct link between lake level and salinity fluctuations and climate change, but we also recognize the possibility that some of these hydroclimate changes in the Summer Lake record may have resulted from episodic drainage captures of the Chewaucan River between the Summer Lake and Lake Abert basins.
ABSTRACT: Mapping and radiocarbon dates of cirque moraines in the Sierra Nevada demonstrate that the last significant pre-Little Ice Age glacier advance in the range, the Recess Peak, resulted from snowline lowering roughly twice that of the Matthes (Little Ice Age) advance, and that the Recess Peak advance is late Pleistocene in age. We mapped Recess Peak and Matthes deposits in 64 cirques along a profile of the main Sierran crest that spans the north-south limits of `Neoglacial' deposits in the range. Equilibrium-line altitudes for the reconstructed Recess Peak glaciers vary greatly but coherently with those of the Matthes advance. The variability of both sets of deposits reflects strong topographic influences on snow accumulation and ablation patterns in their deep cirques.
Tephrochronology and radiocarbon dates from lake-sediment cores provide limits on the timing of the two advances. Previous work documenting the absence of a young, regionally extensive tephra on Matthes moraines in the central Sierra demonstrates that they formed after ~70014 C years BP (~650 cal. years BP). The age of the Recess Peak advance has been less certain; we therefore collected and dated sediment cores from lakes dammed behind terminal moraines correlating to the Recess Peak advance in four widely separated drainages along the Sierran crest (north to south): South Fork American River, Lee Vining Creek, Middle Fork San Joaquin River, and Bishop Creek. Twenty-three high-precision AMS radiocarbon dates on gyttja, peat, and macrofossils from the cores are internally consistent and demonstrate that the Recess Peak advance, previously thought to be of late Holocene age (~2500 years BP), ended before 11,190±7014 C years BP (~13,100±85 cal. years BP). Recess Peak is therefore late Pleistocene in age and probably predates the North Atlantic Younger Dryas climatic reversal. The absence of any glacial deposits on the bedrock between the Recess Peak and Matthes deposits indicates that: (1) any advance related to the Younger Dryas event in central California was smaller than the Matthes advance; (2) the Matthes advance was the most extensive, and possibly the only, Neoglacial event in the range; and (3) climate in the Sierra between ~13,000 cal. years BP and 650 cal. years BP was apparently too warm and/or dry to support glaciers larger than those of the Little Ice Age. Other mapping indicates that the Recess Peak is the first significant glacier advance after retreat of Tioga (local late-Wisconsin maximum) glaciers. These results suggest a regionally variable climate in western North America during the Younger Dryas event, because glaciers appear to have expanded in the Canadian Rockies at that time.
The new Recess Peak age limits, combined with other dated lake cores, indicate that the Sierra was essentially deglaciated by 14,000–15,000 cal. years BP (~12,000–13,00014 C years BP), substantially earlier than previously estimated. This finding indicates that current production rates of some in situ cosmogenic nuclides, calibrated on an assumed deglaciation of the range at 11,000 cal. years BP (~10,00014 C years BP), may be systematically too high by as much as 20%.
ABSTRACT: The highest shoreline features of paleo-Lake Malheur are undated gravelly barrier beaches south of Harney Lake that lie ca. 3.5 m higher than the hydrographic outlet of Harney Basin at Malheur Gap (1254 m). The earliest Quaternary record for Lake Malheur consists of occurrences of water-deposited tephra dated to ca. 70,000–80,000 yr ago. The next identified lake interval is dated by shells with ages of ca. 32,000 and 29,500 yr B.P. No dates are available for the terminal-Pleistocene Lake Malheur. Lake(s) were present between ca. 9600 and 7400 yr B.P., although periodic low levels or desiccation are suggested by a paleosol dated as ca. 8000 yr B.P. The lake system probably dried further after 7400 yr B.P., although dates are lacking for the period between ca. 7400 and 5000 yr B.P. Dune deposits on the lake floor are ca. 5000 yr old and indicate generally dry conditions. Fluctuating shallow lakes have probably characterized the last 2000 years. A date of 1000 yr B.P. gives a maximum age for beach deposits at 1254 m, near the basin threshold elevation. Thus, the Malheur Lake system may have drained to the Pacific Ocean by way of Malheur Gap during the latest Holocene.
G. J. Kukla, M. L. Bender, J. de Beaulieu, G. Bond, W. S. Broecker, P. Cleveringa, J. E. Gavin, T. D. Herbert, J. Imbrie, J. Jouzel, L. D. Keigwin, K. Knudsen, J. F. McManus, J. Merkt, D. R. Muhs, H. Müller, Ri. Z. Poore, S. C. Porter, G. Seret, N. J. Shackleton, C. Turner, P. C. Tzedakis, I. J. Winograd (2002). Last interglacial climates. Quaternary Research 58 (1): 2-13
ABSTRACT: The last interglacial, commonly understood as an interval with climate as warm or warmer than today, is represented by marine isotope stage (MIS) 5e, which is a proxy record of low global ice volume and high sea level. It is arbitrarily dated to begin at approximately 130,000 yr B.P. and end at 116,000 yr B.P. with the onset of the early glacial unit MIS 5d. The age of the stage is determined by correlation to uranium–thorium dates of raised coral reefs. The most detailed proxy record of interglacial climate is found in the Vostok ice core where the temperature reached current levels 132,000 yr ago and continued rising for another two millennia. Approximately 127,000 yr ago the Eemian mixed forests were established in Europe. They developed through a characteristic succession of tree species, probably surviving well into the early glacial stage in southern parts of Europe. After ca. 115,000 yr ago, open vegetation replaced forests in northwestern Europe and the proportion of conifers increased significantly farther south. Air temperature at Vostok dropped sharply. Pulses of cold water affected the northern North Atlantic already in late MIS 5e, but the central North Atlantic remained warm throughout most of MIS 5d. Model results show that the sea surface in the eastern tropical Pacific warmed when the ice grew and sea level dropped. The essentially interglacial conditions in southwestern Europe remained unaffected by ice buildup until late MIS 5d when the forests disappeared abruptly and cold water invaded the central North Atlantic ca. 107,000 yr ago.
ABSTRACT: We correlate oscillations in the hydrologic and/or cryologic balances of four Great Basin surface-water systems with Dansgaard–Oeschger (D–O) events 2–12. This correlation is relatively strong at the location of the magnetic signature used to link the lake records, but becomes less well constrained with distance/time from the signature. Comparison of proxy glacial and hydrologic records from Owens and Pyramid lakes indicates that Sierran glacial advances occurred during times of relative dryness. If our hypothesized correlation between the lake-based records and the GISP2d18 O record is correct, it suggests that North Atlantic D–O stades were associated with relatively cold and dry conditions and that interstades were associated with relatively warm and wet conditions throughout the Great Basin between 50,500 and 27,000 GISP2 yr B.P. The Great Basin lacustrine climate records reinforce the hypothesis that D–O events affected the climate throughout much of the Northern Hemisphere during marine isotope stages 2 and 3. However, the absolute phasing between lake-size and ice-cored18 O records remains difficult to determine.
D. Genty, D. Blamart, R. Ouahdi, M. Gilmour, A. Baker, J. Jouzel, S. Van-Exter (2003). Precise dating of Dansgaard–Oeschger climate oscillations in western Europe from stalagmite data. Nature 421 (6925): 833-837
ABSTRACT: The signature of Dansgaard–Oeschger events—millennial-scale abrupt climate oscillations during the last glacial period—is well established in ice cores and marine records. But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of14 C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using234 U/230 Th ratios. We find rapid climate oscillations coincident with the established Dansgaard–Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel, and deep-sea records, indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 ±0.6 and 67.4 ±0.9 kyr ago.
K. M. Menking, J. L. Bischoff, J. A. Fitzpatrick, J. W. Burdette, R. O. Rye (1997). Climatic/hydrologic oscillations since 155,000 yr B.P. at Owens Lake, California, reflected in abundance and stable isotope composition of sediment carbonate. Quaternary Research 48 (1): 58-68
ABSTRACT: Sediment grain size, carbonate content, and stable isotopes in 70-cm-long (~1500-yr) channel samples from Owens Lake core OL-92 record many oscillations representing climate change in the eastern Sierra Nevada region since 155,000 yr B.P. To first order, the records match well the marined18 O record. At Owens Lake, however, the last interglaciation appears to span the entire period from 120,000 to 50,000 yr B.P., according to our chronology, and was punctuated by numerous short periods of wetter conditions during an otherwise dry climate. Sediment proxies reveal that the apparent timing of glacial–interglacial transitions, notably the penultimate one, is proxy-dependent. In the grain-size and carbonate-content records this transition is abrupt and occurs at ~120,000 yr B.P. In contrast, in the isotopic records the transition is gradual and occurs between 145,000 and 120,000 yr B.P. Differences in timing of the transition are attributed to variable responses by proxies to climate change.
N. G. Pisias, A. C. Mix, L. Heusser (2001). Millennial scale climate variability of the northeast Pacific Ocean and northwest North America based on radiolaria and pollen. Quaternary Science Reviews 20 (14): 1561-1576
ABSTRACT: Radiolaria and pollen abundances in marine sediment cores from the northeast Pacific are used to reconstruct oceanographic and continental climate change during the past glacial cycle (0–150 kyr). These data allow direct comparison of the climate response of continental and oceanic systems. Detailedd18 O and AMS-14 C measurements provide a link into global stratigraphic frameworks. Canonical correlation analysis extracts two modes of variation common to both the Radiolaria and pollen records. The first mode of variation correlates an assemblage of Radiolaria associated with coastal upwelling with increased redwood, western hemlock, and alder pollen. This association is consistent with the modern relationship between coastal upwelling, coastal fog and redwood forests. A second canonical mode relates an oceanic fauna now found in highest abundance in the far North Pacific with reduced pine and western hemlock pollen abundance.
Comparison of these records to an ice cored18 O record suggests that at wavelengths >3000 years, warm events in Greenland are correlated to intervals of increased coastal upwelling off Oregon, decreases in importance of very cold North Pacific fauna (suggesting warming), and increases in pollen associated with wetter coastal environments. Radiolarian based sea-surface temperature estimates suggest that the variability of the northeast Pacific on this time scale is about 2°C. Warming in the coastal regions reflects reduced advection of the California Current, but is moderated by increases in cool coastal upwelling. We infer that the response of the northeast Pacific to millennial scale climate changes is related to changes in atmospheric circulation at mid- to high latitudes.
Preliminary analysis suggests that oceanic variability off Oregon at wavelengths <3000 years is similar to the Dansgaard–Oeschger cycles of the ice cored18 O records. This variability is associated with changes in subtropical faunal elements without similar changes in other faunal elements. This finding suggests that, unlike longer-period millennial scale events, the propagation of the shorter wavelength events to the Northeast Pacific is through subtropical or tropical teleconnections.
ABSTRACT: New cosmogenic surface-exposure ages of moraine-crest boulders from southwestern Colorado are compared with published surface-exposure ages of boulders from moraine complexes in north-central Colorado and in west-central (Fremont Lake basin) Wyoming.10 Be data sets from the three areas were scaled to a single10 Be production rate of 5.4 at/g/yr at sea level and high latitude (SLHL), which represents the average10 Be production rate for two high-altitude, mid-latitude sites in the western United States (US) and Austria. Multiple nuclide ages on single boulders indicate that this10 Be production rate yields ages comparable to those calculated with a commonly used36 Cl production scheme. The average age and age range of moraine-crest boulders on terminal moraines at the southwestern Colorado and Wyoming sites are similar, indicating a retreat from their positions 16.836 Cl ka (Cosmogenic ages in this paper are labeled10 Be or36 Cl ka or just ka when both10 Be or36 Cl ages are being discussed; radiocarbon ages are labeled14 C ka, calibrated radiocarbon are labeled cal ka, and calendar ages are labeled calendar ka. Errors (±1s) associated with ages are shown in tables. Radiocarbon ages were calibrated using the data of Hughen et al. (Science 303 (2004) 202). This suggests a near-synchronous retreat of Pinedale glaciers across a 470-km latitudinal range in the Middle and Southern Rocky Mountains. Hypothetical corrections for snow shielding and rock-surface erosion shifts the time of retreat to between 17.2 and 17.510 Be ka at Pinedale, Wyoming, and between 16.3 and 17.336 Cl ka at Hogback Mountain, Colorado.
ABSTRACT: The demonstration that natural climate variability during the last glacial cycle shifted rapidly between remarkable extremes has dramatically revised the understanding of climate change. To further advance our understanding, research continues into the timings, geographic distribution, and nature of the millennial-scale climate extremes, and into the mechanisms for intra- and inter-hemispheric transmission of variability through the climate/ocean system. Complementing the traditional definition of the timings of millennial-scale climate variability from ice-core '18 O records, we here further narrow down the temporal constraints by determining statistically significant anomalies in the major ion series of the GISP2 ice core. This exercise offers an objective definition of the timing of climatic anomalies in Northern Hemisphere palaeoclimate proxy records of the last 110,000 years that significantly improves the potential for inter-calibration of 'ice-core tuned' chronostratigraphies. We then present a process-oriented synthesis of proxy records from the Northern Hemisphere. This leads to a conclusion that the Dansgaard-Oeschger (D-O) style fluctuations in these records are (virtually) in phase, since all fall within a clear (atmospheric) pattern of concerted relative dominance shifts between polar/westerly dominated winter-type conditions and tropical/monsoon dominated summer-type conditions. Finally, we speculate on a monsoon-related mechanism that could help explain the anomalously long duration of D-O interstadials 12, 8, and 1, which coincided with cooling trends in Antarctic records.
Bond, G., W. Showers, M. Chaseby, R. Lotti, P. Almasi, P. deMinocal, P. Priore, H. Cullen, I. Hajdas, G. Bonani (1997). Pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278 (5341): 1257-1266
INTRODUCTION: Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic's thermohaline circulation.
Cortese, G., A. Abelmann, R. Gersonde (2007). The last five glacial-interglacial transitions: A high-resolution 450,000-year record from the subantarctic Atlantic. Paleoceanography 22 (PA4203): doi:10.1029/2007PA001457
ABSTRACT: A submillennial resolution, radiolarian-based record of summer sea surface temperature (SST) documents the last five glacial to interglacial transitions at the subtropical front, southern Atlantic Ocean. Rapid fluctuations occur both during glacial and interglacial intervals, and sudden cooling episodes at glacial terminations are recurrent. Surface hydrography and global ice volume proxies from the same core suggest that summer SST increases prior to terminations lead global ice-volume decreases by 4.7 ± 3.7 ka (in the eccentricity band), 6.9 ± 2.5 ka (obliquity), and 2.7 ± 0.9 ka (precession). A comparison between SST and benthicd13 C suggests a decoupling in the response of northern subantarctic surface, intermediate, and deep water masses to cold events in the North Atlantic. The matching features between our SST record and the one from core MD97-2120 (southwest Pacific) suggests that the super-regional expression of climatic events is substantially affected by a single climatic agent: the Subtropical Front, amplifier and vehicle for the transfer of climatic change. The direct correlation between warmerDTsite at Vostok and warmer SST at ODP Site 1089 suggests that warmer oceanic/atmospheric conditions imply a more southward placed frontal system, weaker gradients, and therefore stronger Agulhas input to the Atlantic Ocean.
ABSTRACT: Glacial geology and14 C dating in the central Wasatch Mountains indicate: an early canyon-mouth glaciation (Dry Creek till), probably during isotope stage 6; on that till, a paleosol (Majestic Canyon soil) dated at about 26,000 yr B.P.; overriding that soil, a later canyon-mouth glaciation (Bells Canyon till) probably beginning prior to about 19,000 yr B.P.; a midcanyon deglacial pause (Hogum Fork till) prior to 12,300 yr B.P.; an upper-canyon deglacial pause (Devils Castle till) prior to 7500 yr B.P.; and late Holocene periglaciation. Pollen ratios from bog profiles in the mid to upper reaches of the canyon suggest that temperatures cooler than the Holocene average occurred until after about 8000 yr B.P. Warmer and dryer than average conditions were initiated about 8000 to 7500 yr B.P. During the later portion of this Altithermal period conditions became relatively warm and wet. Two subsequent episodes of cooler than average temperatures correspond chronologically to the initial stades of Neoglaciation elsewhere in the Rocky Mountains. However, there is no geomorphic evidence of corresponding glacial activity in the canyon area. Relative moisture during these two periods differs significantly, suggesting that Neoglacial conditions were controlled primarily by changes in summer temperature.
ABSTRACT: Unlike the North Atlantic, the North Pacific Ocean probably remained free of sea ice during the last glacial maximum (LGM), 22,000 to 17,000 BP. Following a eustatic low in sea level of ca. -120 m at 19,000 BP, a marine transgression had flooded the Bering and Chukchi shelves by 10,000 BP. Post-glacial sea-level history varied widely in other parts of the North Pacific coastline according to the magnitude and timing of local tectonism and glacio-isostatic rebound. Glaciers covered much of the continental shelf between the Alaska Peninsula and British Columbia during the LGM. Maximum glacier extent during the LGM was out of phase between southern Alaska and southern British Columbia with northern glaciers reaching their outer limits earlier, between 23,000 and 16,000 BP, compared to 15,000–14,000 BP in the south. Glacier retreat was also time-transgressive, with glaciers retreating from the continental shelf of southern Alaska before 16,000 BP but not until 14,000–13,000 BP in southwestern British Columbia. Major climatic transitions occurred in the North Pacific at 24,000–22,000, 15,000–13,000 and 11,000–9000 BP. Rapid climate changes occurred within these intervals, including a possible Younger Dryas episode. An interval of climate warmer and drier than today occurred in the early Holocene. Cooler and wetter conditions accompanied widespread Neoglaciation, beginning in some mountain ranges as early as the middle Holocene, but reaching full development after 3000 BP.
ABSTRACT: Forty individually named ranges, plateaus, and massifs draining wholly or partly into the Great Basin of the western United States show definite evidence of Pleistocene glaciation. The most obvious deposits are a family of moraines designated, among other names, "Tioga", "Angel Lake", and "Pinedale". Such moraines generally can be traced from range to range away from described type moraines. These deposits have been numerically assigned to Late Wisconsinan glaciation in the Wasatch Range, White Mountains, Boulder Mountain, and Sierra Nevada on the basis of radiocarbon and surface-exposure ages, and have been assigned to Late Wisconsinan time in several other ranges on the basis of relative-age studies. The type Angel Lake moraine, and most other equivalent moraines across the Great Basin, are thick, hummocky, lobate piles of till rather than looping ridges. The thicknesses of the moraines (often 60+m) can be explained by heavy debris loads, and/or glacial advance, retreat, and readvance to the same positions a number of times, which is consistent with recent evidence that multiple Late Wisconsinan advances, possibly related to Heinrich and Dansgaard-Oeschger events, occurred in the Sierra Nevada. Pre-Angel Lake deposits occur in many Great Basin ranges, but it is currently difficult or perhaps impossible to determine if these deposits are equivalent to each other and what their relationship is to pre-Tioga deposits in the Sierra Nevada. Numerical ages are rare and relative-age studies suggest that pre-Angel Lake deposits may be products of more than one glaciation. Mapped pre-Angel Lake glaciers were longer than their Angel Lake counterparts, but the length differences do not translate into large differences in ELA depression. There is evidence of two minor latest Pleistocene or early Holocene advances in some ranges, judging from the presence of overlying Mazama tephra and/or weathering comparisons to local Angel Lake moraines. In the latter part of the Holocene, ELA's were sufficiently high that only the highest, wettest ranges developed Neoglacial glaciers. There does not appear to be a consistent pattern of latest Pleistocene/Holocene glacial fluctuations along an east–west transect through the Cordillera, or even through the Great Basin.
ABSTRACT: A 200 k.y. chronology of river response to climate-related environmental change has been established for northeast Spain using newly developed luminescence dating techniques. This constitutes the best-documented record of late Quaternary river behavior currently available for the North Atlantic region and enables fluvial stratigraphies to be compared with high-resolution ice core and marine oxygen isotope climate series. Pleistocene and Holocene river aggradational episodes coincide with stadial or neoglacial events, while phases of river incision occur during interstadial or interglacial periods. Alluviation and erosion cycles would appear to track variations in sediment supply controlled by vegetation cover and winter storm frequency.
Nowak, C.L., R. S. Nowak, R.J. Tausch, P.E. Wigand (1994). Tree and shrub dynamics in northwestern Great Basin woodland shrub steppe during the late-Pleistocene and Holocene. American Journal of Botany 81 (3): 265-277
ABSTRACT: During the last 12,000 to 30,000 years, a large proportion of the dominant trees and shrubs in modern assemblages of woodland and shrub steppe vegetation in the northwestern Great Basin have undergone relatively small changes in their geographic ranges. A woodland tree,Juniperus osteosperma , has an extensive temporal and spatial fossil record from 11 woodrat midden locales that were sampled in the northwestern Great Basin. Above 1,300 m elevation,J. osteosperma has been continuously present in that fossil record for at least the last 30,000 years. However,J osteosperma was lost at elevations below 1,300 m sometime during the last 10,000 years, during the Holocene. Although the elevational ranges of six shrub taxa show changes during the Holocene, geographic ranges of 11 other shrub taxa have been largely static. Of the woodland and shrub steppe species examined,Pinus monophylla has experienced the greatest change in its geographic range during the late-Pleistocene and Holocene.Pinus monophylla has migrated northward across the Great Basin from Pleistocene refugia in the southern portions of this region. The rate of latitudinal migration was more rapid along the eastern side of the Great Basin than on the western side. Thus, the species that comprise modern woodland and shrub steppe communities of the northwestern Great Basin appear to have two strategies to cope with climate change. First are species, as exemplified byJ. osteosperma , whose geographic ranges were relatively insensitive to climate change and are termed orthoselective species. High genetic variation within species and the formation of coenospecies likely allowed these species to cope with climatic change by genetic adaptation. Secondly, other species, as exemplified byP. monophylla , have experienced shifts in their geographic range during past climate changes and more clearly fit the migration model of species response to climate change.
ABSTRACT: Vegetation records spanning the past 21 kyr in western North America display spatial patterns of change that reflect the influence of variations in the large-scale controls of climate1 . Among these controls are millennial-scale variations in the seasonal cycle of insolation and the size of the ice sheet, which affect regional climates directly through changes in temperature and net radiation, and indirectly by shifting atmospheric circulation. Longer vegetation records provide an opportunity to examine the regional response to different combinations of these large-scale controls, and whether non-climatic controls are important. But most of the longer North American records2, 3 are of insufficient quality to allow a robust test, and the long European records4-9 are in regions where the vegetation response to climate is often difficult to separate from the response to ecological and anthropogenic controls. Here we present a 125-kyr record of vegetation and climate change for the forest/steppe border of the eastern Cascade Range, northwest America. Pollen data disclose alternations of forest and steppe that are consistent with variations in summer insolation and global ice-volume, and vegetational transitions correlate well with the marine isotope-stage boundaries. The close relationship between vegetation and climate beyond the Last Glacial Maximum provides evidence that climate variations are the primary cause of regional vegetation change on millennial timescales, and that non-climatic controls are secondary.
ABSTRACT: Plant macrofossils extracted from fossil woodrat (Neotoma spp.) middens at a single locale in the northwestern Great Basin were used to examine vegetation dynamics during the last 30 000 yr. Although the modern assemblage of xeric species at the study site is a recent occurrence, a large proportion of the modern plant taxa near the study locale were also found 12 000 - 30 000 yr BP. The persistence of extant species through time was likely facilitated by within-species genetic diversity and the formation of coenospecies. The diverse topographic and microhabitat features in the northwestern Great Basin also allowed different species to coexist during glacial periods. Changes in species composition occurred during two time intervals: 20 000 - 30 000 and 10 000 - 12 000 yr BP. Vegetation changes during 20 000 - 30 000 yr BP were cyclic; community composition oscillated between two groups of taxa. Vegetation changes between 10 000 - 12 000 yr BP occurred during the Pleistocene-Holocene transition and were largely directional from the Pleistocene assemblages through two transition assemblages to a Holocene assemblage. These changes in species composition generally reflect changes in climate. The presence of relatively mesic species during 10 000 - 30 000 yr BP is consistent with the regional late-Pleistocene climate, and the gradual loss of relatively mesic species during the Holocene parallels the change to a more xeric climate. Contrasted with other areas of North America and Europe, the magnitude of vegetation changes at our study area were relatively small. Furthermore, the persistence of many species through time at this site in the northwestern Great Basin also differs from results at other study sites in North America and Europe. These differences are probably related to land form characteristics and genetic diversity within species.
ABSTRACT: The 14 papers collected herein treat diverse aspects of the aquatic history of the Great Basin of the western United States and collectively attempt to summarize and integrate portions of the vast body of new information on this subject that has been acquired since the last such compilation was published in 1948. In the first section, four papers (Lowenstein, Negrini, Reheis et al., Sack) focus on the physical aspects of the Great Basin paleolake histories, whereas a fifth paper (Oviatt) summarizes the contributions to the study of Bonneville Basin lacustrine history made by two early giants of the field, Grove Karl Gilbert and Ernst Antevs. In the second section, four papers synthesize perspectives on Great Basin aquatic history provide by diatoms and ostracods (Bradbury and Forester), fishes (Smith et al.), aquatic insects (Polhemus and Polhemus), and aquatic snails (Hershler and Sada), whereas a fifth (Sada and Vinyard) summarizes the conservation status of the diverse aquatic biota that is endemic to the region. In the final section, three papers integrate terrestrial biotic evidence pertaining to Great Basin aquatic history derived from pollen from cores (Davis), floristics (Wigand and Rhode), and the mammal record (Grayson), whereas a fourth (Madsen) examines the relationship between Great Basin lakes and human inhabitants of the region. Although diverse in scope and topic, the papers in this volume are nonetheless linked by an appreciation that integration of geological, biological, and anthropological evidence is a necessary and fundamental key to a mature understanding of Great Basin aquatic systems history.
L. V. Benson, S. P. Lund, J. W. Burdett, M. Kashgarian, T. P. Rose, J. P. Smoot, M. Schwartz (1998). Correlation of late-Pleistocene lake-level oscillations in Mono Lake, California, with North Atlantic climate events. Quaternary Research 49 (1): 1-10
ABSTRACT: Oxygen-18 (18 O) values of sediment from the Wilson Creek Formation, Mono Basin, California, indicate three scales of temporal variation (Dansgaard–Oeschger, Heinrich, and Milankovitch) in the hydrologic balance of Mono Lake between 35,400 and 12,90014 C yr B.P. During this interval, Mono Lake experienced four lowstands each lasting from 1000 to 2000 yr. The youngest lowstand, which occurred between 15,500 and 14,00014 C yr B.P., was nearly synchronous with a desiccation of Owens Lake, California. Paleomagnetic secular variation (PSV) data indicate that three of four persistent lowstands occurred at the same times as Heinrich events H1, H2, and H4.18 O data indicate the two highest lake levels occurred ~18,000 and ~13,10014 C yr B.P., corresponding to passages of the mean position of the polar jet stream over the Mono Basin. Extremely low values of total inorganic carbon between 26,000 and 14,00014 C yr B.P. indicate glacial activity, corresponding to a time when summer insolation was much reduced.
J.D. Ortiz, S.B. O'Connell, J. DelViscio, W. Dean, J.D. Carriquiry, T. Marchitto, Y. Zheng, van Geen, A. (2004). Enhanced marine productivity off western North America during warm climate intervals of the past 52 k.y.. Geology 32 (6): 521-524
ABSTRACT: Studies of the Santa Barbara Basin off the coast of California have linked changes in its bottom-water oxygen content to millennial-scale climate changes as recorded by the oxygen isotope composition of Greenland ice. Through the use of detailed records from a sediment core collected off the Magdalena Margin of Baja California, Mexico, we demonstrate that this teleconnection predominantly arose from changes in marine productivity, rather than changes in ventilation of the North Pacific, as was originally proposed. One possible interpretation is that the modern balance of El Niño–La Niña conditions that favors a shallow nutricline and high productivity today and during warm climate intervals of the past 52 k.y. was altered toward more frequent, deep nutricline, low productivity, El Niño–like conditions during cool climate intervals.