Calibrated radiocarbon dates of organic matter below and above till of the last (Fraser) glaciation provide limiting ages that constrain the chronology and duration of the last advance–retreat cycle of the Puget Lobe in the central and southeastern Puget Lowland. Seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17,420 ± 90 cal yr B.P., which is the closest limiting age for arrival of the glacier near the latitude of Seattle. A time–distance curve constructed along a flowline extending south from southwestern British Columbia to the central Puget Lowland implies an average glacier advance rate of ca. 135 m/yr. The glacier terminus reached its southernmost limit ca. 16,950 yr ago and likely remained there for ca. 100 yr. In the vicinity of Seattle, where the glacier reached a maximum thickness of 1000 m, ice covered the landscape for ca. 1020 yr. Postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16,420 cal yr B.P. and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum. The average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes.

The chemistry of the carbonate-free clay-size fraction of Owens Lake sediments supports the use of total organic carbon and magnetic susceptibility as indicators of stadial–interstadial oscillations. Owens Lake records of total organic carbon, magnetic susceptibility, and chemical composition of the carbonate-free, clay-size fraction indicate that Tioga glaciation began ∼24,500 and ended by ∼13,60014C yr B.P. Many of the components of glacial rock flour (e.g., TiO2, MnO, BaO) found in Owens Lake sediments achieved maximum values during the Tioga glaciation when valley glaciers reached their greatest extent. Total organic carbon and SiO2(amorphous) concentrations reached minimum values during Tioga glaciation, resulting from decreases in productivity that accompanied the introduction of rock flour into the surface waters of Owens Lake. At least 20 stadial–interstadial oscillations occurred in the Sierra Nevada between 52,600 and 14,00014C yr B.P. Total organic carbon data from a Pyramid Lake sediment core also indicate oscillations in glacier activity between >39,500 and ∼13,60014C yr B.P. Alpine glacier oscillations occurred on a frequency of ≤1900 yr in both basins, suggesting that millennial-scale oscillations occurred in California and Nevada during most of the past 52,600 yr.

Temperature profoundly influences the physiology and life history characteristics of organisms, particularly in terms of body size. Because so many critical parameters scale with body mass, long-term temperature fluctuations can have dramatic impacts. We examined the response of a small mammalian herbivore, the bushy-tailed woodrat ( Neotoma cinerea), to temperature change from 20,000 yr B.P. to present, at five sites within the Colorado Plateau. Our investigations focused on the relationship between temperature, plant composition and abundance, and woodrat size. Body size was estimated by measuring fossil fecal pellets, a technique validated in earlier work. We found significant and highly covariable patterns in body mass over the five locations, suggesting that responses to temperature fluctuations during the late Quaternary have been very similar. Although woodrat mass and the occurrence of several plant species in the fossil record weresignificantly correlated, in virtually all instances changes in woodrat size preceded changes in vegetational composition. These results may be due to the greater sensitivity of woodrats to temperature, or to the shorter generation times of woodrats as compared to most plants. An alternative hypothesis is that winter temperatures increased before summer ones. Woodrats are highly sensitive to warmer winters, whereas little response would be expected from forest/woodland plants growing at their lower limits. Our work suggests that woodrat size is a precise paleothermometer, yielding information about temperature variation over relatively short-term temporal and regional scales.

The beginning of the Bølling-Allerød warm period was marked in Greenland ice by an abrupt rise in δ18O, an abrupt drop in dust rain, and an abrupt increase in atmospheric methane content. The surface waters in the Norwegian Sea underwent a simultaneous abrupt warming. At about this time, a major change in the pattern of global rainfall occurred. Lake Victoria (latitude 0°), which prior to this time was dry, was rejuvenated. The Red Sea, which prior to this time was hypersaline, freshened. Lake Lahontan, which prior to this time had achieved its largest size, desiccated. Whereas the chronologic support for the abruptness of the hydrologic changes is firm only for the Red Sea, in keeping with evidence obtained well away from the northern Atlantic in the Santa Barbara Basin and the Cariaco Trench, the onset and end of the millennial-duration climate events were globally abrupt. If so, the proposed linkage between the size of African closed basin lakes and insolation cycles must be reexamined.

An abrupt change in environment from a wetland to a deeper slackwater lake at 20,345 ± 85 yr B.P. occurred in a second-order valley tributary to the Mississippi River near Lomax, Illinois. The age of this shift is associated with the overflow of glacial Lake Milan and diversion of the Mississippi River from the Princeton Valley (Rock Island to Peoria) to its present course (Rock Island to St. Louis). The diversion occurred due to blockage of the Princeton Valley segment of the ancient Mississippi River by the Lake Michigan Lobe, impoundment of glacial Lake Milan, and eventual overflow of the lake southwest of Rock Island across a former drainage divide near Andalusia, Illinois. Fossil ostracode assemblages in the slackwater sediment at Lomax indicate changes in the post-diversion, full-glacial paleohydrology and based on multivariate analysis, hint at the area's paleoclimate. An older part of the succession at Lomax is consistent with glaciation in the upper Iowa River basin about 40,000 yr B.P. Aggradation of sediment rich in coarse silt is attributed to a response of the ancient Iowa River basin associated with deposition of the glacigenic Sheldon Creek Formation by the Des Moines Lobe.

Paleomagnetic secular variation in a portion of the Bonneville Alloformation is compared with secular variation in lacustrine sediments in the Mono Basin, California, and with secular variation in Lake Lahontan sediments in the northwestern Great Basin. The comparison places an age of about 18,000 yr B.P., and a span of 1000 to 3000 yr, on part of a transgressive stage of Lake Bonneville near Delta, Utah, that is coeval with a wet period in the Lahontan Basin.

Three paleodrainage groups are proposed for profiles of the Farmdale–Sangamon soil complex in south-central Indiana. The groups (good, intermediate, and poor) are differentiated on the basis of matrix colors and color patterns. Genetic support for the groupings is provided by morphological trends still evident following > 100,000 yr of pedogenesis and burial by late Wisconsinan loess. Depth of carbonate leaching, solum thickness, and argillic horizon thickness all decrease with poorer drainage. Clay mineralogy also reflects paleodrainage. Illite degradation is intense in all profiles, but profiles with good drainage have poorly crystalline, interstratified expandable minerals, while well crystalline smectites dominate in profiles with poor drainage. Remanent aggregation in former A horizons is stronger in more poorly drained profiles, while the effects of structural overprinting from the modern soil increase with better modern drainage. Soil morphology, mineralogy, and parent material–paleolandscape position of Sangamon profiles are all internally consistent with color development under soil hydrological conditions during the last interglacial to glacial transition. The occurrence frequency of each drainage group indicates that the Sangamon soilscape was better drained than now, and morphology and clay mineral evidence suggests that areas with poor drainage did not experience the extreme seasonal groundwater shifts that the modern landscape experiences.

The S5 is the most prominent paleosol unit in the Chinese Loess Plateau, and is distinguished by its great thickness, dark color, and well-developed clay coatings. Previous studies led to the proposal that the S5 developed under much warmer and wetter conditions than at present, implying a “climatic optimum.” However, the S5 is a paleosol complex corresponding in time to a period of weak glacial–interglacial oscillations, as recorded by marine oxygen-isotope stages 15 through 13. To understand the character of this climatic interval, two profiles of the paleosol S5 in the Guanzhong basin were studied, with most of the emphasis being placed on the uppermost and best-developed pedon, S5-I. Grain-size, mineralogical, and chemical data indicate that the major pedogenic processes experienced by the S5-I included complete decalcification and mechanical translocation of fine particles. Chemical alteration of silicate minerals has been weak. Stable carbon-isotope analyses of organic matter showed that C4 plant remains are abundant throughout the profiles. These results suggest that grasslands dominated the paleovegetation during S5-I development. The prominent development of the S5 paleosol complex may reflect a prolonged period of pedogenesis. In contrast to previous reports, paleoclimatic conditions perhaps even drier than at present are implied by the data for the S5 profiles. The relative strength of the glacial–interglacial oscillation in this region (related to the East Asian monsoon) was similar to that indicated for global climate during S5 time.

Carbonate boulders transported down steep tributary channels by debris flow came to rest on Holocene debris fans beside the Colorado River in Grand Canyon National Park. Weakly acidic rainfall and the metabolic activity of blue-green algae have produced roughly hemispheric dissolution pits as much as 2-cm deep on the initially smooth surfaces of the boulders. The average depth of dissolution pits increases with relative age of fan surfaces. The deepening rate averages 2.4 mm/1000 yr (standard error = 0.2 mm/1000 yr), as calculated from several radiometrically dated surfaces and an archeological structure. This linear rate, which appears constant over at least the past 3000 yr, is consistent with field relations limiting the maximum age of the fans and with the physical chemistry of limestone dissolution. Dissolution-pit measurements ( n= 6973) were made on 617 boulders on 71 fan surfaces at the 26 largest debris fans in Grand Canyon. Among these fan surfaces, the average pit depth ranges from 1.2 to 17.4 mm, and the resulting pit dissolution ages range from 500 to 7300 cal yr B.P. Most (75%) surfaces are younger than 3000 yr, probably because of removal of older debris fans by the Colorado River. Many of the ages are close to 800, 1600, 2300, 3100, or 4300 cal yr B.P. If not the result of differential preservation of fan surfaces, this clustering implies periods of heightened debris-flow activity and increased precipitation.

Clay mineral assemblages of a soil chrono-association comprising five fluvial surface members (QGH1 to QGH5) of the Indo-Gangetic Plains between the Ramganga and Rapti rivers, north-central India, demonstrate that pedogenic interstratified smectite–kaolin (Sm/K) can be considered as a potential indicator for paleoclimatic changes during the Holocene from arid to humid climates. On the basis of available radiocarbon dates, thermoluminescence dates, and historical evidence, tentative ages assigned to QGH1 to QGH5 are <500 yr B.P., >500 yr B.P., >2500 yr B.P., 8000 TL yr B.P., and 13,500 TL yr B.P., respectively. During pedogenesis two major regional climatic cycles are recorded: relatively arid climates between 10,000–6500 yr B.P. and 3800–? yr B.P. were punctuated by a warm and humid climate. Biotite weathered to trioctahedral vermiculite and smectite in the soils during arid conditions, and smectite was unstable and transformed to Sm/K during the warm and humid climatic phase (7400–4150 cal yr B.P.). When the humid climate terminated, vermiculite, smectite, and Sm/K were preserved to the present day. The study suggests that during the development of soils in the Holocene in alluvium of the Indo-Gangetic Plains, climatic fluctuations appear to be more important than realized hitherto. The soils older than 2500 yr B.P. are relict paleosols, but they are polygenetic because of their subsequent alterations.

The desert–loess transitional zone in north-central China has long been thought sensitive to Quaternary climatic change. However, reconstruction of Quaternary climates in this area has been hindered by incompleteness of geological sections. Here we report the analytical results of two recently found sand–loess–soil sections. Both sections have thick eolian deposits from the last interglacial–glacial cycle and can be correlated with one another. Field observations, thermoluminescence dating, and other laboratory analyses show that the last interglacial period produced three paleosols and two intercalated loess layers. Loess from the last glacial period is interbedded with three sand horizons that represent desert extension. The expansion and contraction of desert in northern China may have been forced by the east Asia monsoon.

Pollen analysis of two cores from the Lake Sinnda, located in one of the driest areas of the southern Congo, reveals a history of vegetation and climate in this region during the past 5000 yr. A major change centered around 3000–2500 yr B.P. is indicated by an abrupt decrease in forest pollen and by a corresponding increase in grassland pollen. Concurrent drying up of the lake shows that climate, in particular aridity, was the major cause of this change. This paleoclimatic reconstruction conforms with evidence for drier conditions in other parts of western equatorial Africa, such as the development of isolated enclosed savannas and of heliophilous forested formations. The aridity is recorded more fully at Lake Sinnda than at the previous studied ones. It probably lasted longer, from 4200 to 1300 yr B.P., and was more progressive than previously inferred. The aridity predates agriculture marked by pollen of the oil palm at Lake Sinnda.

The stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern Brazil. Stable oxygen isotope records of the planktonic foraminiferal species Globigerinoides sacculiferand Globigerinoides ruber(pink) reveal sub-Milankovitch changes in sea-surface hydrography during the last 85,000 yr. Warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high XRF intensities of Ti and Fe, and which suggest humid conditions in northeast Brazil. These tropical signals correlate with climatic oscillations recorded in Greenland ice cores (Dansgaard-Oeschger cycles) and in sediment cores from the North Atlantic (Heinrich events). Trade winds may have caused changes in the North Brazil Current that altered heat and salt flux into the North Atlantic, thus affecting the growth and decay of the large glacial ice sheets.

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–∼25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal18O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ13C is 1.7‰ (0.0–1.7‰), and δ13C covaries positively and linearly with δ18O. δ13C values are highest when δ18O values indicate the most arid climatic conditions. This is best explained by expansion of C4grasses at the expense of C3plants at low latitudes during glacial periods when atmospheric pCO2was lowered. C4dominance was overridingly influenced by global change in atmospheric pCO2despite the lowered summer rainfall.

The paleoclimatic record from bottom sediments of Lake Baikal (eastern Siberia) reveals new evidence for an abrupt and intense glaciation during the initial part of the last interglacial period (isotope substage 5d). This glaciation lasted about 12,000 yr from 117,000 to 105,000 yr B.P. according to correlation with the SPECMAP isotope chronology. Lithological and biogeochemical evidence of glaciation from Lake Baikal agrees with evidence for the advance of ice sheet in northwestern Siberia during this time period and also with cryogenic features within the strata of Kazantzevo soils in Southern Siberia. The severe 5d glaciation in Siberia was caused by dramatic cooling due to the decrease in solar insolation (as predicted by the model of insolation changes for northern Asia according to Milankovich theory) coupled with western atmospheric transport of moisture from the open areas of Northern Atlantic and Arctic seas (which became ice-free due to the intense warming during preceeding isotope substage 5e). Other marine and continental records show evidence for cooling during 5d, but not for intense glaciation. Late Pleistocene glaciations in the Northern Hemisphere may have begun in northwestern Siberia.

Archaeological sites at Abeurador and Font-Juvénal have produced extensive charcoal-rich horizons. The results of the charcoal analysis, based on the identification of woody species from anatomical features, contribute to the elaboration of plain phases within the development of vegetation in southern France. They are presented here in a new diagram and are interpreted as four major vegetation phases from 11,00014C yr B.P. to the recent past. In order to interpret the palaeoecological significance of the charcoal assemblages we have used multivariate analysis. The data base consists of 42 taxa and 48 archaeological levels. The division into four vegetation phases based on analytical interpretation is complemented by the more synthetic interpretation based on correspondence factor analysis (CFA). The results reveal the patterns of the vegetation history over the last 11 millennia, each being characterized by a key plant species.

Uranium-series dating and stable isotope analyses of two speleothems from northwest Nelson, New Zealand, record changes in regional climate and local forest extent over the past 31,000 years. Oxygen isotope variation in these speleothems primarily represents changes in the meteoric waters falling above the caves, possibly responding to latitudinal changes in the position of the Subtropical Front in the Tasman Sea. Seven positive excursions can be identified in the oxygen isotope record, which coincide with periods of glacier advance, known to be sensitive to northward movement of the Subtropical Front. Four glacier advances occurred during oxygen isotope stage 2, with the most extreme glacial conditions centered on 19,000 cal yr B.P. An excursion in the oxygen isotope record from 13,800 to 11,700 cal yr B.P. provides support for a previously identified New Zealand glacier advance at the time of the Younger Dryas Stade, but suggests it began slightly before the Younger Dryas as recorded in Greenland ice cores. Carbon isotope variations in the speleothems record changes in forest productivity, closely matching existing paleovegetation records. On the basis of vegetation changes, stage 2 glacial climate conditions terminated abruptly in central New Zealand, from 15,700 to 14,200 cal yr B.P. Evidence of continuous speleothem growth at one site suggests that depression of the local treeline was limited to 600–700 m below its present altitude, throughout the last 31,000 years.

Isotopic and geochemical evidence of paleoclimates, especially for the last glaciation, has been obtained from deep confined groundwaters of southern India. The δ13C, δ18O, chloride, and deuterium analyses of groundwaters show distinct excursions inferred to be related to climatic variations. The arid climatic episode associated with the last glaciation (18,000 ± 2000 yr B.P.) is conspicuously identified by signatures of relatively enriched δ13C (−10 to −12‰ PDB) and δ18O (−5.3 to −4.8‰ SMOW) values, and high chloride concentration (80 to 160 mg/l). The transition from an arid to humid period ca. 12,000–8000 yr B.P. is shown by a decreasing trend in the δ13C (−9.5 to −17‰) and δ18O (−4.5 to −6.3‰) contents of groundwaters. The late Holocene (since 4000 yr B.P.), marked by a more humid but unstable climate, is identified by further depletion of δ13C (−13 to −20‰) and δ18O (−5.2 to −6.3‰). Similar variation between δ18O and chloride values in confined groundwaters further demonstrates two distinct climatic excursions (arid and humid) governed by the “amount effect.” This is the first time that isotopic and geochemical signatures related to changing paleoclimates have been identified in the confined groundwaters of the southern Indian landmass.

Uranium-series dating on a 186-m core (DV93-1) drilled from Badwater Basin in Death Valley, California, and on calcareous tufas from nearby strandlines shows that Lake Manly, the lake that periodically flooded Death Valley during the late Pleistocene, experienced large fluctuations in depth and chemistry over the last 200,000 yr. Death Valley has been occupied by a long-standing deep lake, perennial shallow saline lakes, and a desiccated salt pan similar to the modern valley floor. The average sedimentation rate of about 1 mm/yr for core DV93-1 was punctuated by episodes of more-rapid accumulation of halite. Arid conditions similar to the modern conditions prevailed during the entire Holocene and between 120,000 and 60,000 yr B.P. From 35,000 yr B.P. to the beginning of the Holocene, a perennial saline lake existed, over 70 m at its deepest. A much deeper and longer lasting perennial Lake Manly existed from about 185,000 to 128,000 yr B.P., with water depths reaching about 175 m, if not 330 m. This lake had two significant “dry” excursions of 102–103yr duration about 166,000 and 146,000 yr B.P., and it began to shrink to the point of halite precipitation between 128,000 and 120,000 yr B.P. The two perennial lake periods correspond to marine oxygen isotopic stages (OIS) 2 and 6. Based on the shoreline tufa ages, we do not rule out the possible existence ∼200,000 yr ago of yet a third perennial lake comparable in size to the OIS 6 lake. The234U/238U data suggest that U in tufa owes its origin mainly to Ca-rich springs fed by groundwater that emanated along lake shorelines in southern Death Valley, and that an increase of this spring-water input relative to the river-water input apparently occurred during OIS 6.

Diatoms, magnetic susceptibility, organic content, and14C ages of sediments from a 26-m core suggest that Lake Chalco, in the southern part of the basin of Mexico, went through a series of major fluctuations during the late Pleistocene and the Holocene. Before ca. 39,00014C yr B.P. the lake was very deep (about 8–10 m), alkaline, and saline. It then became shallow (<2 m) for most of the time between ca. 39,000 and 22,500 yr B.P. Chalco deepened to about 4–5 m about the time of a major eruption of nearby Popocatepetl volcano ca. 22,000 yr B.P. The lake remained relatively deep and fresh until ca. 18,500 yr B.P., when lower levels and alternating acidic to freshwater conditions were established. After 14,500 yr B.P. lake level rose slightly, but by ca. 10,000 yr B.P. Chalco became very shallow (<2 m), remaining as a low, alkaline saline marsh until ca. 6000 yr B.P. This period corresponds with the Playa cultural phase, during which the earliest human settlements in the basin were established. After ca. 6000 yr B.P. Chalco became a fresh to slightly alkaline shallow lake a few meters deep.