A 4000-yr sediment core record from Lake Salpetén, Guatemala, provides evidence for Maya-induced forest clearance and consequent soil erosion between ∼1700 cal yr B.C. and 850 cal yr A.D. Radiocarbon ages of wood, seeds, and charcoal support an age-depth model with average errors of ±110 cal yr. Relatively low carbonate δ18O values between 1300 and 400 cal yr B.C. coincide with pollen evidence for forest loss, consistent with increased surface and groundwater flow to the lake. Minimum δ18O values between 400 cal yr B.C. and 150 cal yr A.D. suggest a high lake level, as do 14C-dated aquatic gastropods as much as 7.5 m above the present lake stage. High lake levels resulted from reduced evaporation-to-precipitation ratios, increased hydrologic input caused by anthropogenic deforestation, or both. The Preclassic abandonment (150 A.D.) and Early Classic/Late Classic boundary (550 A.D.) are marked by relatively high δ18O values indicating reduced lake levels. Oxygen isotope composition increased further coincident with the Terminal Classic Maya demographic decline between 800 and 900 A.D. This period of high δ18O may have been caused by the greater aridity that has been documented in northern Yucatán lakes or by decreased hydrologic input to the lake as a consequence of forest recovery. Reduced soil erosion after 850 cal yr A.D. coincided with the Terminal Classic Maya demographic decline and permitted forest recovery and resumption of organic sedimentation.

Due to its location on the eastern North Atlantic seaboard, the Fennoscandian tree-line area is an ideal area to use biological proxies to assess the relative roles of the Scandinavian ice sheet and of oceanic, atmospheric, and astronomic forcings on regional climate history. Here we report pollen-based July mean temperature (Tjul) and annual precipitation (Pann) reconstructions from a sediment core from a high-altitude tree-line lake in northwestern Finland. The reconstructions suggest that at 9600–8300 cal yr B.P. Tjul values were low but steadily rising while Pann was high. The period of warmest summers, with Tjul values ca. 1.8°–1.6°C higher than at present, occurred at ca. 8000–6500 cal yr B.P. Since then climate has become gradually cooler. Tjul values during the “Medieval Warm Period” (ca. 1400–1000 cal yr B.P.) were ca. 0.8°C higher that at present but decreased rapidly to the low “Little Ice Age” levels at 800 cal yr B.P. We compare these results with an earlier pollen-based climate reconstruction from the same region. The reconstructions indicate a similar general Holocene Tjul pattern with lower values in the reconstruction from the high-altitude lake. However, most of the small-scale variations are not synchronous, suggesting that they may represent noise rather than signal in our data.

Previous work in the San Luis Valley of south-central Colorado and northern New Mexico at the Hansen Bluff and SAM Cave localities has allowed reconstruction of the paleoclimate in the region during the interval from ∼0.6 myr to ∼2.6 myr. Surface exposures at Hansen Bluff have been correlated to deep-sea oxygen isotope core stages 18–23 and one of the glacial periods contained therein to the oldest “Nebraskan” till in southwestern Iowa. Deposits in SAM Cave correlate to Hansen Bluff on the basis of paleomagnetics, climate interpretation, and microtine rodent biochronology. In this paper, carbon and oxygen stable isotope data of herbivorous rodent teeth are used as indicators of change in the predominance of C3, C4, and CAM plants and of changes in temperature and precipitation. Taken together with other previously analyzed data from these localities, the evidence indicates a shift in the severity of glacial events that is mainly related to the hydrologic characteristics of the climate. This shift may have been caused by broad regional uplift that may have been instrumental in shifting global climate cycles from the 41,000-yr Matuyama cycles to the Brunhes 100,000-yr-dominated cycles.

The use of shaded digital topographic models to visualizes suites of topographic features, stratigraphy, and field mapping reveals newly recognized multiple moraines associated with oscillations of the remnants of the Cordilleran Ice Sheet in the Fraser Lowland along the western Washington–British Columbia border. Morphologic features show the extent of ice represented by Sumas Drift (ca. 11,600–10,000 14C yr B.P.), following ca. 3000 yr of retreat from the glacial maximum (Fraser Glaciation) positions 80 km south of Seattle and in the Strait of Juan de Fuca. The paleogeography of the ice margin and timing of ice retreat during the Sumas Stade is reconstructed and bracketed by 70 radiocarbon dates (24 on marine shells, 46 on wood and peat), which are secured by morphologic and stratigraphic evidence. Four topographically distinct phases of the Sumas deglaciation model are suggested.

Phase SI: 11,600–<11,400 14C yr B.P., outermost drift, scattered remnants of ice-contact deposits

Phase SII: 11,600–11,400 14C yr B.P., glacier readvance, building prominent moraine followed by glacier retreat

Phase SIII: 10,980–10,250 14C yr B.P., glacier readvance building of multiple moraines followed by glacier retreat

Phase SIV: >10,250–10,000? 14C yr B.P., glacier readvance, building of inner moraine followed by glacier retreat

Holocene sediments from Nelson Lake, on Michigan's eastern Upper Peninsula, provide isotopic, pollen, and charcoal evidence for a two-step sequence of changes in moisture source and increased lake-effect precipitation during the late Holocene. Between 8000 and 5300 cal yr B.P., a warm, dry climate and zonal atmospheric circulation produced enriched stable oxygen and carbon isotopic values in combination with high percentages of pine pollen and sustained influx of charcoal particles. After 5300 cal yr B.P., decreasing isotopic values in marl and increasing pollen percentages of mesic hardwoods and northern white cedar indicate increased meridional air flow and precipitation from cold winter storms generated in Alberta, Canada. After 3000 cal yr B.P., abrupt declines in values of δ13C and δ18O and increased pollen representation of hemlock, American beech, spruce, and aquatic plants indicate paludification from increased lake-effect snowfall. The moisture was derived from the Great Lakes and transported by Alberta cyclonic storms that were steered across Lakes Superior and Michigan by a southward shift in the modal winter position of the polar jet stream.

Two geochemical techniques from soil mineralogy were used with lake sediment to reconstruct soil development in the catchments of two lakes—one on outwash and the other on till—in the Great Lakes region. A sodium pyrophosphate extraction provides information about leaching of exchangeable and organically complexed cations from terrestrial sources, while an acidified ammonium oxalate extraction removes mineral materials without well-developed crystalline structure that are indicative of podzolization. More evidence of pedogenesis is preserved in sediment of the lake on outwash—likely a result of greater retention of material within the soil profile in the catchment on till. This difference between the records suggests that excessively well-drained (leaky) catchments may provide better records of soil development than those with higher water-holding capacity. Evidence of acidification and podzolization appeared first at the lake on till and 4500 years later at the lake on outwash. Both records reveal a late Holocene intensification of weathering that coincided with a regional climatic change to greater moisture availability. The variability in weathering rates observed presents a more complex picture of weathering than the simple exponential functions often assumed in chronosequence studies. Use of lake sediment records provides better temporal resolution of significant events in soil formation than could be achieved using a chronosequence approach.

Computer reconstructions of the bathymetry of the lake were used to quantify variations in the size and form of Lake Agassiz during its final two phases (the Nipigon and Ojibway phases), between about 9200 and 7700 14C yr B.P. (ca. 10,300–8400 cal yr B.P.). New bathymetric models for four Nipigon Phase stages (corresponding to the McCauleyville, Hillsboro, Burnside, and The Pas strandlines) indicate that Lake Agassiz ranged between about 19,200 and 4600 km3 in volume and 254,000 and 151,000 km2 in areal extent at those times. A bathymetric model of the last (Ponton) stage of the lake, corresponding to the period in which Lake Agassiz was combined with glacial Lake Ojbway to the east, shows that Lake Agassiz–Ojibway was about 163,000 km3 in volume and 841,000 km2 in areal extent prior to the final release of lake waters into the Tyrrell Sea. During the Nipigon Phase, a number of catastrophic releases of water from Lake Agassiz occurred as more northerly (lower) outlets were made available by the retreating southern margin of the Laurentide Ice Sheet; we estimate that each of the four newly investigated Nipigon Phase releases involved water volumes of between 1600 and 2300 km3. The final release of Lake Agassiz waters into the Tyrrell Sea at about 7700 14C yr B.P. is estimated to have been about 163,000 km3 in volume.

The chronology of Quaternary paleoenvironment and climate in northeastern Siberia is poorly understood due to a lack of reliable numerical age determinations. The best climatic archives are ice-rich permafrost sequences, which are widely distributed in northeastern Siberia. For this study, 230Th/U-ages were determined by thermal ionization mass spectrometry (TIMS) from frozen peat in a permafrost deposit at the southern cliff of the Bol'shoy Lyakhovsky Island (New Siberian Archipelago), west of the Zimov'e River. These yielded a Pre-Eemian “isochron”-corrected 230Th/U-age of 200,900±3400 yr. This result is reliable because permafrost deposits behave as closed systems with respect to uranium and thorium. Our findings suggest that 230Th/U dating of frozen peat in permafrost deposits is a useful tool for the reconstruction of the Middle Quaternary environment of northern Siberia and of the whole Arctic.

The woodland-steppe ecotone of the southeastern Inner Mongolian Plateau in northern China is located at the northwestern limit of the Pacific monsoon influence, where the landscape may have been a sensitive recorder of past climatic changes. Physical, chemical, and biological analyses of AMS 14C-dated sediment sequences from two lakes of this region were used to reconstruct the Holocene vegetation and desertification history and distinguish four periods: (1) a cold and humid period from 10000 to 8000 14C yr B.P., (2) a warm and humid period from 8000 to 5900 14C yr B.P., (3) a warm and dry period from 5900 to 2900 14C yr B.P., and (4) a cool and dry period from 2900 14C yr B.P. to the present. The increased aridity during the middle Holocene was likely caused by increased winter temperatures and enhanced winter evaporation. The transition from a humid to an arid climate after ∼5900 14C yr B.P. coincided with enhanced aeolian activity, and deciduous woodlands were replaced by pine woodlands and then by steppes in response to the climatic deterioration. These transitions led to the present desertification. The records suggest that a simple association of thermal and moisture conditions, such as warm/wet or cold/dry, may be misleading.

Lateral variations in whole-rock and clay geochemistry of basal Bed II claystones in Olduvai Gorge, Tanzania, reflect water quality differences across the Eastern Lacustrine Plain ∼1.75 myr ago. Bulk Ba/Sr and (Na2O+K2O+MgO)/Al2O3 range from 1.4 to 4.2 and from 0.7 to 1.4, respectively, and indicate leaching of lacustrine claystones beneath freshwater wetlands at times following lake retreat. Bulk MgO/Al2O3 (0.3–1.0) and molar Mg/Al (0.5–3.9) ratios of <0.2-μm clays reflect alteration of Mg-rich lacustrine clays. These indicators point to freshest conditions near Locality 43 of Hay (1976; HWK-East; Leakey, 1971), moderate conditions to the east (Locality 40-MCK), and high salinity and alkalinity to the west (Localities 85-VEK, 45-FLK).

Clay geochemistry and artifact abundances are well correlated (r=−0.67, p<0.005), suggesting a relationship between paleo-water quality and hominid paleoecology. This pattern is consistent with predictions of greatest artifact discard/loss around freshwater sources where scavanging opportunities were greatest for hominids. This quantifies a relationship between artifact density distribution and a paleoecological proxy over landscape scales for the first time in Early Stone Age archaeology. In contrast, fossil bone abundance is uncorrelated (r=0.14, p=0.6), reflecting more complex taphonomic processes. Quantitative tests of landscape-scale land-use models are important for understanding early hominid behavior and its evolution.