Deposits beneath Mubwindi Swamp provide a partial record of vegetation history since at least 43,000 yr ago. We studied pollen from two cores and obtained nine radiocarbon ages from one of these cores and three radiocarbon ages from the other. Pollen deposited before and soon after the last glacial maximum represents vegetation very different from the modern vegetation of the Mubwindi Swamp catchment. Although species now associated with higher altitudes were dominant some elements of moist lower montane forest persisted, possibly because of favorable soils or topography. The pollen data provides evidence for a late glacial montane forest refuge near Mubwindi Swamp. Moist lower montane forest became much more widespread soon after the glacial maximum. The only irrefutably Holocene sediments from Mubwindi Swamp date to the past 2500 yr. During this time a combination of climatic and human-induced changes in vegetation can be seen in the pollen records.

Two century-scale time series in northern Bolivia constrain the ages of abrupt changes in the physical, geochemical, and biological characteristics of sediments obtained from lakes that formed during deglaciation from the late Pleistocene glacial maximum. The watersheds of Laguna Viscachani (16°12′S, 68°07′W, 3780 m) and Lago Taypi Chaka Kkota (16°13′S, 68°21′W, 4300 m), located on the eastern and western slopes of the Cordillera Real, respectively, contain small cirque glaciers. A high-resolution chronology of the lake sediments is provided by 23 AMS14C dates of discrete macrofossils. Late Pleistocene glaciers retreated rapidly, exposing the lake basins between 10,700 and 970014C yr B.P. The sedimentary facies suggest that after 890014C yr B.P. glaciers were absent from the watersheds and remained so during the middle Holocene. An increase in the precipitation–evaporation balance is indicated above unconformities dated to ∼230014C yr B.P. in both Lago Taypi Chaka Kkota and Laguna Viscachani. An abrupt increase in sediment accumulation rates after 140014C yr B.P. signals the onset of Neoglaciation. A possible link exists between the observed millennial-scale shifts in the regional precipitation–evaporation balance and seasonal shifts in tropical insolation.

Both aquatic and land snails are common in the geologic record, but their utility in dating is greatly restricted by their well-documented tendency to yield14C dates inconsistent with true14C ages. In this study, we examine the use of14C ages from (1) small, previously unstudied, terrestrial snails to date hosting spring deposits and from (2) cooccuring aquatic snails to constrain groundwater travel times during the last glacial period. Our study area in the southern Great Basin encompasses Yucca Mountain, site of the proposed high-level nuclear waste repository, where information on the age and extent of past high water tables and on groundwater flow times is crucial to several licensing issues. Our results show that shells of small terrestrial snails belonging toValloniasp. yield14C dates consistent with14C ages of associated carbonized wood. These results imply that these taxa can provide reliable14C age control on the broadly distributed deposits in which they have been described. In contrast, cooccurring aquatic snails from fossil spring deposits yield14C ages generally greater than the control age. This is because the aquatic shells often formed in spring waters that had an initial14C deficiency. However, the magnitude of the deficiency is much less than that observed in nearby modern springs, arguing for much higher average14C contents in late Pleistocene groundwaters in these basins. If representative, this implies shorter groundwater travel times through aquifers in southern Nevada during late-glacial time.

Analyses of phytoliths and carbon isotopes document change in late Quaternary grasslands in the Red Valley of the southern Black Hills. Late Pleistocene grassland composition was equivalent to the C3grass parklands of modern central Alberta. The rise of mixed grassland occurred rapidly between 11,000 and 9000 yr B.P. Early Holocene mixed grasslands included both short and tall C4grasses. A mid-Holocene erosional unconformity (ca. 8000 to 4500 yr B.P.) precludes phytolith or isotope analysis, but suggests lack of vegetation and landscape denudation caused by a drier climatic. Basin-wide stability and soil development followed the erosional episode (ca. 4500 to 3600 yr B.P.). Mesic-adapted C4panicoid grasses increased during this period of soil development. Low-magnitude fluctuation in the C4-dominated mixed grassland occurred throughout the late Holocene (3600 yr B.P. to present). Rise in δ13C values during the last 1000 yr without corresponding change in phytoliths may indicate a decrease in woodlands caused by increased fire frequency.

Fine-interval (∼30–45 yr) sampling of a core from Lake Victoria's Damba Channel shows that numerous abrupt changes in the lake's diatom assemblages have occurred in response to climatic fluctuations over the past 11,40014C yr. Four distinct climatic phases bounded by sudden transitions are inferred: (1) variably dry ∼11,400–10,000 yr B.P., (2) humid ∼10,000–7200 yr B.P., (3) more seasonal ∼7200–2200 yr B.P., and (4) more arid ∼2200–0 yr B.P., with a dry “Little Ice Age” event ∼600–200 yr B.P. The diatom-inferred paleoclimatic history for northern Lake Victoria closely resembles that inferred from a well-dated pollen record from Pilkington Bay. Spectral analysis of the diatom record reveals strong periodicities including globally distributed ∼2360–2550, ∼1400, ∼1030–1130, and ∼500 cal-yr cycles. Repeated, rapid shifts betweenAulacoseira- andNitzschia-dominated diatom assemblages suggest that post-1960 changes in the lake's phytoplankton communities have had earlier, climate-driven analogs.

Benthic foraminifera and stable isotope data from the last interglaciation (Eemian, substage 5e) from a borehole at Skagen, Denmark, provide evidence for major environmental and hydrographic changes during this period. During the first millennium of the Eemian, water masses covering northern Denmark became gradually warmer. Temperate conditions prevailed during most of the interglaciation, but these were interrupted by two periods with decreased water temperatures. The first cooling (Event S-1) was not very distinct at Skagen, but the second (Event S-2), seen in both the foraminiferal and oxygen isotope record, represents a large shift to subarctic conditions. Carbon isotopes indicate a change in ocean circulation during both events. No comparable climate variations are seen within the Holocene record at the site. The final cooling of the water masses associated with the substage 5e/5d boundary occurred within a few hundred years. These last interglacial climatic changes were probably caused by variations in strength and/or position of the North Atlantic Drift, possibly as a result of varying vigor of the Atlantic conveyor. In addition, minor variations in the fossil assemblages also indicate fluctuations in the inflow of Atlantic water to the Skagerrak–Kattegat area during the warm intervals of substage 5e.

Hackberry endocarp (Celtissp.) contains significant amounts (up to 70 wt%) of biogenic carbonate that is nearly pure aragonite (CaCO3). Because of their high mineral content, hackberry endocarps are found abundantly in Tertiary and Quaternary sediments and are very common in many North American archaeological sites. We analyzed the14C content of different components of modern hackberries including the biogenic carbonate in hackberry endocarps collected at known times over the past century. The14C content of the endocarp carbonate accurately records the14C content of the atmosphere.14C dates of fossil endocarp carbonates compared favorably with dates obtained by other means at archaeological and geological sites ranging in age from the late Pleistocene through the early Holocene. We therefore suggest that hackberry endocarp is a suitable substrate for14C dating provided that its morphological and mineralogical integrity is preserved.

Paleolimnological and archaeological records that span 3500 years from Lake Titicaca and the surrounding Bolivian–Peruvian altiplano demonstrate that the emergence of agriculture (ca. 1500 B.C.) and the collapse of the Tiwanaku civilization (ca. A.D. 1100) coincided with periods of abrupt, profound climate change. The timing and magnitude of climate changes are inferred from stratigraphic evidence of lake-level variation recorded in14C-dated lake-sediment cores. Paleo-lake levels provide estimates of drainage basin water balance. Archaeological evidence establishes spatial and temporal patterns of agricultural field use and abandonment. Prior to 1500 B.C., aridity in the altiplano precluded intensive agriculture. During a wetter period from 1500 B.C. to A.D. 1100, the Tiwanaku civilization and its immediate predecessors developed specialized agricultural methods that stimulated population growth and sustained large human settlements. A prolonged drier period (ca. A.D. 1100–1400) caused declining agricultural production, field abandonment, and cultural collapse.

We compare alkenone unsaturation ratios measured on recent sediments from the Indian Ocean (20°N–45°S) with modern sea oceanographic parameters. For each of the core sites we estimated average seasonal cycles of sea surface temperature (SST) and salinity, which we then weighted with the seasonal productivity cycle derived from chlorophyll satellite imagery. The unsaturation index (U37K′) ranges from 0.2 to 1 and correlates with water temperature but not with salinity. TheU37K′versus SST relationship for Indian Ocean sediments (U37K′= 0.033 SST + 0.05) is similar to what has been observed for core tops from the Pacific and Atlantic oceans and the Black Sea. A global compilation for core tops givesU37K′= 0.031 T + 0.084 (R= 0.98), which is close to a previously reported calibration based on particulate organic matter from the water column. For temperatures between 24° and 29°C, however, the slope seems to decrease to about 0.02U37K′unit/°C. For Indian Ocean core tops, the ratios of total C37alkenones/total C38alkenones and the slope of theU37K′-SST relationship are similar to those previously observed for cultures ofEmiliania huxleyibut different from those previously published forGephyrocapsa oceanica.EitherE. huxleyiis a major producer of alkenones in the Indian Ocean or strains ofG. oceanicaliving in the northern Indian Ocean behave differently from the one cultured. In contrast with coccolithophorid assemblages, the ratios of C37alkenones to total C38alkenones lack clear geographic pattern in the Indian Ocean.

Six cores were collected from a small peatland on the Tuktoyaktuk Peninsula, yielding up to 1.8 m of organic material and 1.2 m of ice-rich material. Stratigraphic relationships, radiocarbon dating, and pollen, plant macrofossil, and isotopic analyses are used to reconstruct the developmental history of the peatland. Organic matter began to accumulate in the basin about 7200 yr B.P., during a period when the climate is thought to have been warmer than present. Initially the permafrost table was probably below the bottom of the basin, and an open-water mineral wetland with emergent and submergent aquatic vegetation occupied the site. Enriched δ18O values from ice samples confirm the existence of an open water body with a water balance strongly affected by evaporation. Transformation to a graminoid fen peatland was underway by 6300 yr B.P. Permafrost and ice began to form in the peatland at this time. Low-centered polygons probably began to develop by 4700 yr B.P. and subsequently evolved into the high-centered polygons which characterise the southwestern part of the peatland today. Peat accumulation in the polygons has been extremely slow for the last 4000 years. The eastern, wetter part of the basin has largely remained unchanged since its inception about 7000 yr B.P. Development of the peatland may be linked to permafrost formation that was controlled by regional climate changes during the Holocene.

The origin of Washington State's Mima Mounds has been debated for over a century, with numerous mechanisms being postulated. Subfossil Coleopteran (beetle) remains recovered from the base of a mound at Mima Prairie consist of species that would be expected in rodent burrows and nests; all but one species are obligate burrow inhabitants. These results suggest the past presence of fossorial rodents (probably pocket gophers,Thomomys mazama) in the mounds, although none live there at present. Whether or not the gophers created the mounds, they may well have been instrumental in maintaining mound geometry until very recently.

AMS14C ages of 10 bones of the caribou (Rangifer tarandus), flat-headed peccary (Platygonus compressus), and giant beaver (Castoroides ohioensis) from the Dutchess Quarry Caves, New York, range from 13,840 ± 80 to 11,670 ± 70 yr B.P. No bones from any of these species are demonstrably associated with Paleoindian artifacts (fluted points) or other cultural materials from the sites because the bones lack unequivocal stratigraphic association with artifacts, as well as physical (taphonomic) evidence for human association (e.g., burning, cut marks, distinctive breakage). Together with the Holocene conventional14C dates of charcoal and the varied stratigraphic proveniences of the fluted points and the dated bones, the new AMS14C dates argue that most strata at the Dutchess Quarry Caves contain a mixture of late Pleistocene and Holocene materials. This mixing probably resulted from post-depositional bioturbation (by humans, rodents, carnivores, and scavengers) and cryoturbation (annual freeze–thaw cycles). Rather than being of cultural origin, the bones of caribou, flat-headed peccary, and giant beaver likely were deposited in the Dutchess Quarry Caves by nonhuman predators or scavengers, such as ursids, canids, felids, condors, or eagles.

Today, precipitation over tropical South America is largely controlled by the seasonal movements of the Inter-Tropical Convergence Zone (ITCZ). During the summer, the ITCZ is shifted southward due to the warming of the continent. Paleoclimate data from southeastern Amazonia and the central Andes indicate that these two areas evolved similarly during the last 30,000 yr. However, between 12,400 and 8800 cal yr B.P., eastern Amazonia received substantial moisture whereas the Bolivian Altiplano was arid. This suggests that the ITCZ during summer was then farther north than it is today.