A record of estimated annual (prior August to current July) precipitation derived from a regional bur oak (Quercus macrocarpa Michx.) tree-ring chronology indicates that southern Manitoba's hydroclimate has been relatively stable over the last 200 yr. Although this stability was interrupted briefly by pronounced wet intervals in the late A.D. 1820s and 1850s, hydroclimatic conditions since permanent Euro-Canadian settlement were much less variable and persistent than those prior to A.D. 1790. The reconstruction indicates that the Red River basin experienced extremely dry conditions between A.D. 1670 and 1775, with below-normal precipitation occurring approximately 2 years out of 3. Annual precipitation was estimated at more than two standard deviations below the mean during A.D. 1477, 1485, 1556, 1595, 1612, 1644, 1661, 1743, 1900, and 1980. Comparisons with limnological records from North Dakota and Minnesota suggest that multidecadal fluctuations in regional hydroclimate have been remarkably coherent across the northeastern Great Plains during the last 600 yr. However, individual dry years in the Red River basin were usually associated with larger scale drought across much of the North American interior.

Geostatistical analyses of 35 plant species from 213 packrat middens with combined records spanning the last 40,000 yr indicate that many presumed winter precipitation-dependent taxa that existed in the Sonoran Desert during the last glaciation were expelled by increasing monsoon precipitation instead of waning cool-season moisture. The statistical influence of excessive monsoon rainfall on the distributions of many species probably reflects the simultaneous increase in the magnitude and occurrence of fire. During the early Holocene, results indicate a dramatic decrease in cool-season precipitation and an increase in monsoon rainfall. Levels of temperature and precipitation continued to change linearly until they reached modern values. These conclusions are drawn from a newly developed computer model that determines which climatic factors impede species movement into an unoccupied region. Climatic “limiters,” derived from digital versions of modern plant distributions, elevation, and meteorological data, formed the basis of the reconstructions. Particularly important distribution limiters for the Sonoran Desert include maximum warm-season precipitation and low winter temperatures. The model allows for quantitative estimates of past climatic changes with relatively detailed temporal and spatial resolutions. These results can be used to refine paleoclimatic interpretations based on coarser resolution General Circulation Models.

Past vegetation and climate changes reconstructed from a sediment core from Laguna Atezca, Molango, Mexico, provide new insights into the environmental and cultural histories of the Sierra Madre Oriental during the last 1700 yr. Pollen, microscopic charcoal, sediment chemistry, loss on ignition, and magnetic susceptibility indicate that three phases of human occupation, deforestation, and erosion (ca. A.D. 280–890, ca. A.D. 1030–1420, and ca. A.D. 1680–present) alternate with two phases of abandonment (ca. A.D. 890–1030 and ca. A.D. 1420–1680). Forest composition of the two abandonment phases differed, with cloud forest taxa (Liquidambar, Ostrya/Carpinus, Ulmus, etc.) dominating the pollen record during the first phase, and Quercus and Pinus pollen characterizing phase two. These differences may reflect a climate change in which the second phase was drier than the first; Alternatively, the increase in Pinus and Quercus may have been caused by a human-induced decline in soil fertility. The Laguna Atezca record also differs from several other Mesoamerican paleoenvironmental records in that it shows no evidence of drought at the end of the Classic Period, ca. A.D. 900.

Pollen analysis of two sediment records from a coastal swamp forest site in the Chilean semiarid region (31°50′S; 71°28′W) shows an alternation of dry and wet phases during the past ∼6100 cal yr B.P. The most prominent vegetation changes occur at ∼4200 cal yr B.P., with the expansion of the swamp forest taxa Luma chequen and Escallonia sp., followed by a regression of the forest beginning at ∼3200 cal yr B.P. and ending with its replacement by a xerophytic scrub, between ∼1800 and 1300 cal yr B.P. The swamp forest reexpanded after ∼1300 cal yr B.P. and persisted, with minor variation, until the present. We interpret the establishment of the swamp forest at the study site to be the result of a rising watertable in response to increased rainfalls from ∼4200 cal yr B.P. onward. Our results indicate that in north-central Chile the second half of the Holocene was climatically more variable than previously thought, suggesting significant changes in the position and/or intensity of the westerlies wind belt and possibly in the frequency of El Niño–Southern Oscillation events.

Springs in the 40 to 50 large lake basins (>15 km2) on the southern portion of the Southern High Plains (SHP) were active during periods of aridity in the Holocene when there may have been human habitation of the area. Eolian erosion of the lake floors and lunette accretion occurred as groundwater levels declined in response to decreased groundwater recharge. The declining lake floor associated with eolian erosion allowed groundwater evaporative discharge to continue, thus maintaining a groundwater gradient toward the lake. This hydrologic condition was favorable for a relatively continuous spring discharge to the lake, independent of the elevation of the lake floor. To evaluate the postulated dynamic equilibrium critical to this conclusion, 17 optically stimulated ages were determined from a 17.7-m deep core of a lunette adjacent to Double Lakes, Texas (33°13′15″N, 101°54′08″W). The core yielded sediment accumulation dates of 11,500±1100, 6500±700, and 4900±500 yr B.P., corresponding broadly with periods of aridity known from other evidence. Based on analysis of this lunette, it is concluded that springs in Double Lakes basin probably existed throughout the Holocene with discharges similar to those observed historically. We assumed that similar dynamic equilibrium existed in the other large lake basins in the SHP and that these springs could have provided a continuous source of water for indigenous peoples during periods of prolonged aridity. The dynamic equilibrium that is proposed in this study is applicable not only to other arid and semiarid geographic areas with wind-erodible material but also over different geologic times.

Sediments from Qipisarqo Lake provide a continuous Holocene paleoenvironmental record from southern Greenland. Following deglaciation and glacio-isostatic emergence of the basin from the sea ∼9100 cal yr B.P., proxies of lake paleoproductivity, including biogenic silica and organic matter, increased markedly until 6000 cal yr B.P. and thereafter remained stable over the ensuing warm three millennia. Subsequent decreases in these proxies, most dramatically between 3000 and 2000 cal yr B.P., show the lake's responses to initial Neoglacial cooling. Intervals of ameliorated limnological conditions occurred between 1300 and 900 and between 500 and 280 cal yr B.P., briefly interrupting the decreasing trend in productivity that culminated in the Little Ice Age. Increased lake productivity during the latter half of the 20th century, which reflects the limnological response to circum-arctic warming, still has not reached peak Holocene values. The Neoglacial climate of the last 2000 yr includes the most rapid high-amplitude environmental changes of the past nine millennia. The Norse thus migrated around the North Atlantic Ocean region in the most environmentally unstable period since deglaciation. Lacustrine sediment records provide a context with which to consider future environmental changes in the Labrador Sea region. In particular, any future warming will be superposed on a regional climate system that is currently exhibiting highly unstable behavior at submillennial timescales.

Central Chile (32–35°S) lies at the northern border of strong Westerly influence and thus exhibits a steep precipitation gradient. Therefore, the palaeoclimatic archives in the region are suitable for detecting past moisture changes. The study of Laguna Aculeo (33°50'S, 70°54'W) presents a multiproxy Holocene lake record including sedimentology, geochemistry, mineralogy, pollen, diatoms, and radiocarbon dating (17 dates). Results indicate an arid early to mid-Holocene period (about 9500–5700 cal yr B.P.). After 5700 cal yr B.P. effective moisture increased progressively and around 3200 cal yr B.P., modern humid conditions were established. Numerous intercalated clastic layers reflect flood deposition during rainy winters. A fluvial unit was deposited shortly before 9000 cal yr B.P. Subsequently, flood events were absent until 5700 but have become frequent since 3200 cal yr B.P. The frequency of flood layers possibly points to weak or no El Niño activity during the early and mid-Holocene, with a subsequent increase during the late Holocene. During the early and mid-Holocene, the Westerlies were probably blocked and hence deflected southward by the subtropical high-pressure cell. Higher precipitation during the last 3200 yr seems strongly related to a weakened subtropical high-pressure cell with intensified Westerlies and possibly increased El Niño activity.

Records of Holocene vegetation and climate change at low elevations (<2000 m) are rare in the central Rocky Mountain region. We developed a record of Holocene vegetation and climate change from 55 14C-dated woodrat middens at two low-elevation sites (1275 to 1590 m), currently vegetated by Juniperus osteosperma woodlands, in the northern Bighorn Basin. Macrofossil and pollen analyses show that the early Holocene was cooler than today, with warming and drying in the middle Holocene. During the Holocene, boreal (Juniperus communis, J. horizontalis) and montane species (J. scopulorum) were replaced by a Great Basin species (J. osteosperma). J. osteosperma colonized the east side of the Pryor Mountains 4700 14C yr B.P. Downward movement of lower treeline indicates wetter conditions between 4400 and 2700 14C yr B.P. Increased aridity after 2700 14C yr B.P. initiated expansion of J. osteosperma from the east to west side of the Pryor Mountains.

A continuous record of organic carbon δ13C from a buried soil sequence in south-central Texas demonstrates: 1) strong coupling between marine and adjacent continental ecosystems in the late Pleistocene as a result of glacial meltwater entering the Gulf of Mexico and 2) ecosystem decoupling in the Holocene associated with a reduction of meltwater and a shift in global circulation patterns. In the late Pleistocene, reduction in C4 plant productivity correlates with two well-documented glacial meltwater pulses (∼15,000 and 12,000 14C yr B.P.), indicating a cooler-than-present adjacent continental environment. Increased C4 production between 11,000 and 10,000 14C yr B.P. suggests that the Younger Dryas was a warm interval responding to the diversion of glacial meltwater away from the Mississippi River. With waning meltwater flow, C4 productivity generally increased throughout the Holocene, culminating in peak warm intervals at ∼5000 and 2000 14C yr B.P. Shifts in the abundances of C3–C4 plants through the late Quaternary show no correlation to ecophysiological responses to atmospheric CO2 concentration.

Continuous lacustrine deposits of the Funza-II core from the Bogotá basin, Colombia (5°N74°W) record late Pleistocene climatic variations, providing an opportunity to assess the influence of glacial–interglacial climate changes on alpine ecosystems in equatorial South America. Biogeochemical response of this tropical alpine system to climate change was inferred from changes in elemental concentrations and ratios and isotopic signatures in the upper 120 m of the lacustrine Funza core.

Values of δ13Corg exhibit eight abrupt, positive shifts that are thought to reflect rapid expansions of C4 grasses in the tropical Andes and algal blooms. One of these excursions, interpreted to correspond to C4 vegetation expansion, occurred in sediments accumulated during the last glaciation (∼30,000–50,000 yr B.P.) and implies a downslope shift of the upper Andean treeline, regardless of prevailing temperatures.

Sedimentary carbon/sulfur ratios are low and indicate significant sequestering of sulfur. Monosulfides are the dominant constituent of sedimentary sulfur during relatively humid intervals, when increased supply of iron caused by enhanced weathering favored the formation of monosulfide minerals under strongly reducing conditions. In contrast, organosulfur compounds dominate the sedimentary sulfur-species in relatively drier intervals when mildly reducing conditions and limited iron input promoted the diagenetic incorporation of sulfur in organic matter. Dry events inferred from the sulfur record typically correlate with glacial maxima, whereas glacial terminations are usually associated with wet periods.

So-called annual banding has been identified in a number of speleothems in which the number of bands approximates the time interval between successive U-series dates. The apparent annual resolution of speleothem records, however, remains largely untested. Here we statistically compare variations in band thickness from a late Holocene stalagmite in Carlsbad Cavern, Southern New Mexico, USA, with three independent tree-ring chronologies form the same region. We found no correspondence. Although there may be various explanations for the discordance, this limited exercise suggests that banded stalagmites should be held to the same rigorous standards in chronology building and climatic inference as annually resolved tree rings, corals, and ice cores.

Previous studies report that remains of prairie taxa are common in western Missouri archaeological sites. There is no reported evidence of prairie taxa, such as Bos bison, from sites within the northern Ozark Highlands of Missouri. New data indicate that prairie taxa inhabited the area during the warm and dry mid-Holocene (8500–5000 14C yr B.P.). Evidence suggests prairie taxa were farther east and south than previously documented in central Missouri and underscores the importance of biogeographic data in studies of prehistoric environments.

Recent studies on relict eolian dunes in the Rio Negro basin, northern Amazon, Brazil (00°35′N, 63°14′W), indicate a drier climate regime during the Late Pleistocene–Holocene transition that is different from the present humid climate. The eolian sands form long chains of linear dunes bordering the Rio Negro River and some tributaries. Here, we present thermoluminescence (TL) ages spanning the period 32,000–8000 yr B.P. The final dune stabilization took place after 8,000 yr B.P. and now the bases of the dunes are fixed by vegetation. Clustering of the TL dates suggests that the dry climate in the Amazon Basin occurred in distinct episodes and argues against current opinions that drastic ecological changes did not affect in the Amazon during the last global glaciation.