The time series of volcanically produced sulfate from the GISP2 ice core is used to develop a continuous record of explosive volcanism over the past 110,000 yr. We identified ∼850 volcanic signals (700 of these from 110,000 to 9000 yr ago) with sulfate concentrations greater than that associated with historical eruptions from either equatorial or mid-latitude regions that are known to have perturbed global or Northern Hemisphere climate, respectively. This number is a minimum because decreasing sampling resolution with depth, source volcano location, variable circulation patterns at the time of the eruption, and post-depositional modification of the signal can result in an incomplete record. The largest and most abundant volcanic signals over the past 110,000 yr, even after accounting for lower sampling resolution in the earlier part of the record, occur between 17,000 and 6000 yr ago, during and following the last deglaciation. A second period of enhanced volcanism occurs 35,000–22,000 yr ago, leading up to and during the last glacial maximum. These findings further support a possible climate-forcing component in volcanism. Increased volcanism often occurs during stadial/interstadial transitions within the last glaciation, but this is not consistent over the entire cycle. Ages for some of the largest known eruptions 100,000–9000 yr ago closely correspond to individual sulfate peaks or groups of peaks in our record.

Magnetic susceptibility profiles χ(z) in loess sequences reflect a combination of two climatically modulated processes: dust deposition and pedogenic development. Prominent soils with high χ values, for example, likely reflect periods of slow dust deposition, and warm wet conditions favorable for rapid chemical weathering. To refine our understanding of the climate records contained in loess, we develop a numerical model as a tool for exploring quantitatively the integrated record of the temporal variation in rates of loess aggradation and soil development contained in χ profiles. In our model, the aggrading loess is pedogenically altered in a reactive zone near the ground surface. The strength of the χ signal is dictated by both the depth-dependent intensity of pedogenic processes and the rate of dust accumulation, which dictates the total time a loess parcel spends in the near-surface reactive zone. The model can accommodate both pedogenic production of magnetically susceptible minerals and arrival of magnetically susceptible grains as eolian dust. To explore the model performance and develop a sense of time and length scales implicit in χ profiles, we first examine simple synthetic cases with idealized steady and cyclic climatic forcing. Reported χ profiles in three Chinese loess sequences at varying distances from the western China dust source are then modeled in two illustrative ways: (i) by imposing a specific dust deposition rate history that is proportional to the dust accumulation rate history reported from western Pacific deep-sea cores, allowing the time variation of pedogenic rates to be calculated directly from χ profiles; and (ii) by imposing both dust accumulation and pedogenic rate histories that are independently scaled by the deep-sea δ18O history, which reflects global climate cycles to which regional climate forcing is linked. We find that the zone of pedogenic activity is roughly 0.5–1.0 m thick, both deposition rate and pedogenic intensity have varied dramatically over the last 140,000 yr, the age structure of the Luochuan loess sequence is best fit by driving the model with the δ18O record, and environmental conditions must have been anomalously favorable for pedogenesis during isotope stage 3 at all three sites. Finally, we advocate the assembly of a variety of data types at a suite of sites within any loess field that taken together will better constrain the temporal and spatial patterns of climatically modulated deposition and pedogenic processes.

New evidence has been obtained for the age of the Umı̂vı̂t/Keglen and Ørkendalen moraine systems close to the present ice sheet margin in central West Greenland. The Umı̂vı̂t/Keglen moraine system is dated at 7500 to 6500 14C yr B.P., which is older than the previously assumed date of 7300 to 6000 14C yr B.P. The Ørkendalen system is now dated at 6200 to 5600 14C yr B.P. against earlier estimates of 300 to 700 14C yr B.P. The new age is based on AMS radiocarbon-dated organic material within depressions between morainic ridges belonging to the Ørkendalen system. A major implication of the new age is that ice margin positions prevailed for about 6000 years behind the present ice sheet margin. The retreat behind the present margin could be substantial, and in the light of deglaciation rates prior to the Ørkendalen phase, may be ca. 10's of kilometers rather than kilometers. Circumstantial evidence is found for the retreat of the ice sheet margin behind its present position during the Holocene climatic optimum. The results, placed into a time frame of deglaciation since the last glacial maximum, enable comparison with Greenland ice sheet models and ice core records.

Five pre-Illinoian tills are recognized and named informally in northern Missouri, near the southernmost margin of the pre-Illinoian Laurentide ice sheet. The three youngest tills (McCredie formation) have high (50–60%) expandable clay mineral contents and normal remanent magnetic polarity. The next oldest till (Moberly formation) has low (30–40%) expandable clay content and reversed polarity. The oldest till (Atlanta formation) has very low or no expandable clay minerals. Its remanent magnetic polarity is unknown.

The sequence of four pre-Illinoian tills above the Atlanta formation probably correlates with the similar sequence of four pre-Illinoian “A type” tills in western Iowa and eastern Nebraska. The upper four tills in Missouri are also correlated, in order of increasing age, with the three members of the Wolf Creek Formation and the older Alburnett Formation in eastern Iowa. The oldest till in Missouri possibly correlates with the “C type” till in western Iowa.

Pre-Illinoian Laurentide ice reached the same approximate southern margin at least five times. Those advances deposited tills which maintain characteristic compositions over distances of at least 350 km along flow lines.

Holocene shore-face and beach-face deposits form plains <5 m above present sea level along Taiwan Strait. We measured the 14C ages of detrital mollusk shells and coral in such deposits at the Penghu Islands. Twelve carbonate samples—mainly from the largest island, Makung—were dated. Age measurements for two coral samples and one mollusk sample from the same outcrop imply that the 14C ages of mollusk shells give the best approximation of depositional age. The highest Holocene relative sea level in the Penghu Islands occurred about 4700 years ago with a height about 2.4 m above the present sea level. Thereafter, relative sea level appreciably fell without detectable fluctuations to its recent position. Our sea level data are consistent with other studies from the central and western Pacific, except for the timing of peak sea level position. This variation may reflect local crustal response to hydroisostatic effects on the continental shelf.

The Vedde Ash Bed (mid-Younger Dryas) and the Saksunarvatn Ash (early Holocene) are important regional stratigraphic event markers in the North Atlantic, the Norwegian Sea, and the adjacent land area. It is thus essential to date them as precisely as possible. The occurrence of the Saksunarvatn Ash is reported for the first time from western Norway, and both tephras are dated precisely by AMS analyses of terrestrial plant material and lake sediment at Kråkenes. The Vedde Ash has been previously dated at sites in western Norway to about 10,600 yr B.P. It is obvious in the Younger Dryas sediments at Kråkenes, and its identity is confirmed geochemically. The mean of four AMS dates of samples of Salix herbacea leaves adjacent to the tephra is 10,310 ± 50 yr B.P. The Saksunarvatn Ash is not visible in the early Holocene lake sediment at Kråkenes. After removal of organic material and diatoms, the identity of the tephra particles was confirmed geochemically, and their stratigraphic concentration was estimated. From curve matching of a series of seven AMS dates of terrestrial plant macrofossils and whole sediment, the radiocarbon age of the ash is 8930–9060 yr B.P., corresponding to an age of 9930–10,010 cal yr B.P. (7980–8060 cal yr B.C.).

Although post-glacial marine sediments of late Wisconsinan and early Holocene age are common in eastern Canada and the northeastern United States, remnants of older Pleistocene marine sediments are scarce. A fossiliferous marine clay that predates the classical Wisconsinan was recently discovered in the St. Lawrence Valley. A dominantly estuarine environment is inferred from the geochemistry of the shells (δ18O = −7.1) and from benthic foraminifer and ostracode assemblages. The clay indicates a marine invasion (Cartier Sea) shallower and probably shorter than that during the upper late Wisconsinan Champlain Sea episode (12,000–9,500 yr B.P.). The pollen content shows that regional vegetation during the marine episode began as open tundra, then became a Betula and Alnus crispa forest, reached a climatic optimum with Quercus, Corylus, and Abies, and concluded as a Pinus/Picea boreal forest. A corrected infrared stimulated luminescence age of 98,000 ± 9000 yr is compatible with the epimerization ratio of shells. The Cartier Sea resulted from a post-glacial glacio-isostatic marine invasion in the St. Lawrence lowlands. It probably occurred during late stage 5 and is tentatively assigned to the transition of oxygen isotope substages 5b/5a. This marine episode dates to stage 5 of the preceding continental glacier which extended to middle latitudes in NE America.

A chronology of alluvial surfaces on piedmont slopes below the western Ajo Mountains, southern Arizona, has been obtained using cosmogenic 36Cl accumulation and AMS radiocarbon dating. The apparent 36Cl ages of individual boulders range from 520,000 to 13,000 yr, and the 14C ages of organic material in the two young terraces are 2750–2350 and 17,800 cal yr B.P. The sequence of36Cl ages is consistent with the apparent stratigraphic order, but groupings of similar ages for different surfaces appear to result from repeated reworking of older surfaces associated with the deposition of younger ones. The youngest surface gave a distribution of 36Cl ages about 30,000 yr older than the 14C and soil ages; however, this distribution had36Cl ages that overlapped with 36Cl ages from active channels and hillslopes. We attribute the older-than-expected exposure ages of sampled boulders to inheritance of 36Cl while residing near the surface during very slow erosion on the mountain front. Our results show that although cosmogenic nuclide accumulation can help establish chronologies for surfaces in piedmont settings, care must be used in evaluating the effects of complex exposure histories.

The age of the Sheep Creek tephra (SCt), a widespread marker ash bed in eastern Alaska and western Yukon Territory, has been ambiguous and controversial. We have obtained three reliable thermoluminescence age estimates from bracketing loess near Fairbanks that imply a deposition age of about 190,000 ± 20,000 yr for SCt. Three of six loess samples near and closely bracketing the SCt beds near Fairbanks yielded younger age estimates (∼117,000 and ∼135,000 yr), most likely (based on field aspects) because of reworking and contamination by translocated grains. The new, reliable age assignment of 190,000 yr confirms independent stratigraphic evidence of a pre-last interglaciation age, and stratigraphic evidence from one site (Upper Eva Creek) that SCt is older than the more-widespread 140,000-yr-old Old Crow tephra. The SCt age also has implications for regional correlations of glacial and nonglacial deposits. In particular, it supports the stratigraphic and geomorphic interpretation that the Delta Glaciation in the east-central Alaska Range and the Reid Glaciation in western Yukon Territory are older than the last interglaciation (isotope substage 5e).

Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of ∼1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by 230Th/234U ages of the cave samples, 230Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is ∼50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous 14C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported 14C ages.

Mature desert pavements are traditionally regarded as hallmarks of stability, but their stability is dynamic, not static. In a study aimed at documenting this dynamic stability and its role in healing surface disturbances, experiments were performed over a 5-yr period on small cleared patches, or plats, on pavement surfaces in Panamint Valley, California. These experiments show that stones from plat edges begin to resurface the clearing at rates of about 1% per year on 40-cm-square plats and 10% per year on 10-cm-square plats. Stones contributing to the regenerated pavement have smaller average diameters than stones on the surrounding pavement. Cavities 5–10 cm deep, formed in mature pavement by removal of embedded boulders, fill by ravel and slope failure. After five years, cavity depth has been reduced by as much as 60%. Forty-year-old boulder cavities are nearly completely refilled and have been repaved by smaller than average pavement stones. Gaps caused by removal of small stones (2–3 cm) have completely healed in 5 yr. Displacement of surface stones by small animals is a major component of the healing process.

Lake records from northern Eurasia show regionally coherent patterns of changes during the late Quaternary. Lakes peripheral to the Scandinavian ice sheet were lower than those today but lakes in the Mediterranean zone were high at the glacial maximum, reflecting the dominance of glacial anticyclonic conditions in northern Europe and a southward shift of the Westerlies. The influence of the glacial anticyclonic circulation attenuated through the late glacial period, and the Westerlies gradually shifted northward, such that drier conditions south of the ice sheet were confined to a progressively narrower zone and the Mediterranean became drier. The early Holocene shows a gradual shift to conditions wetter than present in central Asia, associated with the expanded Asian monsoon, and in the Mediterranean, in response to local, monsoon-type circulation. There is no evidence of mid-continental aridity in northern Eurasia during the mid-Holocene. In contrast, the circum-Baltic region was drier, reflecting the increased incidence of blocking anticyclones centered on Scandinavia in summer. There is a gradual transition to modern conditions after ca. 5000 yr B.P. Although these broad-scale patterns are interrupted by shorter term fluctuations, the long-term trends in lake behavior show a clear response to changes in insolation and glaciation.

Radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the 14C content of soil organic matter. Measured 14C ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils. Differences in soil C dynamics due to climate or soil depth will result in significantly different 14C signatures of soil organic matter for soils of the same age. As a result, the deviation of the measured 14C age from the true age of soil formation could differ significantly among different soils or soil horizons. Our model calculations also suggest that 14C ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur. Once a soil or a soil horizon has reached a steady state, 14C dating of soil organic matter will provide no useful information regarding the age of the soil. However, for soils in which steady state has not been reached, it is possible to estimate the age of soil formation by modeling the measured 14C contents of soil organic matter. Radiocarbon dating of buried soils could, in general, overestimate the true age of the burial by as much as the steady-state age of the soil or soil horizon.

Fluvial, marine, and mixed fluvial-marine deposition on the coastal plain of Hanalei Bay on the north shore of Kauai, Hawaii, records a middle- to late-Holocene fall of relative sea level. Radiocarbon dating of the regression boundary preserved in the stratigraphy of the coastal plain documents a seaward shift of the shoreline beginning at least 4800–4580 cal yr B.P. and continuing until at least 2160–1940 cal yr B.P. Marine sands stranded in the backshore and coastal plain environment are buried by fluvial floodplain and channel sands, silts, and muds. In places, erosion at the regression contact exposed older marine sands thus increasing the hiatus at the regression disconformity. The shoreline regression is best explained as the result of a fall in relative sea level. The age and elevation of the cored regression boundary at sites that have not been influenced by erosion are consistent with a middle- to late-Holocene highstand of relative sea level as predicted by geophysical models of whole Earth deformation related to deglaciation.

The continental slope in Orphan Basin, northeast of Newfoundland, is underlain by several seaward-thinning debris-flow wedges alternating with acoustically stratified, regionally extensive, mainly hemipelagic sediments. δ18O stratigraphy and volcanic ash layers in a 11.67-m core indicate that the uppermost debris-flow wedge formed during the last of several sea-level lowstands in isotopic stages 2–4. Similarly, seismic reflection correlation of dated levels at DSDP Site 111 with the Orphan Basin succession suggests that two deeper debris-flow wedges were deposited during oxygen isotopic stages 6 and 8. The oldest of the debris-flow deposits in at least three of the wedges formed well into the corresponding glacial cycle, after ice sheets had reached the edge of the continental shelf. Slower deposition by hemipelagic processes and ice rafting formed the acoustically stratified units, including Heinrich layers. The youngest three debris-flow wedges each have volumes of 1300–1650 km3. Approximately two-thirds of this material is attributed to glacial erosion of Mesozoic and Tertiary strata beneath the Northeast Newfoundland Shelf. The remainder is believed to have been derived by glacial erosion of older bedrock that now forms the island of Newfoundland. The observed sediment volumes and the inferred basal and upper ages of the debris-flow wedges imply an average glacial denudation rate of about 0.13 mm/yr for this older bedrock, and an average of about 60 m of glacial bedrock erosion since oxygen isotope stage 22. This denudation rate is similar to estimates from the Barents Sea region off Norway.

Aspartic acid (Asp) racemization occurs at a significantly higher rate than isoleucine epimerization and consequently provides better temporal resolution of Arctic marine deposits (from Alaska, Spitsbergen, and Baffin Island). Heating experiments (at 100°C) on the bivalves Mya and Hiatella show the Asp racemization rate decreases with increasing D/L values, as is typical for biogenic carbonates. Based on these experimental racemization rates and rates determined from racemization of samples radiocarbon dated to ca. 10,000–12,000 yr B.P., activation energies for Mya and Hiatella are estimated to be 30.6 and 30.0 kcal/mol, respectively, for Asp racemization, and 29.0 and 29.5 for isoleucine epimerization. Analysis of a time series of Plio–Pleistocene Hiatella from the north coast of Alaska shows that last-interglacial mollusks can be readily distinguished from modern samples by Asp but not by isoleucine. D/L Asp values indicate a younger age for the Fishcreekian transgression than does isoleucine epimerization. For Spitsbergen, D/L Asp shows a slight age difference (ca. 12,000 yr) between two units of the “episode B” interstadial and suggests that the age of these units may be closer to 65,000 than to 80,000 yr B.P., two possible ages suggested by other evidence. The age of the Loks Land Interstadial on Baffin Island is likely to be greater than that indicated by radiocarbon ages. Within deposits from each region, D/L Asp values are less variable among individual shells than isoleucine epimerization values. This may indicate better reliability of Asp for geochronology.

Pleistocene glacial-estuarine sediment deposited in an intertidal environment of northeastern Bristol Bay, southwestern Alaska, was dated using a variety of approaches, including infrared stimulated and thermoluminescence (IRSL and TL) techniques. Analysis of modern and 14C-dated Holocene tide-flat mud demonstrates that the bulk of sediment in this environment is reset by solar radiation, thereby lending confidence to ages obtained from similar Pleistocene deposits by luminescence techniques. IRSL seems to be especially well suited for dating, with resolution on time scales of <10,000 yr. The ages of tide-flat mud of the Nushagak Formation, derived from the Ahklun Mountains to the northwest of Bristol Bay, and of Halfmoon Bay drift, derived from the Alaska Peninsula to the southeast, suggest contemporaneous glacial-estuarine deposition related to independent glacial source areas about 75,000–80,000 yr ago. This age is consistent with other geochronological data that indicate a pre-late-Wisconsin and post-substage-5e age, including nonfinite 14C ages, a lack of interglacial indicators, and Old Crow tephra (∼140,000 yr) atop the drift, normal paleomagnetic inclinations, and amino acid (isoleucine) epimerization ratios (aIle/Ile). AIle/Ile ratios in Portlandia arctica(0.052 ± 0.003) from a marine-lag horizon at South Naknek beach, which separates Halfmoon Bay drift above from older glacial-estuarine drift below, are only slightly higher than in Mya truncata(0.041 ± 0.007) from last-interglacial Pelukian deposits at Nome. As laboratory heating experiments show that the two genera epimerize at similar rates, these data imply correlation of the marine lag at South Naknek beach with Pelukian deposits. Hence, glaciers on the Alaska Peninsula experienced major pre-late-Wisconsin advances both before and after the last interglaciation. Shells reworked into Halfmoon Bay drift yield aIle/Ile ratios of 0.028 ± 0.005 for Portlandiaat Second Point and 0.027 ± 0.001 for Hiatella arcticaat Etolin Point. Together with assumptions about the postdepositional temperature history, these ratios indicate that the shells are at least 55,000 yr, and probably closer to ∼90,000 yr, although the uncertainty in this age estimate is broad. The amino acid and luminescence data converge on an age between about 75,000, and 90,000 yr, late during oxygen-isotope stage 5, for a major ice advance far beyond late-Wisconsin limits.

Palynological, paleopedological, and glacial geomorphological evidence from the area of the high plain of Bogotá in the Colombian Eastern Cordillera indicates a significant climatic warming around 18,000 14C yr B.P. Comparison of dated vegetation changes, pedogenic episodes, and glacier fluctuations provides the basis for defining the so-called “La Laguna Interstadial” that lasted from 19,500 to 17,000 yr B.P. During this interstadial period, mean annual temperatures in the tropical Andes were up to 4°C higher than during the preceding and following stadial periods, when full-glacial conditions prevailed and temperatures were up to 8°C colder than at present.

The vegetation and climate of the Pampa grassland, Argentina, during the late Quaternary are reconstructed from pollen recovered from dated stratigraphic sections from arroyo walls and from archaelogical excavations. Prior to 10,500 yr B.P., herbaceous psammophytic steppe existed in the central part of the Pampa grassland while xerophytic woodland associated with psammophytic and halophytic steppe occurred in the southwestern part of the Pampa. These types of vegetation and the continental conditions that prevailed in the area of the present-day coast (38°S), indicate subhumid-dry climate and annual precipitation 100 mm lower than present. A subsequent change toward a vegetation characteristic of ponds, swamps, and foodplains, or toward environments with locally more effective moisture, occurred ca. 10,500 yr B.P. suggesting annual precipitation close to modern levels or a higher availability of water in the central part of the Pampa grassland, this type of vegetation existed until 8000 yr B.P., when it was replaced by grassland communities that lasted until 7000 yr B.P. In the southwestern part of the Pampa grassland, this vegetation developed before 7000 yr B.P. and persisted until ca. 5000 yr B.P. Sea level higher than the present ca. 6200 yr B.P. is consistent with sharp modification of the vegetation and development of local halophytic communities dominant at 38°S. A return to subhumid-dry conditions occurred after 5000 yr B.P. The late Holocene vegetation is characterized by pollen assemblages similar to the psammophytic and halophytic communities of the Southern pampa grassland, associated with communities with more edaphic conditions. At the same time, at 38°S a sea level regression is suggested by the dominance of fresh-water pollen assemblages and micropaleontological remains. The trend toward humid, temperate conditions ca. 1000 yr B.P. suggested by vertebrate remains characteristic of temperate and humid conditions, as well as a very short but dry episode during the 18th century suggested by the geology, are not clearly evidenced in the pollen sequences. Vegetational and climatic changes are explained by the latitudinal shifts and changes in intensity of the southern atmospheric circulation and changes in sea level.

Analyses of pollen, plant macrofossils, macroscopic charcoal, mollusks, magnetic susceptibility, and geochemical content of a sediment core from Farewell Lake yield a 11,000-yr record of terrestrial and aquatic ecosystem changes in the northwestern foothills of the Alaska Range. Between 11,000 and 8500 yr B.P., the regional landscape was dominated by a Betula shrub tundra, in which Populus-Salix communities were common. Abundant charcoal in sediments indicates that fires were common in the lake catchment during this period, and high mineral accumulation rates, allogenic elemental content, and magnetic susceptibility suggest intense soil erosion. In addition, mollusks, pollen and macrofossils of aquatic macrophytes, and biogenic silica provide evidence that the lake was substantially shallower and more productive 11,000–8500 yr B.P. than later. Low lake level and high aquatic productivity might have been caused by warm and dry summers associated with early postglacial insolation maximum in northern high latitudes. About 8000 yr B.P., Picea glauca arrived within the lake catchment, forming a forest tundra association until ca. 6000 yr B.P. Alnus shrub thickets established in the region ca. 6500 yr B.P., and Betula papyrifera arrived ca. 6000 yr B.P. Closed P. glauca forests developed ca. 6000 yr B.P. Picea mariana became important subsequently and replaced P. glauca as the dominant tree species in the region ca. 4000 yr B.P. An increase in authigenic Fe/Mn ratios suggests that the development of waterlogged soils accompanied this vegetation change. Fires increased in importance at this time and might have accelerated soil erosion. The establishment of P. mariana forests probably reflected complex responses of forest ecosystems to the onset of cooler and wetter climate conditions during the late Holocene.