In situ-produced cosmogenic helium (3Hec) provides a new tool for constraining histories of Quaternary geomorphic surfaces. Before general application of the technique, however, the systematics and production rates of 3Hec must be well understood. In a companion study, 3He and 10Be data from sandstone and granite boulders in the Dry Valleys region of Antarctica have been used to constrain the ages of an important moraine sequence formed by the Taylor Glacier. Data from these deposits also provide information about the systematics of 3He in quartz that has important implications for geochronology based on 3Hec. In contrast to previous results from olivine and clinopyroxene, crushing quartz in vacuo releases helium with high 3He/4He ratios (up to 148 × Ra, where Ra is the atmospheric 3He/4He ratio), indicating that crushing cannot be used to determine the isotopic composition of trapped (i.e., noncosmogenic) helium in quartz. Analysis of 3He in different size fractions of the same samples indicates significant 3 He loss not predicted by existing 3He diffusion data for quartz. The origin of the discrepancy is not clear, but loss from these samples is not as significant as suggested by the limited data of previous studies.

A microparticle concentration peak in a GISP2 ice core contains volcanic glass shards of rhyolitic composition that correspond in age to the 1479-1480 A.D. Mt. St. Helens Wn eruption. These glass shards are compositionally similar to the Wn tephra and constitute 83% of the total particle population. The shards are very coarse-grained (up to 40 μm diameter), suggesting rapid transport from their source to Greenland. A major sulfate peak in the ice occurs approximately 4 months after deposition of the glass shards. This difference in depositional timing suggests primarily tropospheric transport of the ash and stratospheric transport of the sulfate aerosol. Large-scale climatic perturbations following this eruption were evidently negligible. Glaciochemical seasonal indicators suggest a late-fall to early-winter 1479 A.D. eruption.

The progressive development of unusual rock-weathering features and soils and minimum-limiting radiocarbon dates provide a basis for subdividing four groups of late Pleistocene moraines on the west side of the Cordillera Blanca, northern Peru (9°20′1°000′S, 77°10′-77°30′W). Boulders on the youngest late Pleistocene moraines have 10 to 14-cm-tall weathering posts; soils on these moraines yield mean profile development index (PDI) values of 0.05 ± 0.04 (±1σ). These moraines date between ca. 13,500 and 9700 ± 500 yr B.P., older than previously postulated. The next older moraines have boulders with weathering-post heights between 20 and 25 cm and soils with PDI values of 0.08 ± 0.07, and were deposited prior to 13,280 ± 190 yr B.P., probably during the last glacial maximum (marine isotope stage 2). Moraines from an older glaciation have boulders with weathering posts between 39 and 50 cm high, soils that yield PDI values of 0.21 ± 0.07, and are older than 19,700 ± 340 yr B.P. Boulders on moraines from a still older glaciation have lost ca. 50% of their above-ground volume, and have weathering posts between 62 and 70 cm high. PDI values for soils on these moraines are 0.32 ± 0.06. Linear and logarithmic models of weathering-post and soil development with time are used to estimate minimum and maximum ages for the two oldest moraine groups. Linear models suggest that the second oldest moraines are between ca. 20,500 and 46,500 yr B.P., and that the oldest moraines are between ca. 29,000 and 72,000 yr B.P. In contrast, logarithmic models suggest ages of greater than ca. 75,500 yr B.P. and greater than ca. 500,000 yr B.P., respectively.

In situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits. In situ3He and 10Be in quartz sandstone boulders from Arena Valley, southern Victoria Land, Antarctica, provide chronological constraints for a sequence of moraines ("Taylor II-IVb" moraines) related to expansions of Taylor Glacier and the East Antarctic Ice Sheet. Mean 3He ages are 113,000 ± 45,000 yr, 208,000 ± 67,000 yr, 335,000 ± 187,000 yr, and 1.2 ± 0.2 myr, for Taylor II,III,IVa, and IVb moraines, respectively (mean ± 1σ). Corresponding mean 10Be ages for Taylor II and IVb moraines are 117,000 ± 51,000 yr and 2.1 ± 0.1 myr. For the older deposits the 3He ages are probably lower limits due to diffusive loss. Although the exposure ages appear consistent with the few previous age estimates, particularly with an isotope stage 5 age for Taylor II, each moraine exhibits a broad age distribution. The distribution probably results from a variety of factors, which may include prior exposure to cosmic rays, 3He loss, erosion, postdepositional boulder movement, and radiogenic production of 3He. Nonetheless, the exposure ages provide direct chronological constraints for the moraine sequence, and suggest a maximum thickening of Taylor Glacier relative to the present ice surface of ∼500 m since the late Pliocene-early Pleistocene.

The Little Ice Age was the most extensive Neoglacial glacier advance in the Canadian Rockies. Evidence of earlier, less-extensive Neoglacial glacier advances is based on wood recovered from several glacier forefields. Wood flushed out of Athabasca Glacier (7550-8230 yr B.P., three dates) and Dome Glacier (6120-6380 yr B.P., two dates) indicates that forests occurred upvalley of present glacier termini during the Hypsithermal. Detrital logs from Peyto (14 dates), Saskatchewan (3 dates), Robson (3 dates), and Yoho (1 date) Glaciers, plus in situ slumps at Peyto and Robson Glaciers, have yielded 14 C dates between 2490 and 3300 yr B.P. (12 dates between 2800 and 2990). This wood is derived from sources at or upvalley from present glacier termini and represents forests overridden by glaciers between ca. 3100 and 2500 yr B.P. (Treeline was higher than present immediately prior to this advance.) This advance, which did not extend beyond the Little Ice Age maximum position, is designated the Peyto Advance and correlated with the Tiedemann Advance in western British Columbia. Earliest Little Ice Age advances at Peyto and Robson Glaciers are dated ca. 800-600 yr B.P. at positions ca. 500 m upvalley from Little Ice Age limits.

Optical dating of late Quaternary quartz dune sands from the Clear Creek portion of Ferris dune field, Wyoming, demonstrates the considerable potential of the technique as a chronostratigraphic tool. A sequence of radiocarbon-dated Holocene interdune strata permit optical dating of the intercalated dune sand to be tested; the concordance is good. The optical dates for the aeolian deposits not datable by radiocarbon suggest that aeolian sedimentation at Clear Creek peaked during two relatively short phases at ca. 8500 and 4000 yr B.P. The dates indicate that aeolian accumulation maxima (at least in the Clear Creek area) may not be synchronous with previously defined phases of marked aridity.

Moraines in the canyons of Mono Basin are separable into relative-age groups on the basis of clast-sound velocities in exposed boulders, moraine morphology, and weathering features on boulder surfaces. Tioga, Tenaya, Tahoe, and Mono Basin moraine deposits each have distinct weathering characteristics and therefore constitute different relative-age groups. The Tioga glacial episode at June Lake may postdate ∼25,200(?)-yr-old basaltic lavas, and the Tenaya episode may have occurred ∼30,700 yr ago. A comparison of the glacial and lacustrine records of Mono Basin over the past 40,000 yr, based on new interpretations of radiometric ages, is consistent with the hypothesis that maximum glacial and maximum pluvial periods were not necessarily synchronous.

Accelerator mass spectrometer radiocarbon ages of the Roxana Silt (loess) along the Upper Mississippi Valley of Wisconsin and Minnesota indicate that loess sedimentation of the Roxana Silt occurred between about 55,000 and 27,000 14 C yr B.P. However, due to local environmental controls, the basal age at any given site may range from 55,000 to 35,000 14C yr B.P. The radiocarbon ages presented here are in agreement with previous radiocarbon ages for the Roxana Silt in its type area of west-central Illinois, but indicate that long-term sedimentation rates along the bluffline of the Upper Mississippi Valley were very slow (4-8 cm/1000 yr) compared to long-term sedimentation rates along the bluffline of the type area (40-70 cm/1000 yr). Comparison of radiocarbon ages for midcontinent middle Wisconsinan loess deposits indicates that sedimentation along the Mississippi River valley may have preceded loess sedimentation along the Missouri River valley by as much as 20,000 yr or that basal ages for middle Wisconsinan loess along the Missouri Valley are erroneously young. The bracketing ages for the Upper Mississippi Valley Roxana Silt indicate that the Mississippi River valley was receiving outwash sedimentation between 55,000 and 27,000 14C yr B.P.

A review of published stratigraphic records of pollen, sediment grain size, diatoms, and organic matter composition from Lake Biwa, Japan, identifies four pre-Holocene episodes of milder climate, increased surface runoff, and enhanced aquatic productivity, indicating intervals of warmer and wetter conditions which are interpreted as being interglacial. Correlation of these episodes to times of marine interglacial periods revises the age scale of the Lake Biwa sediment sequence which has been based on fission-track dating. The revised chronostratigraphic scale proposes an age of ca. 430,000 yr B.P. for the base of the 250-m-thick T Bed instead of the former age of ca. 700,000 yr B.P.

Two processes contribute to inaccurate 14C age estimates of carbonates precipitated within the Lahontan basin, NevadaCalifornia: low initial 14C/C ratios in lake water (reservoir effect) and addition of modern carbon to calcium carbonate after its precipitation. The mast reliable set of 14C ages on carbonates from elevations > 1310 m in the Pyramid and Walker Lake subbasins indicate that lakes in all seven Lahontan subbasins coalesced ∼14,200 14C yr B.P. forming Lake Lahontan. Lake Lahontan achieved its 1330-m highstand elevation by ∼13,800 14 C yr B.P. and receded to 1310 m by ∼13,700 14C yr B.P. Calculations, based on measured carbonate-accumulation rates, of the amount of time Lake Lahontan exceeded 1310 and 1330 m (500 and 50 yr) are consistent with this chronology. The timing of the Lake Lahontan highstand is of interest because of the linkage of highstand climates with proximity to the polar jet stream. The brevity of the Lahontan highstand is interpreted to indicate that the core of the southern branch of the polar jet stream remained only briefly over the Lahontan basin.

Geomorphic and sedimentologic evidence in the Grand Valley, which drained the retreating Saginaw Lobe of the Laurentide Ice Sheet and later acted as a spillway between lakes in the Huron and Erie basins and in the Michigan basin, suggests that at least one drainage event from glacial Lake Saginaw to glacial Lake Chicago was a catastrophic outburst that deeply incised the valley. Analysis of shoreline and outlet geomorphology at the Chicago outlet supports J H Bretz's hypothesis of episodic incision and lake-level change. Shoreline features of each lake level converge to separate outlet sills that decrease in elevation from the oldest to youngest lake phases. This evidence, coupled with the presence of boulder lags and other features consistent with outburst origin, suggests that the outlets were deepened by catastrophic outbursts at least twice. The first incision event is correlated with a linked series of floods that progressed from Huron and Erie basin lakes to glacial Lake Saginaw to glacial Lake Chicago and then to the Mississippi. The second downcutting event occurred after the Two Rivers Advance of the Lake Michigan Lobe. Outbursts from the eastern outlets of glacial Lake Agassiz to glacial Lake Algonquin are a possible cause for this period of downcutting at the Chicago outlets.

Streamflow since 1560 A.D. for four rivers within the Sacramento River Basin, California, has been reconstructed dendroclimatically. Both the highest and the lowest reconstructed streamflows occurred during the historical period, with high flows from 1854 to 1916 and low flows from 1917 to 1950. Prolonged (decade-scale) excursions from the mean have been the norm throughout the reconstructed period. The periods of high and low streamflow in the Sacramento Basin are generally synchronous with wet and dry periods reconstructed by dendroclimatic studies in the western United States. The record indicates a number of asynchronous droughts or wet years. The strongest contrasts are developed between northern (western Washington and Oregon or the Columbia Basin) and southern (the Sacramento Basin or central California) climate regions. These asynchronous events may be due to variation in the latitude of the subtropical high and in the latitudinal position of winter storms coming off the Pacific. No association was found with El Niño-Southern Oscillation events.

Although the Tibetan Plateau greatly influences the atmospheric circulation of the Nortbern Hemisphere, few continuous paleoclimatic records are available from the plateau. A 13,000-yr pollen and diatom record from the Sumxi-Longmu Co basin in western Tibet gives information on major changes both in regional vegetation and in local hydrology. After the basin first filled ca. 13,000 yr B.P., a dry spell occurred about 10,500 yr B.P. within the interval spanned by the European Younger Dryas chronozone. A major environmental change occurred suddenly at ≈10,000 yr B.P., with the establishment of wet conditions, and was followed by a long-term trend toward maximum aridity, which lasted approximately 6000 yr. Short-term oscillations are superimposed on this general climatic change with a major reversal event about 8000 yr B.P. and a second wet pulse leading to a maximum lake volume ca. 7500-6000 yr B.P. Maximum aridity occurred 4300 yr B.P. The major environmental fluctuations recorded at Sumxi-Longmu Co appear in phase with climatic changes recognized in north tropical Africa, suggesting that the 8000 to 7000-yr-B.P. event was caused by an abrupt disequilibrium in the climatic system, as was the Younger Dryas and possibly the 4300-yr-B.P. event.

Taking China as a whole, lake expansion occurred 500 B.C. (?)-0 A.D., 650-950 A.D., and 1250-1650 A.D. over the past 3000 yr. The expansions were mainly due to climatic fluctuations and correspond to frequent flood reports. Three regional characteristics of lake development have been recognized. (1) Fluctuations of near-coastal lakes, and those in other regions as well, were out of phase during 500 B.C.-500 A.D., which possibly resulted from out-of-phase fluctuations of climate and sea level on 10- to 100-yr time scale. (2) As the lower Yellow River channel migrated southward, the lakes in the south of the North China Plain formed and reached their largest sizes much later than those in the north. (3) For the lakes in the middle and lower Yangtze River valley, the higher a lake basin lies above a neighboring river and the further the lake is from the river's mouth, in general, the younger the lake is. Through land reclamation, the ancient Chinese have changed most of the lakes greatly. However, human activities were also constrained in certain ways by the natural changes of the lakes. Reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding.

The Baxie loess section, just east of the Tibetan Plateau, contains evidence showing that the Asian monsoon climate experienced an abrupt reversal near the end of the last glacial age. Rapid deposition of dust under cool, dry full-glacial conditions gave way to an interval of soil development and reduced dust influx attributed to a strengthening of the warm, moist summer monsoon. A subsequent abrupt increase in dust deposition, a response to a weakening of the summer monsoon, was later followed by renewed soil formation as summer monsoon circulation again intensified during the early Holocene. By one interpretation, the thin upper loess is a manifestation of the European Younger Dryas oscillation; however, in this case the available 14C ages require either that (1) onset of loess deposition lagged the beginning of the Younger Dryas event in Europe by as much as 2000 calibrated 14C years or (2) all the 14C ages are too young, possibly due to contamination. Alternatively, the late-glacial paleosol, the top of which is synchronous with the abrupt end of the late-glacial δ18O anomaly in the Dye 3 Greenland ice core, records the Younger Dryas event. Such an interpretation is consistent with general circulation model simulations of Younger Dryas climate that show strong seasonality and a strengthened summer monsoon, and with marine cores from the western Pacific Ocean that contain evidence of pronounced cooling of surface waters during Younger Dryas time.

The Farmdale Soil occurs below late Wisconsinan loess throughout the U.S. Midwest. At the La Sena site in the central Great Plains, humates in the Farmdale Interstadial Soil have a corrected age of 21,000 yr B.P. Humates in a buried Bt horizon and a bulk sample of overlying loess 2.5 m above the Farmdale Interstadial Soil have ages of 17,000 and 19,000 yr B.P., respectively. In the Republican River Valley Picea (spruce) charcoal is common in the lower meter of Peoria loess. Near Bloomington, Nebraska, humates from burned organic matter only 60 cm above the base of Peoria loess have a corrected age of ca. 19,000 yr B.P.

Sediment cores from three lakes in the Upper Kolyma region, northeast Russia, provide the first well-dated continuous record of late Quaternary vegetation change from far southwestern Beringia. The oldest pollen zone, tentatively assigned to the Karginsk (mid-Wisconsinan) Interstade, indicates an Artemisia shrub tundra with Pinus pumila, Betula, and Alnus at mid- to low elevations. With the onset of the Sartan (late Wisconsinan) Stade, Pinus disappeared, probably indicating severely cold, dry winters and cool summers. As conditions deteriorated further, an Artemisia -Gramineae tundra developed. Selaginella rupestris and minor herb taxa indicate the presence of poor soils and disturbed ground. This herb tundra was replaced by a short-lived (< 1000 yr) Betula-Alnus shrub tundra followed by the rapid establishment of a Larix dahurica forest with a Betula exilis-ericales-lichen understory. Populus suaveolens and Chosenia may have formed limited hardwood gallery forests at this time. Modern vegetation associations probably developed during the early Holocene with the arrival of Pinus pumila ca. 9000 yr B.P. This shrub became important in the forest understory and, with B. exilis, formed a belt of shrub tundra beyond altitudinal treeline. Comparison of the Upper Kolyma and Alaskan pollen records indicates that important differences in vegetation types and timing of vegetation change occurred across Beringia during the late Quaternary.

Detailed mapping and provisional numerical age determinations of glacial deposits in the South Chiatovich Creek Basin of the White Mountains provide an opportunity to evaluate the ability of conventional soil parameters to discriminate first- and second-order glacial events. Sampling and analytical procedures were designed to minimize variation in climate and lithology. When lithology and climate are similar among sites, age trends are more pronounced in both field and chemical soil properties. Profile development indices (PDIs), adjusted by removing melanization and pH, systematically increase with greater soil age, and discriminate first-order, but not second-order, glacial events. The best-fit curve for adjusted PDI data assumes an exponential form and suggests that the rate of soil formation in this region decreases over time, similar to the rate of weathering-rind development. Variation in eolian influx and surface erosion, which are dominant processes affecting soils of the basin, cause major uncertainties in establishing soil age and, hence, soil-development rates. Even on the youngest glacial deposits, soil age is probably significantly less than deposit age due to these geomorphic processes. Soil and weathering parameters imply that these field techniques can be inexpensively employed to define relative chronologies and to assess surface degradation and its impact on surface exposure ages. Results from this study indicate that site-selection strategy for establishing glacial chronologies should be reevaluated. Working with stable residual bedrock surfaces and associated low-relief outwash fans and terraces may prove more productive than focusing on relatively unstable moraine surfaces in tectonically active mountain systems.

A 34.5 m borehole, which was drilled near the Dead Sea coast (altitude -394 m) in the southern part of the fan delta of Wadi Zeelim, reveals the geological history of that area from the latest Pleistocene to present. The depositional time frame is based on six 14C dates and two U-Th dates. An erosional (or nondepositional) period is implied by the hiatus between 21,100 yr B.P. (U-Th age, depth 33 m) and 11,315 yr B.P. (14C age, depth 32 m). A subsequent arid phase is recorded by a 6.5-m-thick layer of halite; based on 14C dates this phase relates to the abrupt Younger Dryas cold period reported in temperate to polar regions. The fragility of the environment in this region is indicated by the fact that the region experienced such a severe, short aridification phase (less than 1000 yr), evidence of which is found widely in the desert fringes of the Middle East and North Africa. The aragonite found in most of the Holocene section indicates that the well site was covered by the lake for most of the Holocene. Exceptions are the intervals at 0-3 and 10-14 m depths which represent low stands of the lake.

Pollen evidence from Lake Shayema, Mianning County, was obtained to examine postglacial vegetation and climatic change in southwestern Sichuan, China. The sclerophyllous character of the region's warm temperate vegetation today is a reflection of extreme drought in spring before the onset of the Asian monsoons. The pollen record displays several changes in the vegetation over the last 11,000 yr. From 11,000 to 9100 yr B.P., cold-tolerant species, such as Abies , Betula, and deciduous oaks, dominated the vegetation. Between 9100 and 7800 yr B.P., the abundance of deciduous oaks decreased and evergreen oaks increased, as did Tsuga and mesic deciduous species. This change suggests a warming climate with increased precipitation. From 7800 to 4000 yr B.P., sclerophyllous species increased at the expense of mesic deciduous species, an indication that precipitation was becoming more seasonal. Except for increased disturbance starting ca. 1000 yr B.P., the predominance of sclerophyllous vegetation continued until today. The pollen results are compatible with proposed global circulation hypotheses of a strengthened monsoon system during the early to mid Holocene.