bluuebunny/crossref_metadata_2025_split huggingface dataset:
Late Weichselian Ice Sheet of Northern Eurasia
Authors: Grosswald M. G. 
Published: July 2006

AbstractA considerable portion of Northern Eurasia, and particularly its continental shelf, was glaciated by inland ice during late Weichsel time. This was first inferred from such evidence as glacial striae, submarine troughs, sea-bed diamictons, boulder trains on adjacent land, and patterns of glacioisostatic crustal movements. Subsequently, the inference was confirmed by data on the occurrence and geographic position of late Weichselian end moraines and proglacial lacustrine deposits.The south-facing outer moraines in the northeastern Russian Plain, northern West Siberia, and on Taimyr Peninsula are underlain by sediments containing wood and peat, the radiocarbon dating of which yielded ages of 22,000 to 45,000 yr B.P. The youngest late-glacial moraines are of Holocene age: the double Markhida moraine in the lower Pechora River basin, presumably associated with “degradational” surges of the Barents Ice Dome, is underlain by sediments with wood and peat dated at 9000 to 9900 yr B.P.: this suggests that deglaciation of the Arctic continental shelf of Eurasia was not completed until after 9000 yr B.P.The reconstructed ice-front lines lead to the conclusion that the late Weichselian ice sheet of Northern Eurasia (proposed name: theEurasian Ice Sheet) extended without interruptions from southwestern Ireland to the northeastern end of Taimyr Peninsula, a distance of 6000 km: it covered an area of 8,370,000 km2, half of which lay on the present-day continental shelves and a quarter on lowlands that were depressed isostatically below sea level. Hence, the ice sheet was predominantly marine-based.A contour map of the ice sheet based both on the dependence of the heights of ice domes upon their radii and on factual data concerning the impact of bedrock topography upon ice relief has been constructed. The major features of the ice sheet were the British, Scandinavian, Barents, and Kara Ice Domes that had altitudes of 1.9 to 3.3 km and were separated from one another by ice saddles about 1.5 km high. At the late Weichselian glacial maximum, all the main ice-dispersion centers were on continental shelves and coastal lowlands, whereas mountain centers, such as the Polar Urals and Byrranga Range, played only a local role.The portions of the ice sheet that were grounded on continental shelves some 700 to 900 m below sea level were inherently unstable and could exist only in conjunction with confined and pinned floating ice shelves that covered the Arctic Ocean and the Greenland and Norwegian Seas.The Eurasian Ice Sheet impounded the Severnaya Dvina, Mezen, Pechora, Ob, Irtysh, and Yneisei Rivers, and caused the formation of ice-dammed lakes on the northern Russian Plain and in West Siberia. Until about 13,500 yr B.P. the proglacial system of lakes and spillways had a radial pattern; it included large West Siberian lakes, the Caspian and Black Seas, and ended in the Mediterranian Sea. Later, the system became marginal and discharged proglacial water mainly into the Norwegian Sea.
Canyon Creek: A Late Pleistocene Vertebrate Locality in Interior Alaska
Authors: Weber Florence R. , Hamilton Thomas D. , Hopkins David M. , Repenning Charles A. , Haas Herbert 
Published: July 2006

AbstractThe Canyon Creek vertebrate-fossil locality is an extensive road cut near Fairbanks that exposes sediments that range in age from early Wisconsin to late Holocene. Tanana River gravel at the base of the section evidently formed during the Delta Glaciation of the north-central Alaska Range. Younger layers and lenses of fluvial sand are interbedded with arkosic gravel from Canyon Creek that contains tephra as well as fossil bones of an interstadial fauna about 40,000 years old. Solifluction deposits containing ventifacts, wedge casts, and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of Donnelly Glaciation about 25,000–17,000 years ago. Overlying sheets of eolian sand are separated by a 9500-year-old paleosol that may correlate with a phase of early Holocene spruce expansion through central Alaska. The Pleistocene fauna from Canyon Creek consists of rodents (indicated by burrows), Mammuthus primigenius (woolly mammoth), Equus lambei (Yukon wild ass), Camelops hesternus (western camel), Bison sp. cf. B. crassicornis (large-horned bison), Ovis sp. cf.O. dalli (mountain sheep), Canis sp. cf. C. lupus (wolf), Lepus sp. cf. L. othus or L. arcticus (tundra hare), and Rangifer sp. (caribou). This assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites. Camelops evidently was present in eastern Beringia during the middle Wisconsin interstadial interval but may have disappeared during the following glacial episode. The stratigraphic section at Canyon Creek appears to demonstrate that the Delta Glaciation of the north-central Alaska Range is at least in part of early Wisconsin age and was separated from the succeeding Donnelly Glaciation by an interstadial rather than interglacial episode.
Periglacial Wedges and the Late Pleistocene Environment of Wyoming's Intermontane Basins
Authors: Mears Brainerd 
Published: July 2006

AbstractNonsorted polygons in the uppermost 2 to 3 m beneath Pleistocene surfaces indicate permafrost at 1340 m and higher elevations in the intermontane and piedmont plains of Wyoming during the Wisconsin, and perhaps earlier, glacial maxima. The polygons, as much as 10 m in diameter, are delineated by wedges that vary in depths, range from narrow to moderately flared forms, and deform host materials. The wedges have silty fine-to-medium sand matrices (largely eolian) with pebbles or clasts from hosts of gravel or bedrock. Some wedges may reflect seasonal cracking in a periglacial active zone, but most are either permafrost sand-wedge relics or, less commonly, ice-wedge casts. Alternative explanations are rejected largely because similar features are apparently lacking in the lower and warmer plains from eastern Colorado southward. The wedges suggest an arid, windy, periglacial environment whose mean-annual temperatures are conservatively estimated as some 10° to 13°C colder than those at present. Although late Wisconsin-early Holocene floral and faunal evidence indicates lowered montane biotic zones, the eolian and periglacial features indicate a lack of extensive forest cover on the basin floors. In conjunction with vertebrate-fossil associations of grazing and tundra animals, the wedges may provide a parallel line of evidence for a former periglacial steppe, or “steppe-tundra”, in the Wyoming basins.
Modeling of Pleistocene European Ice Sheets: Some Experiments with Simple Mass-Balance Parameterizations
Authors: Oerlemans J. 
Published: July 2006

AbstractA vertically integrated ice-flow model suitable for use in climate studies is formulated. Large continental ice sheets may be characterized by two fundamental quantities: the height-to-width ratio, and the steepness of the edge. So it is natural to develop a model containing two parameters that can be chosen to give the right values of those characteristic quantities. The result is a model that is close to M. A. W. Mahaffy's (Journal of Geophysical Research,81, 1059–1066 (1976)). The model is used to study glaciation in Europe. Dropping the level of zero mass balance creates small stable ice caps in the Alps and the Scandinavian mountains. If the drop exceeds 600 m (with respect to present-day conditions), the feedback between ice-sheet height and mass balance becomes dominating and the Fennoscandian Ice Sheet keeps growing. It does not reach an equilibrium state within 60,000 yr. An experiment simulating rapid onset of a glacial cycle shows that the growth of ice volume in Europe is smaller than that in northern America (J. T. Andrews and M. A. W. Mahaffy,Quaternary Research,6, 167–183 (1976)). After 10,000 yr, the volume of the Fennoscandian Ice Sheet (2 × 1015m3) is about half the volume of the Laurentide Ice Sheet. This leaves the “observed” sea-level lowering in the period 125,000–115,000 yr B.P. (estimates center around 50 m) unexplained.
Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: A Comparison of Methods
Authors: Meierding Thomas C. 
Published: July 2006

AbstractSix methods for approximating late Pleistocene (Pinedale) equilibrium-line altitudes (ELAs) are compared for rapidity of data collection and error (RMSE) from first-order trend surfaces, using the Colorado Front Range. Trend surfaces computed from rapidly applied techniques, such as glaciation threshold, median altitude of small reconstructed glaciers, and altitude of lowest cirque floors have relatively high RMSEs (97–186 m) because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability. Surfaces computed for accumulation-area ratios (AARs) and toe-to-headwall altitude ratios (THARs) of large reconstructed glaciers show that an AAR of 0.65 and a THAR of 0.40 have the lowest RMSEs (about 80 m) and provide the same mean ELA estimate (about 3160 m) as that of the more subjectively derived maximum altitudes of Pinedale lateral moraines (RMSE = 149 m). Second-order trend surfaces demonstrate low ELAs in the latitudinal center of the Front Range, perhaps due to higher winter accumulation there. The mountains do not presently reach the ELA for large glaciers, and small Front Range cirque glaciers are not comparable to small glaciers existing during Pinedale time. Therefore, Pleistocene ELA depression and consequent temperature depression cannot reliably be ascertained from the calculated ELA surfaces.
Magnetic Polarity Stratigraphy of the Middle Pleistocene (Ensenadan) Tarija Formation of Southern Bolivia
Authors: MacFadden Bruce J. , Siles Oscar , Zeitler Peter , Johnson Noye M. , Campbell Kenneth E. 
Published: July 2006

AbstractThe Tarija Formation of southern Bolivia, which is well known for its classic vertebrate faunas, is of prime importance in understanding of the chronology of the Ensenadan Land Mammal Age. This formation consists of well-exposed and relatively fossiliferous sections of clays, clayey silts, sands, gravels, and tuffs which were deposited in a predominately fluviatile regime in a Pleistocene structural basin. Four stratigraphic sections, each measuring 110 m or less, were studied to establish a magnetic polarity stratigraphy. Paleomagnetic samples were collected from the finer-grained sediments at 100 sites spaced at stratigraphic intervals of 5 m or less. All paleomagnetic specimens were demagnetized in alternating fields of least 250 oersteds (oe). Some specimens were also thermally demagnetized at 200°C or more. Of the 100 sites, 77 were ultimately used to determine the magnetic polarity zonation. Based on the four sections sampled, the Tarija Formation spans a time interval from about 1 my to about 0.7 my B.P. or perhaps younger. The lower half of the composite section is of reversed polarity punctuated by a short normal event. This sequence probably represents the late Matuyama chron with the Jaramillo subchron. The upper part of the section is of normal polarity and represents early Brunhes time. A tuffaceous unit 43 m above the Brunhes-Matuyama boundary yielded a fission track (zircon) age of 0.7 ± 0.2 by B.P. These data indicate that the classic Tarija fauna is middle Pleistocene Ensendan in age.
Regional Significance of pre-Wisconsinan Till from Nantucket Island, Massachusetts
Authors: Oldale R. N. , Eskenasy D. M. 
Published: July 2006

AbstractA major pre-Wisconsinan glacial event is the only possible source of the lower till on Nantucket Island, Massachusetts. The till occurs near the late Wisconsinan drift border and below fossiliferous marine beds of oxygen-isotope stage 5 (Sangamonian) age. It is considered to be Illinoian in age, but the evidence is tenuous. The till is correlated with the lower till of New England, and its presence supports the view that the New England upper and lower tills represent two glaciations. The pre-Wisconsinan (lower) till in New England may correlate with older tills elsewhere in the northeastern United States and in southeastern Canada that are considered to be early Wisconsinan or older in age.
New Evidence for the Age of the Gubik Formation Alaskan North Slope
Authors: Repenning Charles A. 
Published: July 2006

AbstractAt several Alaskan North Slope localities south of the shore of the Arctic Ocean the Gubik Formation, herein regarded as latest Pliocene and Pleistocene in age, contains a marine unit at its base. Near Ocean Point and near Teshekpuk Lake this basal unit, or the lowest exposed marine unit, of the Gubik contains unusual, relatively warm-water marine mammals. Although these mammals have poorly known fossil histories, consideration of what is known suggests that the basal marine unit near Ocean Point is of latest Pliocene age, between 2.2 and 1.7 my old, and that the marine unit near Teshekpuk Lake is probably late Pleistocene, most likely correlating with the Sangamon Interglaciation and about 120,000 yr old.
Radiocarbon and Cation-Radio Ages for Rock Varnish on Tioga and Tahoe Marainal Boulders of Pine Creek, Eastern Sierra Nevada, California, and their Paleoclimatic Implications
Authors: Dorn Ronald I. , Turrin Brent D. , Jull A. J. Timothy , Linick Timothy W. , Donahue Douglas J. 
Published: July 2006

AbstractAccelerator mass spectrometry 14C analyses of organic matter extracted from rock varnishes on morainal boulders yield limiting minimum ages for three crests of the Tioga glaciation. At Pine Creek in the eastern Sierra Nevada, varnish started to form on boulders of the outermost Tioga moraine before 19,000 yr B.P., and varnish originated on the innermost Tioga moraine before 13,200 yr B.P. Comparisons with lake-level, paleohydrological, paleoecological, colluvial, and rock varnish micromorphological data indicate that central-eastern California and western Nevada experienced a moisture-effective period during the late Pleistocene but after the Tioga maximum, and perhaps as Tioga glaciers receded from the mouth of Pine Creek canyon. Varnishes on Tahoeage morainal boulders at Pine Creek have cation-ratio ages of about 143,000–156,000 yr B.P., suggesting that the Tahoe glaciation should not be correlated with oxygen-isotope stage 4 in the early Wisconsin, but rather with stage 6. Varnishes on morainal boulders of an older glaciation at Pine Creek are dated by cation ratio at about 182,000–187,000 yr B.P.
Climatic Changes in the Qinghai-Xizang (Tibetan) Region of China during the Holocene
Authors: Fu-Bao Wang , Fan C. Y. 
Published: July 2006

AbstractClimatic changes in the Qinghai-Xizang Plateau of China were studied by analyzing the composition of peat and layers of sand and gravel distributed along the southern slopes of Nianqing-Tanggula and Gangdise Mountains, cross sections of deposits near a number of interior lakes in Xizang, past glacial variations on the southern slope of Nianqing-Tanggula Mountain, and landform changes south of the Yaluzangbu River. Such geologic evidence suggests a division of five climatic periods since the beginning of the Holocene: (1) The Wumadung interval, 10,000–7500 yr B.P., slightly cold and dry; (2) Qilongduo interval, 7500-3000 yr B.P., warm and moist; (3) the mid-Neoglacial period, 3000-1500 yr B.P., cold, except between 2500 and 200 yr B.P. when it was warmer; (4) the Dawelong interval, 1500-300 yr B.P., mild; and (5) the Little Ice Age, 300-0 yr B.P., cold. These changes progressed in a similar but not identical pattern as those in the northeastern part of China and in the northern region of Europe.
Pollen Analysis of Hyena Coprolites and Sediments from Equus Cave, Taung, Southern Kalahari (South Africa)
Authors: Scott Louis 
Published: July 2006

AbstractEquus Cave, in Quaternary tufa near Taung in the semiarid woodland of the southern Kalahari, yielded 2.5 m of sediment in which a rich assemblage of bones and coprolites was preserved. The fossils were accumulated mainly by hyenas during the late Pleistocene and Holocene. Pollen from coprolites reflects diet as well as vegetation over relatively large areas visited by hyenas, while pollen from sediments represents more local sources. The pollen sequence derived from coprolites and sediments demonstrates how the vegetation evolved from open grassland with small shrubs and occasional trees during the late Pleistocene, to open savanna with more small shurbs, then, during the last 7500 yr, to modern savanna. Temperatures were not more than 4°C cooler and it was slightly moister than today during the late Pleistocene phase; it became gradually warmer but relatively dry before optimal temperature and moisture conditions developed around 7500 yr B.P. Climatic conditions slightly less favorable for woodland occurred during part of the late Holocene.
Late Quaternary Lacustrine History and Paleoclimatic Significance of Pluvial Lake Cochise, Southeastern Arizona
Authors: Waters Michael R. 
Published: July 2006

AbstractDuring the latest Quaternary, freshwater pluvial lakes intermittently formed in the topographically closed Willcox basin, Arizona. A lacustrine sequence of six separate high stands of Lake Cochise is documented by stratigraphic studies, 19 radiocarbon ages, and supplementary evidence. Two stands of pluvial Lake Cochise, older than 14,000 yr B.P., reached elevations above 1290 m. The prominent 1274-m shoreline of Lake Cochise, which circumscribes the basin, was largely created during a high stand between 13,750 and 13,400 yr B.P. During the Holocene, water filled the Willcox basin three times to an elevation slightly below the crest of the 1274-m shoreline. This occurred once during the early Holocene around or before 8900 yr B.P. and twice during the later part of the middle Holocene. Since the middle Holocene, only shallow ephemeral lakes have occupied the deflated central portion of ancient Lake Cochise, a depression known as the Willcox Playa. The lacustrine sequence of Lake Cochise provides an independent evaluation of late Quaternary paleoclimatic reconstructions for southern Arizona and the American Southwest.
On the Formation of the Tunnel Valleys of the Superior Lobe, Central Minnesota
Authors: Moores Howard D. 
Published: July 2006

AbstractWhen considering the formation of tunnel valleys, most researchers have appealed to basal melting as the primary source of water. Erosion of the tunnel valleys can then be accomplished by steady-state drainage over a long period or by storage and subsequent catastrophic release. However, field relationships in a large system of tunnel valleys formed by the Superior lobe in central Minnesota indicate that another source of water must be considered. The Minnesota tunnel valleys are composed of individual segments 10–20 km long, and the segments commonly terminate in subaerial outwash fans at recessional ice margin positions. Eskers, associated with the tunnel valleys, are also composed of short segments, frequently beginning at moulin kames and terminating at the head of outwash fans. The dominant source of the water responsible for tunnel-valley formation was seasonal meltwater from the glacier surface that reached the bed through moulins and crevasses. The apparent continuity of the valleys resulted from the headward development of the englacial drainage system during ice retreat.
New Magnetostratigraphic Dates of LantianHomo erectus
Authors: Zhisheng An , Kun Ho Chuan 
Published: July 2006

AbstractSkeletal remains ofHomo erectusfound in Pleistocene loess at two sites near Lantian in central China are of greatly different geologic age. The cranium found in the fossil-bearing strata at Gongwangling is about 1.15 myr old whereas the remains found at the Chenjiawo locality in middle Pleistocene loess are about 0.65 myr old. The dating is based on new paleomagnetic polarity determinations and on the lithostratigraphic position of the fossils in the loess-paleosol sequence. Our results confirm that both localities are older than the first occupation of Zhoukoudian. New dates, palaeoenvironmental settings, and morphological features of the hominids from Lantian localities have significant bearing on the understanding of adaptive radiations of the middle and late hominids in Asia.
Freshwater Diatom Influx in Intertropical Atlantic: Relationships with Continental Records from Africa
Authors: Gasse Françoise , Stabell Bjørg , Fourtanier Elizabeth , van Iperen Yolanda 
Published: July 2006

AbstractDiatom assemblages from modern West African rivers and from lacustrine sediments subjected to deflation represent the present-day sources of continental diatoms to the sea. Diatom productivity in large rivers is high, especially for the genus Melosira. Windblown diatoms derive mainly from the central and northern Sahara (summer dust plume) where saline chloride-water assemblages are widespread, or from the southern edge of the Sahara (winter dust plume) where Melosira-rich assemblages of dilute water predominate. Freshwater diatom peaks in Atlantic cores may reflect (1) phases of increased river influx, correlated with humid episodes on the continent or (2) phases of enhanced deflation and wind transport during arid episodes (the single hypothesis for fine sediments from mid-ocean sites). Genus Melosira dominates the freshwater assemblages of many modern and fossil marine samples, whatever the transport agent is. Therefore, it is not an accurate paleoclimate indicator by itself, but associated taxa may provide information on the environmental and geographical origin of the displaced diatoms. This tentative approach shows that freshwater diatoms in Atlantic cores may be a good tool for reconstructing paleoclimates and for establishing continent-ocean correlations if species analyses are made and if the continental distribution of the taxa encountered is considered.
Some Geological Implications of Average Quaternary Glacial Conditions
Authors: Porter Stephen C. 
Published: July 2006

AbstractAnalyses of Quaternary landscape evolution tend to focus on events associated with culminations of glacial and interglacial ages, but for most of Quaternary time, environments were intermediate in character. Average Quaternary glacial conditions, based on assessment of the marine isotope record, approximated those near the stage 2/stage 1 transition and during substages 5b and 5d; isotope stages 3 and 4 and substages 5a-d lie within one standard deviation of the mean value. Under average glacial conditions, ice sheets lying over northern North America and Europe were much more contracted than their full-glacial counterparts, and the distribution of mountain glaciers reflected a snowline depression of some 500 m. Geomorphic processes operating under average Quaternary conditions contributed importantly to landscape evolution. Examples of landscapes that may represent such average conditions include cirques and fluvial deposits of the Pacific Northwest, the fjords and strandflat of western Norway, and atolls of the tropical oceans. By examining the geologic record from the perspective of average conditions, rather than those of climatic extremes, added insight can be gained regarding the evolution of Quaternary landscapes.
Late Quaternary Vegetation and Climate of the Sahel
Authors: Lézine Anne-Marie 
Published: July 2006

AbstractPollen and phytogeographic evidence provides a vegetational history of the Sahel for the period 0–18,000 yr B.P. The zonal vegetation fluctuated latitudinally and its most extreme positions occurred at 18,000 and 8500 yr B.P. The first involved a southward shift of the Sahelian wooded grassland to 10°N under the arid conditions of the last glacial maximum. The second change shows a rapid northward migration of humid vegetation: Guinean elements reach 16°N and Sahelo-Sudanian elements extend to the southern margin of the modern Sahara (21°N) when the Atlantic monsoon flux increased. In the middle Holocene the extensive spread of Sudanian elements into the modern Sahelian zone suggests the appearance of a markedly dry season. The modern Sahelian semiarid conditions appeared abruptly at 2000 yr B.P.