bluuebunny/crossref_metadata_2025_split huggingface dataset:
Vegetation Dynamics During the Last Deglaciation, Southeastern Great Basin, U.S.A.
Authors: Spaulding W. Geoffrey 
Published: July 2006

AbstractVegetation changes in the southeastern Great Basin between 17,500 and 8000 yr B.P. can be reconstructed from plant macrofossil assemblages in ancient packrat (Neotomaspp.) middens. Vegetation instability after ca. 16,000 yr B.P. involved a dispoportionate number of local immigrations (arrivals) relative to apparent local extinctions (departures). At a site in modern woodland, less than 50% of the arrivals were of transients, species that subsequently went locally extinct; but at a low-elevation desert site almost all arrivals before 10,000 yr B.P. were transient species. Thus, by the close of the last glacial age, higher elevations supported vegetation that resembled present woodland, while lower elevations supported desertscrub that bore little resemblance to modern thermophilous desertscrub.Differences in the pace of plant community “modernization” in different elevational zones can be attributed to migrational lag. Woodland species survived the last glacial age at low elevations nearby, and began arriving at sites within current woodland before 11,700 yr B.P. Warm-desert plants were displaced far to the south, and began arriving after 9500 yr B.P. Moreover, their staggered arrival times suggest that low winter temperatures did not inhibit their migration during the early Holocene. These data suggest that biotic factors, particularly variable dispersal distances and consequent migrational lag, did affect deglacial vegetation change.
Quaternary Transgressive/Regressive Cycles in the Gulf of Argos, Greece
Authors: van Andel Tjeerd H. , Zangger Eberhard , Perissoratis Constantine 
Published: July 2006

AbstractBorings in the Argive Plain reveal cycles of marine incursions, each ending with a Mediterranean soil profile and followed by a prograded fluvial and coastal wedge. The sediment prism of the Gulf of Argos shelf, visible in high-resolution seismic reflection profiles, also consists of transgressive and regressive depositional sequences identified by onlap, downlap, and truncation of deposits. At least four major reflectors, recognizable by their high acoustic impedance and erosional features, can be correlated across the shelf. The sediments between each pair of reflectors represent the seaward part of a set of transgressive and regressive marine deposits. They can be matched to the stratigraphic sequence on land where each marine unit is topped by a soil. Corrected for subsidence, the terminations of the onlapping and downlapping units define a local sea-level history; its time scale can be derived from a comparison with the eustatic sea-level history deduced from ocean cores. Thus, marine seismic reflection data can be used for the correlation of Quaternary oceanic and terrestrial chronologies.
Late Pleistocene Temperature Depression and Vegetation Change in Ecuadorian Amazonia
Authors: Bush Mark B. , Colinvaux Paul A. , Wiemann Michael C. , Piperno Dolores R. , Liu Kam-biu 
Published: July 2006

AbstractPaleoecological (pollen, phytolith, and wood) analyses of sediments, radiocarbon dated 33,000 to 26,000 yr B.P., from two sites in Ecuadorian Amazonia provide data that suggest a cooling of ca. 7.5°C below present in equatorial lowlands from 33,000 to 30,000 yr B.P. A period of warning followed in which novel species assemblages, a blend of montane and lowland floral components, persisted for at least 4000 years. These data of forest community change, from sites lying within the postulated glacial rain forest Napo refugium, provide the strongest paleoecological refutation of the refugial hypothesis yet obtained. The large temperature depression at ca. 30,000 yr B.P. allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum (LGM) of the northern hemisphere, freezing temperatures would have been experienced in parts of lowland Amazonia between 25,000 and 18,000 B.P.
Reconstruction of Precipitation in Morocco Since 1100 A.D. Based onCedrus AtlanticaTree-Ring Widths
Authors: Till Claudine , Guiot Joël 
Published: July 2006

AbstractAnnual (October through September) precipitation from 1100 A.D. to modern times is reconstructed for Morocco, usingCedrus atlantica(Endl.) Carrière tree-ring chronologies. Both multiple regression on principal components and the bootstrap method are use to calibrate tree-ring width with precipitation; precipitation variation is reconstructed for three climatically distinct areas: the humid, subhumid, and arid regions of Morocco. A series of successive wet and dry periods is identified for the past 1000 years; the maximum length of the 13 dry periods (during which precipitation was at least 1σ below normal) is 6 years. Twenty-one years are identified during which precipitation fell more than 2σ below normal. We are unable to identify significant correspondence in climatic variation in Morocco, Europe, and the Sahel during this time period.
Middle Pleistocene age of the Nome River glaciation, northwestern Alaska
Authors: Kaufman Darrell S. , Walter Robert C. , Brigham-Grette Julie , Hopkins David M. 
Published: July 2006

AbstractDuring the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion 40Ar/39Ar analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 ± 190,000 yr (±1σ). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous 40Ar enrichments that rendered the mean of conventional K-Ar dates on larger bulk samples of the same flow too old (700,000 ± 570,000 yr). While the 40Ar/39Ar analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional K-Ar date on a basaltic lava flow that underlies the Nome River drift at nearby Lave Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene.
Late quaternary snowline reconstructions for the southern and central Sierra Nevada, California and a reassessment of the “Recess Peak Glaciation”
Authors: Burbank Douglas W. 
Published: July 2006

AbstractIn the Sierra Nevada, the “Recess Peak Glaciation” has been previously defined on the basis of deposits exhibiting relative-age characteristics intermediate between those of the Little Ice Age deposits and those of early Holocene or older moraines. In the absence of reliable chronological control, the Recess Peak deposits were assigned an early Neoglacial age. Although numerous moraines in the central and southern Sierra have been attributed to this interval, regional snowline gradients reconstructed from these deposits lack internal consistency and appear to represent several distinctly different episodes of glacier advance. As a basis for comparison with the Recess Peak data, modern and late Pleistocene regional snowlines were reconstructed using accumulation-area ratios and cirque-floor altitudes. These reconstructions display regionally consistent gradients, rising gradually southward and more steeply eastward. Based on these data, the full-glacial late Pleistocene snowline depression is estimated to have been ≥800 m. Estimates of Recess Peak snowline depression vary widely, ranging from 140 to 500 m, and a reconstructed regional gradient rises northward, in opposition to the late Pleistocene and modern snowlines. Limited radiocarbon dating and the irregular pattern derived from the Recess Peak snowline data suggest that, even in the type area, these deposits resulted from both pre- and post-Hypsithermal glacier advances.
Revised age of deglaciation of Lake Emma based on new radiocarbon and macrofossil analyses
Authors: Elias Scott A. , Carrara Paul E. , Toolin L. J. , Jull A. J. T. 
Published: July 2006

AbstractPrevious radiocarbon ages of detrital moss fragments in basal organic sediments of Lake Emma indicated that extensive deglaciation of the San Juan Mountains occurred prior to 14,900 yr B.P. (Carrara et al., 1984). Paleoecological analyses of insect and plant macrofossils from these basal sediments cast doubt on the reliability of the radiocarbon ages. Subsequent accelerator radiocarbon dates of insect fossils and wood fragments indicate an early Holocene age, rather than a late Pleistocene age, for the basal sediments of Lake Emma. These new radiocarbon ages suggest that by at least 10,000 yr B.P. deglaciation of the San Juan Mountains was complete. The insect and plant macrofossils from the basal organic sediments indicate a higher-than-present treeline during the early Holocene. The insect assemblages consisted of about 30% bark beetles, which contrasts markedly with the composition of insects from modern lake sediments and modern specimens collected in the Lake Emma cirque, in which bark beetles comprise only about 3% of the assemblages. In addition, in the fossil assemblages there were a number of flightless insect species (not subject to upslope transport by wind) indicative of coniferous forest environments. These insects were likewise absent in the modern assemblage.
Environmental, ecological, and paleoanthropological implications of the late Pleistocene mammalian fauna from Equus Cave, northern Cape Province, South Africa
Authors: Klein Richard G. , Cruz-Uribe Kathryn , Beaumont Peter B. 
Published: July 2006

AbstractThe late Pleistocene deposits of Equus Cave, northern Cape Province, South Africa, have provided more than 30,000 taxonomically identifiable mammal bones from 48 species. Context, associations, and features of the bone assemblage implicate brown hyenas as the main accumulators. The fauna is significant mainly because (1) it supplements previous evidence that regional climate was cooler and possibly also somewhat moister during part(s) of the late Pleistocene, but deviated less from the historic norm than in areas farther south; (2) it shows that Bond's springbok, which became extinct in the early Holocene, differed from the surviving common springbok not only in important morphological respects but also in reproductive pattern; and (3) it sustains earlier suggestions that an abundance of carnivores, a paucity of small hard bones, and increase in the cranial/postcranial ratio with species size, and exclusively attritional mortality profiles are features that tend to differentiate assemblages accumulated by brown hyenas from those accumulated by people. In addition, pending firmer dating, the fragmentary human fossils from Equus Cave may support an exclusively African origin for anatomically modern humans.
Rates of Soil Development from Four Soil Chronosequences in the Southern Great Basin
Authors: Harden Jennifer W. , Taylor Emily M. , Hill Cindy , Mark Robert K. , McFadden Leslie D. , Reheis Marith C. , Sowers Janet M. , Wells Steven G. 
Published: July 2006

AbstractFour soil chronosequences in the southern Great Basin were examined in order to study and quantify soil development during the Quaternary. Soils of all four areas are developed in gravelly alluvial fans in semiarid climates with 8 to 40 cm mean annual precipitation. Lithologies of alluvium are granite-gneiss at Silver Lake, granite and basalt at Cima Volcanic Field, limestone at Kyle Canyon, and siliceous volcanic rocks at Fortymile Wash. Ages of the soils are approximated from several radiometric and experimental techniques, and rates are assessed using a conservative mathematical approach. Average rates for Holocene soils at Silver Lake are about 10 times higher than for Pleistocene soils at Kyle Canyon and Fortymile Wash, based on limited age control. Holocene soils in all four areas appear to develop at similar rates, and Pleistocene soils at Kyle Canyon and Fortymile Wash may differ by only a factor of 2 to 4. Over time spans of several millennia, a preferred model for the age curves is not linear but may be exponential or parabolic, in which rates decrease with increasing age. These preliminary results imply that the geographical variation in rates within the southern Great Basin-Mojave region may be much less significant than temporal variation in rates of soil development. The reasons for temporal variation in rates and processes of soil development are complexly linked to climatic change and related changes in water and dust, erosional history, and internally driven chemical and physical processes.
Late Quaternary Environments in Ruby Valley, Nevada
Authors: Thompson Robert S. 
Published: July 2006

AbstractPalynological data from sediment cores from the Ruby Marshes provide a record of environmental and climatic changes over the last 40,000 yr. The modern marsh waters are fresh, but no deeper than ∼3 m. A shallow saline lake occupied this basin during the middle Wisconsin, followed by fresh and perhaps deep waters by 18,000 to 15,000 yr B.P. No sediments were recovered for the period between 15,000 and 11,000 yr B.P., possibly due to lake desiccation. By 10,800 yr B.P. a fresh-water lake was again present, and deeper-than-modern conditions lasted until 6800 yr B.P. The middle Holocene was characterized by very shallow water, and perhaps complete desiccation. The marsh system deepened after 4700 yr B.P., and fresh-water conditions persisted until modern times. Vegetation changes in Ruby Valley were more gradual than those seen in the paleolimno-logical record. Sagebrush steppe was more widespread than at present through the late Pleistocene and early Holocene, giving way somewhat to expanded shadscale vegetation between 8500 and 6800 yr B.P. Shadscale steppe contracted by 4000 yr B.P., but had greater than modern coverage until 1000 to 500 yr ago. Pinyon-juniper woodland was established in the southern Ruby Mountains by 4700 yr B.P.
Paleoecological Interpretation and Age of an Interstadial Lake Bed in Western New York
Authors: Miller Norton G. , Calkin Parker E. 
Published: July 2006

AbstractPollen assemblages from a 6.6-m exposure of Pleistocene lake silt in central western New York consist of anomalous mixtures dominated by spruce, pine, and sedge but with a significant representation of deciduous forest elements. Leaves ofDryas integrifoliaand plants of the terrestrial mossDistichiumfrom the lake silt yielded AMS ages of 24,900 ± 1000 and 24,180 ± 900 yr B.P., indicating that the silt was deposited during the middle Wisconsinan Plum Point interstade and that the pollen of beech, hickory, and other deciduous trees was recycled from an interglacial deposit. Caution therefore must be exercised in the paleoecological interpretation of interstadial lake deposits. Plant macrofossil assemblages (seeds, fruits, mosses) from the silt also probably are mixtures of primary and secondary fossils. Scanning electron microscopy of surface features of fruits and seeds revealed two classes of preservation. The best-preserved fossils had intact surface microfeatures and are considered of primary origin. Those with the outer cell layer degraded or absent probably have been recycled. The distributions of species represented in the latter category center on the Great Lakes region and southward, whereas those of the primary fossils are more northern and indicate tundra and spruce-jack pine forest. The late Wisconsinan Kent ice margin advanced across central western New York after ca. 24,500 yr B.P. into a mosaic of tundra and conifer forest.
Rapid Climatic Change in Coastal Southern California Inferred from Pollen Analysis of San Joaquin Marsh
Authors: Davis Owen K. 
Published: July 2006

AbstractPollen analysis and five radiocarbon dates of a 687-cm core provide a detailed chronology of environmental change in a marsh at the head of Newport Bay, Orange County, California. Sediment deposition kept pace with sea-level rise during the early history of the marsh. From ca. 7000 to 4500 yr B.P. the site was a freshwater marsh, trees were more abundant than today, and grassland was the regional vegetation. As sea level rose, salt marsh gradually invaded the site. Brief periods of freshwater marsh 3800, 2800, 2300, and after 560 yr B.P. correlate with episodes of global cooling during the Neoglacial. The historic period is marked by the appearance of exotic species (particularly Erodium cf. cicutarium and Eucalyptus) and the spores of fungi (Sporormiella and Thecaphora). Peak influx of pollen, spores, and charcoal probably reflect greater frequency of flooding and erosion ca. 5000 yr B.P. and during the last 1000 yr.
Dating Late Pleistocene Pluvial Events and Tephras by Correlating Paleomagnetic Secular Variation Records from the Western Great Basin
Authors: Negrini Robert M. , Davis Jonathan O. 
Published: July 2006

AbstractPaleomagnetic records are used to correlate sedimentary sequences from pluvial Lakes Chewaucan and Russell in the western Great Basin. This correlation is the basis for age control in the relatively poorly dated sequence from Lake Chewaucan. The resulting chronology supports a lack of sedimentation in Lake Chewaucan during the interval 27,400 to 23,200 yr B.P., an assertion supported by the presence of a lag deposit at the corresponding stratigraphic horizon. Because the Lake Chewaucan outcrop (near Summer Lake, Oregon) is near the bottom of the lake basin, we conclude that Lake Chewaucan was at a lowstand during this time interval. The Chewaucan lowstand is coeval with the lowstand accompanying the Wizard's Beach Recession (isotope stage 3) previously seen in the geologic record from nearby pluvial Lake Lahontan. The ages of six tephra layers, including the Trego Hot Springs tephra, were also estimated using the paleomagnetic correlation. Together, the new age of the Trego Hot Springs tephra (21,800 yr B.P.) and the lake surface level prehistory of Lake Chewaucan imply a revised model for the lake surface level prehistory of Lake Lahontan. The revised model includes a longer duration for the Wizard's Beach Recession and the occurrence of a younger lowstand of short duration soon after the lowstand corresponding to the Wizard's Beach Recession.
Influence of Terrestrial Vegetation on Sediment-Forming Processes in Kettle Lakes of West-Central Minnesota
Authors: Almquist-Jacobson Heather , Almendinger James E. , Hobbie Sarah 
Published: July 2006

AbstractThe composition, magnetic characteristics, and pollen content of sediments from two small kettle lakes near the prairie-forest border in west-central Minnesota were used to infer changes in terrestrial vegetation, shoreline erosion, eolian inputs, carbonate deposition, and aquatic productivity. Several important changes in sediment stratigraphy coincide closely with changes in the terrestrial pollen assemblage. Accumulation rates of organic, inorganic, and carbonate sediment fractions as well as the concentration of magnetic particulates increased abruptly as prairie replaced pine forest, and decreased gradually throughout the subsequent transitions to oak scrub, and later, mixed woodland. Results suggest that by its mediation of wind exposure, local vegetation may affect (1) influx of eolian particulates, (2) erosion of shorelines, (3) water circulation, and (4) carbonate equilibria. In addition, low rates of accumulation of organic matter and increased humidicity during periods of conifer vegetation suggest that humic matter leached from forest soils may influence lake-water chemistry and reduce productivity. Changes in lake level caused by differential rates of evapotranspiration of the various vegetation types (conifer forest, deciduous hardwood forest, brush scrub, and prairie) could not be detected. Sediment-forming processes can therefore be altered by changes in local terrestrial vegetation through several mechanisms that are independent of changes in lake level.
Time Dependence of Magnetic Susceptibility of Soil Chronosequences on the California Coast
Authors: Singer M. J. , Fine P. , Verosub K. L. , Chadwick O. A. 
Published: July 2006

AbstractAs the age of the soils in a chronosequence on the California coast increases, the difference between the magnetic susceptibility of eluvial and illuvial horizons increases, and the residual susceptibility after extraction with citrate-bicarbonate-dithionite (CBD) decreases. Enhanced susceptibility results from the conversion of nonferrimagnetic minerals to secondary ferrimagnetic forms (most likely maghemite) and the preferential accumulation of inherited and pedogenic magnetic minerals. Little enhancement occurs for pedons younger than 40,000 yr. By 124,000 yr, most of the magnetic susceptibility can be attributed to forms soluble in CBD. Magnetic susceptibility appears to vary systematically over time for three chronosequences from areas with mean annual precipitation ranging from 650 to 1500 mm yr−1. Magnetic susceptibility enhancement may be a useful parameter for estimating soil age in certain climates.
Quantification of the Biogenic Silica Dissolution in Southern Ocean Sediments
Authors: Pichon Jean-Jacques , Bareille Gilles , Labracherie Monique , Labeyrie Laurent D. , Baudrimont Annick , Turon Jean-Louis 
Published: July 2006

AbstractA transfer function has been established to quantify the dissolution of diatom silica in Southern Ocean sediments. The relationship between the amount of silica dissolution and changes in diatom species distribution is built by controlled progressive dissolution of biogenic silica in five recent sediment samples from box-core tops, each representative of a modern diatom species sediment assemblage. The amount of dissolved silica was measured for each experiment. The resulting data set of species abundances (42 samples containing 32 diatom species and 2 silicoflagellate genera) was added to the modern data base of diatom species distributed over the Southern Ocean (124 core tops). Q-mode factor analysis individualizes four factors explaining 83% of the variance. The first three factors are controlled by surface water properties (mostly temperature). The fourth factor is the only one correlated with loss of silica in the reference samples (R = 0.900). We quantified the dissolution factor using this correlation: superficial sediments of the Southeast Indian Ocean are characterized, from low to high latitudes, by a decrease in silica loss by dissolution (from >50 to 10%) from the Subantarctic Zone (40°S) to around 55°S, followed by an increase of silica loss to values larger than 60% between 63° and 68°S. Application of the dissolution factor in two cores from the Southern Ocean (≈44° and 55°S) shows enhanced opal dissolution during the last glaciation, particularly during Emiliani's stage 3 (from 40,000 to 30,000 yr B.P.).