bluuebunny/crossref_metadata_2025_split huggingface dataset:
Late Wisconsin Vegetational and Climatic History at Kylen Lake, Northeastern Minnesota
Authors: Birks H. J. B. 
Published: July 2006

AbstractKylen Lake, located within the Toimi drumlin field, is critically positioned in relation to Late Wisconsin glacial advances, for it lies between the areas covered by the Superior and St. Louis glacial lobes between 12,000 and 16,000 yr B.P. The pollen and plant-macrofossil record suggests the presence of open species-rich “tundra barrens” from 13,600 to 15,850 yr B.P. Small changes in percentages ofArtemisiapollen between 14,300 and 13,600 yr B.P. appear to be artifacts of pollen-percentage data. Shrub-tundra with dwarf birch, willow, andRhododendron lapponicumdeveloped between 13,600 and 12,000 yr B.P. Black and white spruce and tamarack then expanded to form a vegetation not dissimilar to that of the modern forest-tundra ecotone of northern Canada. At 10,700 B.P. spruce and jack pine increased to form a mosaic dominated by jack pine and white spruce on dry sites and black spruce, tamarack, and deciduous trees such as elm and ash on moist fertile sites. At 9250 yr B.P. red pine and paper birch became dominant to form a vegetation that may have resembled the dry northern forests of Wisconsin today. The diagram terminates at 8410 ± 85 yr B.P. Climatic interpretation of this vegetational succession suggests a progressive increase in temperature since 14,300 yr B.P. This unidirectional trend in climate contrasts with the glacial history of the area. Hypotheses are presented to explain this lack of correspondence between pollen stratigraphy and glacial history. The preferred hypothesis is that the ice-margin fluctuations were controlled primarily by changes in winter snow accumulation in the source area of the glacier, whereas the vegetation and hence the pollen stratigraphy were controlled by climatic changes in front of the ice margin.
Recent Sediment Flux and Erosional Processes in a Welsh Upland Lake-Catchment Based on Magnetic Susceptibility Measurements
Authors: Dearing John A. , Elner Janet K. , Happey-Wood Christine M. 
Published: July 2006

AbstractSingle sample magnetic susceptibility measurements are used to correlate synchronous levels in 16 cores of dated (210Pb, 137Cs) recent sediment taken from the deep and oligotrophic Llyn Peris, N. Wales, in order to provide a basis for calculating total sediment and chemical influx through time. Results show that sediment influx has steadily increased since c. 1750 A.D. to reach peak levels in the period 1966–1976 A.D., equivalent to a rise in erosion in the catchment from c. 5 t km−2 yr−1 to c. 42 t km−2 yr−1. Comparison of down-core susceptibility fluctuations with sediment pollen and organic pigment data indicates that maximum susceptibility values relate to periods of channel erosion and minimum susceptibility values relate to periods of slate debris inwash from spoil tips in the catchment. A study of historical records reveals that pre-twentieth-century peak levels of erosion were due to the effects of extractive industries, while twentieth-century erosion has been caused by overgrazing, increased trampling pressure, and heavy construction works in the catchment.
Vegetation East of the Rocky Mountains 18,000 Years Ago
Authors: Wright H.E. 
Published: July 2006

AbstractThe various lobes and segments of the southern periphery of the Laurentide ice sheet reached their maximum extension at different times between 21,000 and 14,000 yr ago, but the CLIMAP date of 18,000 yr ago is taken as a reference level to review the distribution of major vegetational formations in central and eastern United States. Tundra was apparently confined to a narrow belt peripheral to the ice margin only in the Minnesota area and from northern Pennsylvania to New England, with extensions down the crest of the Appalachian Highlands at least as far as Maryland. Some areas south of the Great Lakes may later have been marked by treeless vegetation briefly as the ice retreated. The boreal forest to the south in the central United States was dominated by spruce; the jack pine that had prevailed during previous times was apparently eliminated by the time the ice reached its maximum. In the Appalachian Highlands and the Atlantic Coastal Plain, however, jack pine occurred along with spruce, which decreased in importance southward. The southern limit of the boreal forest in the Southeast was perhaps somewhere in southern Georgia and Alabama. Oak and other temperate deciduous trees were minor components of the boreal coniferous forests especially in the southern Appalacchians, but there is no evidence yet in the southeastern states for a relic mixed mesophytic forest 18,000 yr ago similar to the rich modern deciduous forests of the region, except possibly in the Lower Mississippi Valley. The climate in much of the Southeast was apparently dry as well as cool at that time; in Florida oak/pine scrub and prairie-like openings prevailed, and all but the deepest lakes dried up.
Pollen and Oxygen Isotope Analyses on Late- and Post-Glacial Sediments of the Tourbière de Chirens (Dauphiné, France)
Authors: Eicher U. , Siegenthaler U. , Wegmüller S. 
Published: July 2006

AbstractThe sequence of vegetation phases in the late Glacial was studied in a sediment section from the bog Tourbière de Chirens by means of pollen analysis.18O/16O ratios of samples of lacustrine marl, obtained from the same profile, reflect variations of18O/16O in precipitation and thus provide an additional, independent paleoclimatic record. The observed18O/16O variations agree well with the climatic history as deduced from pollen analysis. The climatic transition from the Oldest Dryas to the Bølling periodsensu lato, as well as the beginning and end of the Younger Dryas cold phase, is marked by abrupt changes in the18O/16O ratio which were observed also in other regions of the Alps. These drastic climatic changes probably took place simultaneously over large areas of Central Europe and occurred within short time spans.
The Sedimentary Framework of the Southern Basin of Lake George, New York
Authors: Hutchinson D.R. , Ferrebee W.M. , Knebel H.J. , Wold R.J. , Isachsen Y.W. 
Published: July 2006

AbstractInformation from 240 km of high-resolution seismic reflection profiles has been analyzed to show the bathymetric and subsurface configuration of southern Lake George in the southeastern corner of the Adirondack Mountains, New York. Three units have been identified and sampled in 13 piston cores as long as 7 m and 4 grab samples; they are glacial drift, glaciolacustrine nonorganic clay, and Holocene lake mud rich in organic material. Three deep bedrock basins controlled glacial, glaciolacustrine, and postglacial deposition within the lake. Glaciolacustrine clay is more than 30 m thick in these basins but is generally absent in water depths less than 20 m. An unconformity separates glaciolacustrine clay from overlying Holocene mud in water depths less than 30 m, but the contact is conformable and transitional in deeper water. The unconformity may have originated from subaqueous or subaerial erosion during a low stage of lake level which probably occurred between 10,000 and 700 yr B.P. Holocene lake mud is thin to absent in the shallower waters separating the three basins, but reaches 15-m thickness near the entrance to The Narrows. A new radiocarbon date of 6950 ± 60 yr B.P. was obtained from a wood fragment which was found in the Holocene lake mud. We found no clear evidence of postglacial tectonic disturbances of the lake sediments although recent releveling profiles suggest that the Adirondack Mountains are undergoing contemporary uplift.
Paleoclimatic Estimates from Water and Energy Budgets of East African Lakes
Authors: Hastenrath Stefan , Kutzbach John E. 
Published: July 2006

AbstractThe Turkana, Nakuru-Elmenteita, and Naivasha basins in the Rift Valley of East Africa experienced high water levels during the period of 10,000–7000 yr B.P. Analyses of the modern hydrologic and energy budgets for these basins, along with that of Lake Victoria, were used to infer the amount of precipitation that would have been required to maintain the enlarged paleolakes of the early Holocene. Precipitation must have been at least 150–300 mm/yr (15 to 35%) above the modern average. The precipitation estimates were fairly consistent among the various basins, but no quantitative estimate was made for the additional precipitation required to account for overflow from the Rift Valley lake basins. Discharge from the Lake Victoria basin around 1880 A.D. was considerably above the more recent average, and the increased discharge into the White Nile for that period might have been similar to that of the early Holocene. A sensitivity analysis showed that temperature changes were probably not too important for changing the hydrologic-energy budget; changes of albedo, Bowen ratio, and cloudiness were likely to have been of greater importance.
Age, Palaeoenvironments, and Climatic Significance of Late Pleistocene Konya Lake, Turkey
Authors: Roberts Neil 
Published: July 2006

AbstractThe Konya Basin, a closed depression at the southern margin of the Anatolian Plateau, contained a shallow but extensive lake during the late Quaternary. Nearshore sands and gravels containingDreissenashells form prominent depositional terraces around the basin edges at elevations between 1005 and 1020 m, and the highest has been dated at more than 30,000 yr B.P. The last major phase of limnic sedimentation occurred between 23,000 and 17,000 yr B.P., after which the lake fragmented into a number of subbasins or secondary depressions which evolved to become either marshes or playas. A final expansion of these residual lakes has been dated to ca. 11,000 yr B.P. in one secondary depression. Archaeological and other evidence indicate that only minor lake-level fluctuations occurred during the Holocene. The dominant sedimentary process over the last 10,000 years has been alluviation at the basin margin. The Konya sequence confirms that the most recent period of high-lake levels in the northern Near East was of the last glacial age. The palaeolakes of Anatolia and Iran were, however, more a product of reduced evaporation brought about by temperature depression than of changes in precipitation, and thus belie their often-used description as an indicator of “pluvial” climatic conditions.
Toward a Comprehensive Upper Quaternary Tephra and Ignimbrite Stratigraphy in New Zealand using Electron Microprobe Analysis of Glass Shards
Authors: Froggatt P. C. 
Published: July 2006

AbstractNew Zealand Quaternary marine and terrestrial sequences contain numerous tephras, or volcanic-ash horizons, that are the distal correlatives of voluminous welded ignimbrite sheets, erupted from central North Island. Electron microprobe analyses of glass shards from the distal tephras demonstrate their homogeneity and are shown to identify each tephra examined. By matching tephras from stratigraphically controlled sequences, the first comprehensive tephra stratigraphy spanning from 50,000 to 700,000 yr ago and covering the New Zealand region is advanced.Analyses on glass shards from the unwelded base of ignimbrite sheets are comparable to the distal tephra analyses and allow correlation between ignimbrites and to the distal tephras. The better exposed tephra record constrains the number of separate eruptive events and the stratigraphy of the ignimbrites, both of which were previously confused by lack of outcrop.Samples from pumiceous marine sediments were found to contain two or more chemically distinct populations of glass. The pumice is in cross-bedded sands or sand lenses within conglomerate, attesting to a shallow high-energy environment where reworking could occur. However, each glass population could be matched to older, known tephras.
Late Wisconsin Macrofossil Records of Desert Vegetation in the American Southwest
Authors: Spaulding W. Geoffrey 
Published: July 2006

AbstractUntil recently, the oldest-known packrat (Neotoma spp.) midden records of desert scrub vegetation were less than 10,500 yr old and were restricted to altitudes below 300 m in the northern Sonoran Desert. Recent discovery of macrofossil assemblages in the Mojave Desert extends the record of desert vegetation back to 14,800 yr ago and to altitudes as high as 910 m. Although xerophytic conifer woodland was widespread in current desert habitats during the Late Wisconsin and early Holocene, the development of extensive desert vegetation was not delayed until the beginning of the middle Holocene. A regional vegetation mosaic of desert scrub and woodland existed at altitudes below 1000 m in the Mojave Desert during the last part of the Late Wisconsin.
Late-Quaternary Vegetational Dynamics and Community Stability Reconsidered
Authors: Delcourt Paul A. , Delcourt Hazel R. 
Published: July 2006

AbstractDefining the spatial and temporal limits of vegetational processes such as migration and invasion of established communities is a prerequisite to evaluating the degree of stability in plant communities through the late Quaternary. The interpretation of changes in boundaries of major vegetation types over the past 20,000 yr offers a complementary view to that provided by migration maps for particular plant taxa. North of approximately 43°N in eastern North America, continual vegetational disequilibrium has resulted from climatic change, soil development, and species migrations during postglacial times. Between 33° and 39°N, stable full-glacial vegetation was replaced by a relatively unstable vegetation during late-glacial climatic amelioration; stable interglacial vegetation developed there after about 9000 yr B.P. Late-Quaternary vegetation has been in dynamic equilibrium, with a relatively constant flora, south of 33°N on upland interfluves along the northern Gulf Coastal Plain, peninsular Florida, and west-central Mexico.
Palynologic and Geomorphic Evidence for Environmental Change During the Pleistocene-Holocene Transition at Point Reyes Peninsula, Central Coastal California
Authors: Rypins Steven , Reneau Steven L. , Byrne Roger , Montgomery David R. 
Published: July 2006

AbstractThree sites on the Point Reyes Peninsula, California, provide evidence of major environmental change during the Pleistocene-Holocene transition. A 12,300-yr-long pollen record from Coast Trail Pond reveals a change from closed canopyPseudotsuga-Abiesforest to coastal scrub and grassland about 10,000 yr B.P. The same change is also evident in pollen records from two sea cliff exposures that contain a series of buried soils interstratified with fluvial and debris-flow deposits. Eight radiocarbon dates from these exposures indicate that aggradation at both sites began ca. 12,000 yr B.P. and that the change from forest to scrub and grassland occurred between 10,300 and 9400 yr B.P. High percentages of alder pollen and bracken fern spores reflect repeated geomorphic disturbance at ca. 10,400 yr B.P. The occurrence of major aggradation accompanied by recurrent debris flows between ca. 12,000 and 10,000 yr B.P. suggests that this was a period of more frequent high-intensity storms.
A Multivariate Mixing Model for Identifying Sediment Source from Magnetic Measurements
Authors: Yu Lizhong , Oldfield Frank 
Published: July 2006

AbstractA sequential method for quantitative identification of sediment source components, based on magnetic measurements, has been developed and tested for sediments from the Rhode River, Maryland. Simulated mixing tests and multiple regression were employed to establish numerical relationships between source component proportions and the magnetic measurements of mixtures. On the basis of these multivariate mixing models, source components of three estuarine sediment cores were estimated by linear programming. The results strongly support the previous studies on this catchment which indicated a dramatic change in sediment source some 150 to 200 yr ago. Quantitative calculations are more useful and informative than purely qualitative descriptions.
500,000-Year Paleomagnetic Record from New Zealand Loess
Authors: Pillans Brad , Wright Ian 
Published: July 2006

AbstractA paleomagnetic record of secular variation has been obtained from a 17.4-m-long core drilled from loess in North Island, New Zealand. Three dated rhyolitic tephra occur in the loess core at depths of 1.5, 3.4,and 13.0 m, with ages of 22,500, ≧42,000, and 370,000 years, respectively. The base of the core is estimated to be 500,000 years old, which makes this the longest continuous loess sequence yet described from the Southern Hemisphere. Two periods of anomalously low inclination in the core are correlated with the Mungo Event (ca. 35,000 years ago) and the Emperor Event (ca. 490,000 years ago); the Blake Event (ca. 115,000 years ago) is not recorded.
Discriminating Texturally Similar Tills in Central Minnesota by Graphical and Multivariate Techniques
Authors: Mooers Howard D. 
Published: July 2006

AbstractThe sequence of glaciation in the midwestern United States has been defined mainly by till stratigraphy. Correlation of tills over long distances, however, is complicated by spatial variation of sediment characteristics. In east-central Minnesota three till units were previously defined. Two were considered to be lithologically identical and were correlated with the late Wisconsin advance of the Rainy lobe, whereas a third till was assigned to the contemporaneous advance of the Superior lobe. This interpretation was inconsistent with subsequent glaciological reconstructions of the Rainy and Superior lobes. In an effort to define the parent lobe of the deposits in central Minnesota, the tills were sampled, analyzed, and compared by graphical and multivariate techniques. Graphical comparisons of sedimentological parameters were inadequate to distinguish important differences among the groups. However, principal component analysis of the analytical data indicates that the original interpretation is untenable and that one of the tills formerly assigned to the Rainlylobe should now be attributed to the Superior lobe. Problems encountered with correlation of tills exposed at the surface illustrate the complexity and problems associated with subsurface stratigraphic correlation.
Rainfall in the Negev Desert during the Middle Holocene, Based on13C of Organic Matter in Land Snail Shells
Authors: Goodfriend Glenn A. 
Published: July 2006

AbstractAnalysis of stable carbon isotope ratios (13C/12C) of organic matter in land snail shells is used to infer middle Holocene rainfall amounts in the Negev Desert by reconstructing the distribution of C4plants in the family Chenopodiaceae. The organics are derived from the diet of the snails, which consists of plant material, and are enriched in13C where C4plants are present. A survey of modern plant communities indicates that in areas receiving ≥300 mm mean annual rainfall, nearly all plant communities consist of C3species only (no C4chenopodes), whereas in areas under ≤230 mm rainfall, most plant communities contain one or more C4chenopode species. In between is a transition zone consisting of a mosaic of both pure C3and mixed C3+ C4plant communities. Isotopic results for fossil land snails indicate a consistent geographic pattern throughout the middle Holocene, from ca. 6500 to 3000 yr B.P., with the transition zone located ca. 20 km south of its present position. This implies a near doubling of rainfall within this region as compared to present.
Late Deglaciation of the Central Labrador Coast and Its Implications for the Age of Glacial Lakes Naskaupi and McLean and for Prehistory
Authors: Clark Peter U. , Fitzhugh William W. 
Published: July 2006

AbstractThe age of the marine limit and associated deglaciation has been estimated from relative sea-level curves for the Hopedale and Nain areas of the central Labrador coast as approximately 7600 ± 200 and 8500 ± 200 yr ago, respectively. These ages indicate that the ice margin remained on the coast for up to 3000 yr longer than previously estimated. Because the central coast is due east of glacial lakes Naskaupi and McLean, the earliest the lakes could have formed was <8500 ± 200 yr ago, with their largest phases being fully established only after 7600 ± 200 yr ago. This suggests that the age of the lakes, and associated deglaciation of the central Labrador-Ungava region, is younger by at least 1500 yr than previously estimated. A late-glacial marine-based ice mass in Ungava Bay that dammed the lakes collapsed ca. 7000 yr ago. Within this time frame, therefore, the glacial lakes only existed for <500 yr. The persistence of the Laurentide Ice Sheet margin on the central Labrador coast until 7600 yr ago probably restricted the northward movement of early prehistoric people into northern Labrador.
Paleoclimatic implications of late quaternary lacustrine sediments in Western Nubia, Sudan
Authors: Pachur Hans-Joachim , Hoelzmann Philipp 
Published: July 2006

AbstractOwing to the hypercontinental location of Western Nubia, secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern Sahara. The study area is crossed by the 20-mm isohyet; between 9300 and about 4000 yr B.P., however, there were widespread lake and swamp environments with freshwater molluscs, ostracods, and diatoms, and a species-rich savanna mammal fauna. The center of the West Nubian Basin (approx. 18°N), an area of about 20,000 km2, was occupied by a semiaquatic landscape which was situated at the same latitude as Paleolake Chad. From extensive lake carbonates up to about 4 m thick, a long-term rise of the groudwater table is inferred. Environments developed that now exist at about latitude 13°N. Radiocarbon dates from lake sediment sequences cluster between 30,000 and 21,000 yr B.P., indicating a Pleistocene wet phase. A gap in radiocarbon dates between 21,000 and 11,000 yr B.P. signals a phase of hyperaridity, similar to the present hyperarid phase, with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation.
Comparative Uranium-Thorium and Thermoluminescence Dating of Weathered Quaternary Alluvium in the Tropics of Northern Australia
Authors: Nanson Gerald C. , Price David M. , Short Stephen A. , Young Robert W. , Jones Brian G. 
Published: July 2006

AbstractThermoluminescence (TL) age determinations of alluvial sediments in the tropics are evaluated by comparison with U/Th age determinations of pedogenic accumulations in the alluvium of the lower Gilbert River, a large fan delta in the wet-dry tropics of northern Queensland, Australia. This study extends U/Th dating by applying it not only to calcretes, but also to Fe/Mn oxyhydroxide/oxide accumulations. While a direct correlation cannot be made between U/Th dates from secondary minerals and TL dates from the host sediments, both sets of data show broad consistency. In addition to providing a minima for acceptable TL ages, U/Th dates are useful for determining the chronology of pedogenesis/diagenesis. They show that calcretes and ferricretes have formed under similar climatic conditions in the wet-dry tropics of northern Australia during the late pleistocene. Beneath about 5–12 m the Gilbert fan delta consists of an extensive sand body older than 85,000 yr and probably about 120,000 yr in age, representative of a period of major fluvial activity not repeated since this time. Above are muds and fine sandy muds that extend uninterrupted to the present surface except in the downstream fan where they are bisected by a thin unit of medium sand that TL dates at 40,000–50,000 yr B.P. A system of sandy distributary channels over the fan surface represents an early Holocene fluvial phase probably more active than at present.