Book Description
This digital document is a journal article from Chemie der Erde - Geochemistry - Interdisciplinary Journal for Chemical, published by Elsevier in 2006. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

Description:
Pedogenic needle-fiber calcite was studied regarding its morphology, texture and stable isotope composition from the paleosol of the Quaternary Varhegy travertine (Budapest, Hungary). The needle-fiber calcite is composed of 40-200@mm long monocrystals. Smooth rods as well as serrated-edged crystals with calcite overgrowths were identified by SEM. Needles have several textural varieties: randomly distributed crystals in vugs and pores with calcite hypocoatings, bundles of subparallel crystals forming coatings around grains and alveolar structure with bridging needles in vugs. The morphological study of needle-fiber calcite suggests that needles are calcified fungal sheaths and produced by fungal biomineralization, a common process in recent and fossil soils and calcretes. The stable isotope composition of needle-fiber calcite (average: @d^1^8O=-7.1%% and @d^1^3C=-7.3%% vs. V-PDB) indicates significant incorporation of organically derived CO"2 and probably biological influence on needle genesis. Dissolved host rock travertine and/or atmospheric CO"2 could also contribute some carbon to the acicular calcite. ... Read more

Book Description
This digital document is a journal article from Quaternary Science Reviews, published by Elsevier in 2004. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

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Palynological analysis of cave sediments, in particular the analysis of pollen from speleothems is not a widely adopted approach to palaeoenvironmental studies in the British Isles. This disregard of speleopalynology may be due to the lack of understanding of the cave pollen taphonomy, and uncertainty and suspicion as to what cave pollen spectra actually represent. However, a number of studies of pollen from caves have been carried out, particularly in Belgium, France and Spain which have demonstrated speleothem pollen spectra to be representative of contemporary vegetation above the cave, with pollen generally well preserved. Speleothems have the distinct advantage over other pollen sources that they can be dated by high precision TIMS U-Th dating, up to an order of magnitude further back in time than ^1^4C dating of organic sediments would allow. This potentially allows previously unconstrained Middle Pleistocene palynological records to be precisely and accurately dated, thus providing a geochronologically controlled terrestrial record of palaeoenvironmental change. This paper presents a timely review of speleothem palynology, aiming to eliminate biases and misconceptions and illustrate the enormous potential this unique source of palaeoenvironmental data holds, particularly in providing accurately dated pollen records from previously understudied environments and time periods. ... Read more

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This study represents a comprehensive and quantitative examination of the paleoecological evidence for forest community development in the Temperate Zone of Eastern North America since the last glacial maximum 20,000 years ago. A computerized fossil-pollen data set, compiled from 162 radiocarbon-dated paleoecological sites were used. Their fossil pollen spectra were transformed into an estimate of past dominance for 19 major tree taxa. Ecological ordination of the data allowed evaluation of the degree of persistence. Measurement of the rates of spread along major migration routes give new insights into the biogeography of temperate and boreal trees. These extensive studies are of interest to graduate students and researchers in all fields of ecology, as well as biogeography. ... Read more

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Produced by the Minnesota Geological Survey ... Read more

Book Description
This digital document is a journal article from Global and Planetary Change, published by Elsevier in . The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

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Models that allow vegetation to respond to and interact with climate provide a unique method for addressing questions regarding feedbacks between the ecosystem and climate in pre-Quaternary time periods. In this paper, we consider how Dynamic Global Vegetation Models (DGVMs), which have been developed for simulations with present day climate, can be used for paleoclimate studies. We begin with a series of tests in the NCAR Land Surface Model (LSM)-DGVM with Eocene geography to examine (1) the effect of removing C"4 grasses from the available plant functional types in the model; (2) model sensitivity to a change in soil texture; and (3), model sensitivity to a change in the value of pCO"2 used in the photosynthetic rate equations. The tests were designed to highlight some of the challenges of using these models and prompt discussion of possible improvements. We discuss how lack of detail in model boundary conditions, uncertainties in the application of modern plant functional types to paleo-flora simulations, and inaccuracies in the model climatology used to drive the DGVM can affect interpretation of model results. However, we also review a number of DGVM features that can facilitate understanding of past climates and offer suggestions for improving paleo-DGVM studies. ... Read more

Book Description
This digital document is a journal article from Quaternary Science Reviews, published by Elsevier in 2004. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

Description:
Global estimates of sea-surface temperature (SST) distributions represent an important prescribed boundary condition for atmosphere-only general circulation models (AGCMs) which aim to simulate the behaviour of the past climate system. We investigate the use of objective interpolation techniques, based on Delaunay triangulation and gridded-averaging, in translating point-based SST data into global maps. Existing global SST datasets generated by CLIMAP for the Last Glacial Maximum (LGM) and PRISM for the middle Pliocene are used as vehicles for evaluation. Results indicate that the sample quantity and array of the CLIMAP sites are sufficient to faithfully reconstruct major surface water patterns of the LGM. However, as problems with sample quantity and distribution become more acute in pre-Quaternary periods, interpolators are limited in their ability to produce reasonable estimates in data sparse regions. In such cases, potential biases associated with interpolation become significant enough to bring into doubt the overall validity of the global SST reconstruction. This limits the datasets use as a prescribed boundary condition in AGCM simulations. On a regional scale, objective methods offer a valuable means of assessing data quality and constraining future subjective analysis. Primary rather than interpolated SST data remain extremely valuable as a tool to evaluate output from coupled ocean-atmosphere models rather than a prescribed boundary condition in AGCM simulations. ... Read more

Book Description
This digital document is a journal article from Quaternary Science Reviews, published by Elsevier in 2006. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

Description:
The existing subglacial megaflood explanation of glacial sediment deposition and landform production in south-central Alberta relies upon four sets of observations and associated interpretations: (1) a subglacial genesis for all glacilacustrine sediments lying stratigraphically below a local melt-out till or ''ice stagnation facies''; (2) a melt-out genesis for the ''ice stagnation facies''; (3) a syn-depositional status for the glacitectonic disturbance visible in most exposures due to periodic ice-bed re-coupling; and (4) a subglacial megaflood origin for a ''truncation surface'' and clast lag that occurs at the top of the sedimentary exposures and the carving of hummocky terrain and flutes in the area. We provide alternative interpretations of the sediment-landform assemblages of the area and demonstrate that alternative, simpler interpretations of the evidence can be proposed. Specifically, we propose: (1) a proglacial rather than subglacial origin of glacilacustrine sediments; (2) a glacilacustrine origin for the ''melt-out till'' and its subsequent glacitectonic disturbance by minor glacier readvances; (3) postdepositional rather than syn-depositional glacitectonic disturbance due to a significant advance by glacier ice after the accumulation of a thick sequence of glacilacustrine sediments; (4) that some linear chains of hummocks are the morphological expression of glacitectonic folding and thrusting; (5) that the use of the truncation surfaces to propose a subglacial fluvial origin for hummocky moraine is flawed logic, because they do not constitute the tops of hummocks but rather occur on the interfluves that separate the numerous small coulees incised into the valley sides during postglacial time. Truncation surfaces and clast lags occur locally on the beds of spillway tracts and can be explained as the products of erosion by subaerial spillway water. The mega-flood explanation of flutings and hummocky moraine in the region is also questioned because these landforms record a regional ice flow imprint of mis-aligned and cross-cutting flow sets terminating at series of inset and locally cross-cutting and lobate transverse ridges (moraines). These landform associations preclude a subglacial flood origin for the flutings and demonstrate that the transverse ridges are more appropriately interpreted as recessional push moraines. The similarity of the glacial landforms of this region to the active, temperate glacial landsystem produced by historical glacier recession in modern glacierized catchments, where sheetfloods have not been influential in subglacial landform production, allows us to apply a modern analogue to landscape interpretation that does not include subglacial sheetflooding. ... Read more