Melt loss and the preservation of granulite facies mineral assemblages

The loss of a metamorphic fluid via the partitioning of H₂O into silicate melt at higher metamorphic grade implies that, in the absence of open system behaviour of melt, the amount of H₂O contained within rocks remains constant at temperatures above the solidus. Thus, granulite facies rocks, composed of predominantly anhydrous minerals and a hydrous silicate melt should undergo considerable retrogression to hydrous upper amphibolite facies assemblages on cooling as the melt crystallizes and releases its H₂O. The common occurrence of weakly retrogressed granulite facies assemblages is consistent with substantial melt loss from the majority of granulite facies rocks. Phase diagram modelling of the effects of melt loss in hypothetical aluminous and subaluminous metapelitic compositions shows that the amount of melt that has to be removed from a rock to preserve a granulite facies assemblage varies markedly with rock composition, the number of partial melt loss events and the P–T conditions at which melt loss occurs. In an aluminous metapelite, the removal of nearly all of the melt at temperatures above the breakdown of biotite is required for the preservation of the peak mineral assemblage. In contrast, the proportion of melt loss required to preserve peak assemblages in a subaluminous metapelite is close to half that required for the aluminous metapelite. Thus, if a given proportion of melt is removed from a sequence of metapelitic granulites of varying composition, the degree of preservation of the peak metamorphic assemblage may vary widely.     

Cosmogenic-nuclide ages for New England coastal moraines, Martha's Vineyard and Cape Cod, Massachusetts, USA

Cosmogenic-nuclide exposure ages for 13 glacially transported boulders atop the Martha's Vineyard moraine, MA, USA, indicate that the southeastern margin of the Laurentide ice sheet reached its maximum extent during the last glaciation 23,200±500 yr ago. Another 10 age determinations from the younger Buzzards Bay moraine near Woods Hole, MA, indicate that this moraine complex was formed 18,800±400 yr ago. These ages correlate approximately with the terminations of cooling cycles recorded in Greenland ice cores and coeval ice-rafting events, suggesting that the marginal position of this sector of the ice sheet was tightly coupled to North Atlantic climate during the last glacial maximum.     

Probability Density Functions as Botanical-Climatological Transfer Functions for Climate Reconstruction

We present a new procedure, the pdf method (pdf=probability density function), for reconstructing Quaternary climate utilizing botanical data. The procedure includes the advantages of the indicator species method by considering the fossil and modern presence and absence of taxa rather than their frequencies, thus avoiding the need for modern analog plant communities. Overcoming the problematic use of absolute limits to describe climate response ranges is the main progress of the pdf method in comparison to the indicator species method. This advantage results from estimating probability density functions (pdfs) for monthly mean January and July temperature conditional on the present day occurrence of single taxa. Gaussian distributions sufficiently approximate pdfs of many, although not all, studied taxa. On the assumption of statistical independence, the procedure calculates a joint pdf as the product of the pdfs of the individual taxa. This algorithm weights each taxon according to the extent of its climate response range expressed by its covariance structure. We interpret the maximum of the resulting pdf as the most likely climate and its confidence interval as the uncertainty range. To avoid an artificial reduction of uncertainty arising from the use of numerous similar pdfs, a preselection method is proposed based on the Mahalanobis distance between pdfs. The pdf method was applied to the Carpinus phase of a profile from Gröbern, Germany, that spans the last interglaciation (Eemian). The reconstructed most probable January and July temperatures of about 0.0°C and 18.4°C barely differ from the modern values of −0.5°C and 18.3°C.     

Cathodoluminescence of authigenic albite

Authigenic albites in carbonate rocks typically grow in a high-grade diagenetic to low-grade metamorphic environment and often show Roc-Tourné-twinning sensu Füchtbauer. Based on an investigation of four Middle to South European occurrences, they show Mn²⁺- and Fe³⁺-activated cathodoluminescence (CL), as revealed by combined high resolution spectroscopy of cathodoluminescence emission (HRS-CL), electron paramagnetic resonance (EPR), and proton-induced X-ray emission (μPIXE).     

A comparison of early juvenile red drum densities among various habitat types in Galveston Bay, Texas

Seagrass meadows are often cited as important nursery areas for newly settled red drum even though many estuaries, such as Galveston Bay, Texas, support large numbers of red drum and have limited seagrass cover, suggesting the use of alternate nursery areas. We examined patterns of habitat use for newly settled red drum at six sampling areas in Galveston Bay; two areas had seagrass beds and four areas had no seagrass. We measured densities in different habitat types using epibenthic sleds and enclosure samplers. Peak recruitment of young red drum to the estuary occurred during September through December. Highest densities of new settlers were found in seagrass meadows (primarilyHalodule wrightii), but when seagrass was absent, the highest densities of red drum occurred along theSpartina alterniflora marsh edge interface. Densities were relatively low on nonvegetated bottom away from the marsh edge. We also examined density patterns in other habitat types at selected sampling areas and found no red drum within marsh vegetation away from the marsh edge interface (5 and 10 m into the marsh interior). Oyster reefCrassostrea virginica was sampled using lift nets, and we found no red drum using this habitat, although adjacent seagrass and marsh interface habitats were used. Even though red drum densities in marsh edge were low relative to seagrass, the large areal extent of marshes in the bay complex probably makes marsh edge the most important nursery habitat for red drum in Galveston Bay.     

δC of low-molecular-weight organic acids generated by the hydrous pyrolysis of oil-prone source rocks

Low-molecular-weight (LMW) aqueous organic acids were generated from six oil-prone source rocks under hydrous-pyrolysis conditions. Differences in total organic carbon-normalized acid generation are a function of the initial thermal maturity of the source rock and the oxygen content of the kerogen (OI). Carbon-isotope analyses were used to identify potential generation mechanisms and other chemical reactions that might influence the occurrence of LMW organic acids. The generated LMW acids display increasing ¹³C content as a function of decreasing molecular weight and increasing thermal maturity. The magnitudes of observed isotope fractionations are source-rock dependent. These data are consistent with δ¹³C values of organic acids presented in a field study of the San Joaquin Basin and likely reflect the contributions from alkyl-carbons and carboxyl-carbons with distinct δ¹³C values. The data do not support any particular organic acid generation mechanism. The isotopic trends observed as a function of molecular weight, thermal maturity, and rock type are not supported by either generation mechanisms or destructive decarboxylation. It is therefore proposed that organic acids experience isotopic fractionation during generation consistent with a primary kinetic isotope effect and subsequently undergo an exchange reaction between the carboxyl carbon and dissolved inorganic carbon that significantly influences the carbon isotope composition observed for the entire molecule. Although generation and decarboxylation may influence the δ¹³C values of organic acids, in the hydrous pyrolysis system described, the nondestructive, pH-dependent exchange of carboxyl carbon with inorganic carbon appears to be the most important reaction mechanism controlling the δ¹³C values of the organic acids.     

Climate change impacts on U.S. Coastal and Marine Ecosystems

Increases in concentrations of greenhouse gases projected for the 21st century are expected to lead to increased mean global air and ocean temperatures. The National Assessment of Potential Consequences of Climate Variability and Change (NAST 2001) was based on a series of regional and sector assessments. This paper is a summary of the coastal and marine resources sector review of potential impacts on shorelines, estuaries, coastal wetlands, coral reefs, and ocean margin ecosystems. The assessment considered the impacts of several key drivers of climate change: sea level change; alterations in precipitation patterns and subsequent delivery of freshwater, nutrients, and sediment; increased ocean temperature; alterations in circulation patterns; changes in frequency and intensity of coastal storms; and increased levels of atmospheric CO₂. Increasing rates of sea-level rise and intensity and frequency of coastal storms and hurricanes over the next decades will increase threats to shorelines, wetlands, and coastal development. Estuarine productivity will change in response to alteration in the timing and amount of freshwater, nutrients, and sediment delivery. Higher water temperatures and changes in freshwater delivery will alter estuarine stratification, residence time, and eutrophication. Increased ocean temperatures are expected to increase coral bleaching and higher CO₂ levels may reduce coral calcification, making it more difficult for corals to recover from other disturbances, and inhibiting poleward shifts. Ocean warming is expected to cause poleward shifts in the ranges of many other organisms, including commercial species, and these shifts may have secondary effects on their predators and prey. Although these potential impacts of climate change and variability will vary from system to system, it is important to recognize that they will be superimposed upon, and in many cases intensify, other ecosystem stresses (pollution, harvesting, habitat destruction, invasive species, land and resource use, extreme natural events), which may lead to more significant consequences.     

G-Probe-1 - An International Proficiency Test for Microprobe Laboratories - Report on Round 1 : February 2002 (TB-1 Basaltic Glass)

Results are presented for round one of a new international proficiency testing programme designed for microprobe laboratories involved in the routine analysis of silicate minerals. The sample used for this round was TB-1, a basaltic glass fused and prepared by the USGS. Thirty nine laboratories contributed data to this round, the majority of major element results being undertaken by EPMA and the majority of trace elements by LA-ICP-MS. Assigned values were derived from the median of results produced by nine selected laboratories that analysed powdered material by conventional ICP-MS, INAA and XRF techniques using bulk powders of the sample. Submitted microprobe results were evaluated using a target precision calculated using the Horwitz function, adopting the same criteria as those used for "applied" geochemistry laboratories in the companion GeoPT proficiency testing programme for laboratories involved in the routine bulk analysis of silicate rocks. An evaluation of results from participating microprobe laboratories indicated that overall, data were compatible with this precision function. A comparison between the performance of bulk and microprobe techniques used in the analysis of the basaltic glass showed remarkably good agreement, with significant bias only observed for the major oxide MgO.     

Book Review: Upscaling and Downscaling Methods for Environmental Research. By Marc F. P. Bierkens,Peter A. Finke,and Peter de Willigen. Kluwer Academic Publishers,Dordrecht, The Netherlands, 2000, 190 p. + CD-ROM, $88.00 lpar;U.S.), hardback,ISBN 0-7923-6339-6

Mathematical Geosciences -     

Integrated Two-Dimensional Modeling of Fluid Flow and Compaction in a Sedimentary Basin

In this paper we employ mixed finite elements and numerically study an integrated two-dimensional model of fluid flow and compaction in a sedimentary basin. This model describes a single phase incompressible flow in a two-dimensional section of a sedimentary basin with vertical compaction. At each time step, an iterative algorithm is used to solve this model. The determination of the grid movement is based on the mass conservation and movement of sediments in the basin, while the mixed method is utilized to solve the fluid flow over the moving grid. Numerical experiments are presented to verify this iterative algorithm and show representative solutions for the model under consideration.