Short Note: Precision and Convergence of a Steady Two-Dimensional Ice Sheet Flow Model

Ice-sheet flow is described by the Navier–Stokes equations. We present here an analytical solution for a very simple configuration of two-dimensional ice sheet flow. It is obtained for an imposed flat surface elevation and for a linear flow law. This analytical solution is used here to estimate the performance and precision of a two-dimensional ice sheet flow model. In particular, the comparison of this 2D ice sheet flow model with the analytical solution has allowed to test all the terms involved in the mechanical equations. This analytical solution may be very useful to test similar types of models.     

L'ı̂le de la Dominique, à l'origine des avalanches de débris les plus volumineuses de l'arc des Petites AntillesThe Island of Dominica,site for the generation of the most voluminous debris avalanches in the Lesser Antilles Arc

Results from recent fieldwork and the Aguadomar marine survey in the Lesser Antilles clearly indicate that the volcanic field of southern Dominica has experienced three major edifice collapse events. This led to formation of the most voluminous debris avalanches known in the Caribbean Arc. Submarine hummocky morphology with plurikilometric megablocks is characteristic of debris avalanche deposits. We propose that steep slopes on the western Caribbean side of the island and intense hydrothermal alteration lead to recurrent large-scale edifice collapses. Therefore islands in the Lesser Antilles face a non-negligible risk from generation of tsunamis associated with potential future edifice collapse. To cite this article: A. Le Friant et al., C. R. Geoscience 334 (2002) 235–243.     

Using helicopter electromagnetic data to predict groundwater quality in fractured crystalline bedrock in a semi-arid region, Northeast Brazil

Geostatistical modeling, using airborne and borehole electromagnetic data, was used to estimate electrical conductivity in groundwater within fractured paragneisses and migmatites in a semi-arid climate in northeastern Brazil. Despite the geologic heterogeneity of crystalline aquifers, the use of high resolution helicopter electromagnetic (HEM) data enabled the characterization of groundwater electrical conductivity where data from drilled wells were insufficient. The tacit assumption is that HEM measurements can be used to relate rock electrical conductivity to groundwater electrical conductivity. In this study, the HEM data were used as an external drift variable in non-stationary estimation and stochastic simulation to identify the variability of groundwater electrical conductivity. Validation tests, comparing predicted values for groundwater conductivity with measurements in new wells, confirmed the success of these models in locating fresh groundwater sources in crystalline bedrock.     

A large metabolic carbon contribution to the δC record in marine aragonitic bivalve shells

It is well known that the incorporation of isotopically light metabolic carbon (C_M) significantly affects the stable carbon isotope (δ¹³C) signal recorded in biogenic carbonates. This can obscure the record of δ¹³C of seawater dissolved inorganic carbon (δ¹³C_DIC) potentially archived in the shell carbonate. To assess the C_M contribution to Mercenaria mercenaria shells collected in North Carolina, USA, we sampled seawater δ¹³C_DIC, tissue, hemolymph and shell δ¹³C. All shells showed an ontogenic decrease in shell δ¹³C, with as much as a 4‰ decrease over the lifespan of the clam. There was no apparent ontogenic change in food source indicated by soft tissue δ¹³C values, therefore a change in the respired δ¹³C value cannot be the cause of this decrease. Hemolymph δ¹³C, on the other hand, did exhibit a negative relationship with shell height indicating that respired CO₂ does influence the δ¹³C value of internal fluids and that the amount of respired CO₂ is related to the size or age of the bivalve. The percent metabolic C incorporated into the shell (%C_M) was significantly higher (up to 37%, with a range from 5% to 37%) than has been found in other bivalve shells, which usually contain less than 10%C_M. Interestingly, the hemolymph did contain less than 10%C_M, suggesting that complex fractionation might occur between hemolymph and calcifying fluids. Simple shell biometrics explained nearly 60% of the observed variability in %C_M, however, this is not robust enough to predict %C_M for fossil shells. Thus, the metabolic effect on shell δ¹³C cannot easily be accounted for to allow reliable δ¹³C_DIC reconstructions. However, there does seem to be a common effect of size, as all sites had indistinguishable slopes between the %C_M and shell height (+0.19% per mm of shell height).     

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (P_i), silicate (Si)), nutrient ratios (DIN/P_i), alkaline phosphatase activity, chlorophyll and ¹⁴C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed P_i limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/P_i and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33?C7.01 g C m^?2 d^?1), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 ??M DIN and Pi). The results of this study indicate that P_i limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.     

Manganese-reducing Pseudomonas fluorescens-group bacteria control arsenic mobility in gold mining-contaminated groundwater

Previous studies show the importance of iron- and arsenate-reducing bacteria in mobilizing arsenic in groundwater. Here the authors present experimental evidence of arsenic mobilization in connection with bacterially mediated manganese reduction in groundwater affected by mining activities. Manganese-reducing Pseudomonas species were enriched, isolated and identified by 16S rRNA gene phylogeny from groundwater containing high co-dissolved arsenic (as As^III) and manganese. Enrichment cultures dissolved synthetic birnessite and hausmannite efficiently, but Mn reduction by isolates was reduced at the upper range of environmental levels of dissolved As^III. Results suggest either a self-limiting release of arsenic coupled to bacterial manganese reduction, in the absence of other electron donors like sulfide, or increased arsenic resistance conferred to Mn-reducing bacteria in consortia.