Tectonic wedging along the rear of the offshore Taiwan accretionary prism

The structural geometry, kinematics and density structure along the rear of the offshore Taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling. Deformation between the offshore prism and forearc basin at the point of incipient collision, and southward into the region of subduction, has been interpreted as a tectonic wedge, similar to those observed along the front of mountain ranges. This tectonic wedge is bounded by an east-dipping roof thrust and a blind, west-dipping floor thrust. An east-dipping sequence of forearc-basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge. Section restoration of the roof sequence yields an estimate of 4 km of shortening, which is small compared with that inferred in the collision area to the north, based on the variation in distance between the apex of the prism and the island arc.Previous studies propose that either high-angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the Central Range in the collision zone on land. To better constrain the initial crustal configuration, we tested 350 crustal models to fit the free-air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land. The gravity anomaly, observed in the region of subduction (20.2°N), can be modeled with the arc basement forming a trenchward-dipping backstop that is overlain by materials with densities in the range of sedimentary rocks. Near the point of incipient collision (20.9°N), however, the free-air gravity anomaly over the rear of the prism is approximately 40 mgal higher, compared with the region of subduction, and requires a significant component of high density crustal rocks within the tectonic wedge. These results suggest that the forearc basement may be deformed along the rear of the prism, associated with the onset of collision, but not in the subduction region further to the south.     

Stable isotope indicators of movement and residency for brown shrimp (<Emphasis Type="Italic">Farfantepenaeus aztecus</Emphasis>) in coastal Louisiana marshscapes

Brown shrimp (Farfantepenaeus aztecus) are an important commercial aquatic species experiencing loss of inshore marsh nursery habitat in coastal Louisiana. To study inshore brown shrimp movements and identify aspects of essential habitat important for sustaining brown shrimp populations, we collected juvenile brown shrimp in April and May 2000, the time of annual maximum brown shrimp abundance, in a small 1-km² marsh area on the central Louisiana coast. Drop sampling showed average shrimp densities of 1.6–2.4 m⁻² in shallow marsh ponds and seining indicated lower densities of 0.5–0.9 m⁻² in nearby shallow channel and open bay sites. Smaller shrimp (< 50 mm) fed disproportionately on benthic diatoms and small harpacticoid copepods, while large shrimp fed more frequently on larger-bodied amphipods and tanaids. We developed novel chemical approaches to estimate patterns of shrimp residency and movement using carbon and nitrogen stable isotopic determinations. Resident shrimp had isotopic values similar to average foods and showed consistent isotopic spacings between fast and slow turnover tissues. Residency was highest (47–55%) in ponds and shallow channel habitats and much less in open bays and deep channels (4–27%). There was sparse evidence for dietary specialization among individull shrimp. The results support the view that small 10–20 mm postlarval and juvenile brown shrimp arriving in estuaries from offshore waters continue movement through sub-optimal habitats (deep channels and open bays), but exhibit much less movement once an optimal habitat (marsh ponds or shallow channel margins) is reached. This study also indicated that combining estimates of shrimp densities, residency, growth rate, and mortality allows evaluation of the importance of different habitat types for shrimp production. Shallow ponds that in many ways resemble fertile aquaculture ponds appear to be hot spots for brown shrimp production, and coastal preservation and restoration efforts should focus on these areas as important for sustaining shrimp fisheries.     

Correlation and interpretation of paleosols and loess across European Russia and Asia over the last interglacial-glacial cycle

Loess-paleosol sequences of the last interglacial-glacial cycle are correlated from European Russia to central Siberia and the Chinese Loess Plateau. During cold periods represented by marine oxygen isotope stages (OIS) 2 and 4, loess deposition dominated in the Russian Plain and the Loess Plateau. In central Siberia, loess deposition took place also, but five to seven thin, weakly developed paleosols are identified in both stages. OIS 3, in the Chinese Loess Plateau near Yangchang, consists of a loess bed that is flanked by two weakly developed paleosols. At Kurtak, Siberia, OIS 3 is represented by two distinct, stacked paleosols with no loess bed separating the paleosols. In the Russian Plain, OIS 3 consists of a single, possibly welded paleosol, representing upper and lower stage-3 climates. Brunisols and Chernozems dominate the profiles in China and Siberia, whereas Regosols, Luvisols, and Chernozems are evident in the northern and southern Russian Plain, respectively. OIS 5 is represented in China and the Russian Plain by pedo complexes in a series of welded soils, whereas in contrast, the Kurtak site consists of six paleosols with interbedded loess. The paleosols consist largely of Brunisols and Chernozems. Although the three areas examined have different climates, geographical settings, and loess source areas, they all had similar climate changes during the last interglacial-glacial cycle.     

Sulfur isotopic studies of continental flood basalts in the Noril’sk region: implications for the association between lavas and ore-bearing intrusions

Previous studies of both ore and non-ore-bearing intrusives in the Permo-Triassic flood basalts of the Siberian platform in the Noril’sk area have shown that high-grade Ni-Cu-platinum group elements (PGE) mineralization is associated with anomalously high δ³⁴S values of ∼8 to 12‰. In addition, several researchers have proposed that observed depletions in the Cu, Ni, and PGE content of basaltic lavas of the Nadezhdinsky (Nd) Formation are related to diffusional exchange with, and upgrading in metal tenor of, sulfides in the volcanic conduit system. Sulfur isotopic studies of the lavas at Noril’sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident, and if the Nd lavas in particular were characterized by an anomalous isotopic signature. δ³⁴S values of the lavas range from −4.5 to 8.7‰ Vienna Cañon Diablo Troilite (VCDT), with S concentrations from <40 to 1373 ppm. The majority of δ³⁴S values range from 0 to 4‰, and are similar to those from S-poor intrusions in the Noril’sk area. Although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas, recrystallization may have erased expected mineralogical and textural evidence. Mineralogical data indicate that hydrothermal alteration of the lavas has occurred, but S redistribution has been restricted to localized areas and δ³⁴S values have not been affected. The relatively low S concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption. Our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of ∼900°C at QFM leads to only minor ³⁴S-depletion of sulfur remaining in the melt, and decreases in δ³⁴S values of less than 2‰ at 90% degassing. For this reason all lavas with δ³⁴S values in excess of ∼ 2‰ require a contribution of ³⁴S-enriched country rock sulfur. Because of the high S content and δ³⁴S value (∼ 16-20‰) of evaporites in the country rocks at Noril’sk, contamination of less than 0.5% is required to explain the most ³⁴S-enriched lavas. The Nd lavas have an average δ³⁴S of 2.9‰, but show no difference in S isotopic composition relative to the other lavas, suggesting that metal depletion involved only limited S transfer, or that exchange between mantle-derived S and S of crustal origin buffered δ³⁴S values to less than ∼5‰. Anomalously positive δ³⁴S values, similar to those of the ore-bearing intrusives in the Noril’sk region, are not consistently found in low-S rocks, either lavas or intrusives. Although the mechanism for the derivation of sulfide in the ore-bearing intrusions remain speculative, it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin.     

Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation

Published experimental data for Al(III) and Fe(III) binding by fulvic and humic acids can be explained approximately by the Humic Ion-Binding Model VI. The model is based on conventional equilibrium reactions involving protons, metal aquo ions and their first hydrolysis products, and binding sites ranging from abundant ones of low affinity, to rare ones of high affinity, common to all metals. The model can also account for laboratory competition data involving Al(III), Fe(III) and trace elements, supporting the assumption of common binding sites. Field speciation data (116 examples) for Al in acid-to-neutral waters can be accounted for, assuming that 60-70 % (depending upon competition by iron, and the chosen fulvic acid : humic acid ratio) of the dissolved organic carbon (DOC) is due to humic substances, the rest being considered inert with respect to ion binding. After adjustment of the model parameter characterizing binding affinity within acceptable limits, and with the assumption of equilibrium with a relatively soluble form of Fe(OH)₃, the model can simulate the results of studies of two freshwater samples, in which concentrations of organically complexed Fe were estimated by kinetic analysis.The model was used to examine the pH dependence of Al and Fe binding by dissolved organic matter (DOM) in freshwaters, by simulating the titration with Ca(OH)₂ of an initially acid solution, in equilibrium with solid-phase Al(OH)₃ and Fe(OH)₃. For the conditions considered, Al, which is present at higher free concentrations than Fe(III), competes significantly for the binding of Fe(III), whereas Fe(III) has little effect on Al binding. The principal form of Al simulated to be bound at low pH is Al³⁺, AlOH²⁺ being dominant at pH >6; the principal bound form of Fe(III) is FeOH²⁺ at all pH values in the range 4-9. Simulations suggest that, in freshwaters, both Al and Fe(III) compete significantly with trace metals (Cu, Zn) for binding by natural organic matter over a wide pH range (4-9). The competition effects are especially strong for a high-affinity trace metal such as Cu, present at low total concentrations (∼1 nM). As a result of these competition effects, high-affinity sites in humic matter may be less important for trace metal binding in the field than they are in laboratory systems involving humic matter that has been treated to remove associated metals.     

The Homogeneous Finite-Difference Formulation of the P-SV-Wave Equation of Motion

Two different approaches to finite-difference modeling of the elastodynamic equations have been used: the heterogeneous and the homogeneous. In the heterogeneous approach, boundary conditions at interfaces are treated implicitly; in the homogeneous, they are explicitly discretized. We present a homogeneous finite-difference scheme for the 2-D P-SV-wave case. This scheme represents a generalization of earlier such schemes, being able to model media with arbitrary non-uniformities, provided only that all interfaces are aligned with the numerical grid. We perform a detailed comparison of the generalized homogeneous scheme with the analogous heterogeneous scheme, and show the two schemes to be identical for media with a spatially constynt Poisson's ratio. For media where Poisson's ratio is spatially varying, the schemes differ by terms first-order in the spatial step size. However, a comparison of the numerical results produced by the two schemes shows that the resulting differences are negligible for a wide range of values of the Poisson's ratio contrast.     

铬酸根离子在羟基铁离子-蒙脱石体系中的吸附行为研究

采矿、电镀、制革等行业废物排放及含Cr矿物风化可造成一些地区土壤和地下水的Cr污染 .Cr主要以三价和六价形式存在 ,其中Cr(Ⅵ )有强的迁移能力 ,对动植物均有很强的毒害性 .研究了在氧化、酸性条件下 ,土壤及土壤溶液中的蒙脱石和羟基铁离子共存时对铬酸根离子 (主要以HCrO2 - 4和CrO2 - 4等Cr(Ⅵ )形式存在 )的吸附行为 .进行了蒙脱石、羟基铁离子、Cr(Ⅵ )离子添加顺序不同的 3个系列的实验 ,重点研究了Cr的初始质量浓度、溶液 pH值、环境温度、吸附时间、溶液离子强度对 3个系列Cr吸附行为的影响 ,并与羟基铁离子体系进行了对比 .结果表明 ,蒙脱石 -羟基铁离子体系的Cr吸附能力明显强于蒙脱石而低于羟基铁离子 ,其Cr吸附率随Cr初始质量浓度、温度的升高和吸附时间的延长而降低 ,随离子强度的升高而升高 ,而pH值对不同系列的Cr吸附率有不尽相同的影响 .     

Book Review: Spatial Statistics: Methodological Aspects and Applications. By Marc Moore (ed.). Lecture Notes in Statistics 159. Springer,New York, 2001, 282 p., $69.95 (U.S.), paperback,ISBN 0-387-95240-3

Mathematical Geosciences -     

The Use of Seasonal Climate Forecasting in Policymaking: Lessons from Northeast Brazil

This article examines the use of seasonal climate forecasting in public and private efforts to mitigate the impacts of drought in Ceará, Northeast Brazil. Here, forecasts have been directed towards small scale, rainfed agriculturalists as well as state and local level policymakers in the areas of agriculture, water management, and emergency drought relief. In assessing possibilities and constraints of forecast application in Ceará, the present analysis takes into account three types of variables: (a)characteristics of the forecasts; (b) policymaking systems; and (c)institutional environments. We conclude that, on the one hand, several factors in the Ceará case have limited the effectiveness of seasonal climate forecast use. First, the current level of skill of the forecasts is inadequate for the needs of policy development and farmer decisionmaking. Second, forecast information application has been subject to distortion, misinterpretation and political manipulation. Third, focus on the forecast as a product until recently neglected to take into account end users' needs and decisionmaking behavior. On the other hand, climate forecasting has the potential to offer a dramatic opportunity for state and local level bureaucracies to embark on a path of proactive drought planning.