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391.
Moritz F. Lehmann Daniel M. Sigman Julie Granger Greg Cane 《Geochimica et cosmochimica acta》2007,71(22):5384-5404
We report 15N/14N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong 15N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration decreases (δ15N generally reaches 25-35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction (εcell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of εcell, while uncertain, show a negative correlation with bottom water [O2]; we propose that this relates to the at the depth of denitrification. The N isotope effect at the scale of nitrate sediment-water exchange (εapp) is ∼0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. εapp is much lower than εcell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O2] tends to yield more complete expression of εcell at the sediment-water scale, due to higher at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O2] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss (εsed, equivalent to εapp but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean εsed of ∼4‰ (∼2‰ if restricted to seafloor below 1 km depth). 相似文献
392.
Caswell JA 《Marine pollution bulletin》2006,53(10-12):650-656
Reducing marine-based public health risk requires strict control of several attributes of seafood products, often including location and conditions of catch or aquaculture, processing, and handling throughout the supply chain. Buyers likely will also be interested in other attributes of these products such as eco-friendliness or taste. Development of markets for improved safety, as well as for other quality attributes, requires an effective certification and tracking of these attributes as well as their communication to buyers. Several challenges must be met if labeling, particularly consumer labeling, is to support the development of markets for improved seafood safety. 相似文献
393.
Chedliya Ghemari Christophe Waterlot Anas Ayari Julie Leclercq Francis Douay Karima Nasri-Ammar 《Environmental Earth Sciences》2017,76(17):623
In this study, data on several metals (Cd, Pb, Zn and Cu) in soil and isopod Porcellio laevis taken at 21 sites from the most important industrial areas in Tunisia (Bizerte, Nabeul, Zaghouan, Sfax and Gabes) were presented. Heavy metal concentrations in both soil samples and isopods were determined using atomic absorption spectrometry. Soil contamination was estimated using the contamination factor (CF). On the other hand, the bioaccumulation factor (BAF) was determined to estimate metal accumulation in isopods. The CF values show that the level of contamination varies between sampled soils, which may be due to the source of pollution at each site. The BAF values allow defining the order of accumulation in P. laevis which was classified for the majority of the sampled sites as a macro-concentrator of Cu and Zn and a deconcentrator of Cd with some exceptions. A principal component analysis (PCA) was conducted between soil properties (pH, OM and CaCO3) and metal concentrations in soils. Through PCA, we obtained four groups in which soils were distinguished by their physicochemical properties and their metal concentrations. Moreover, linear multiple regressions with a downward stepwise procedure were conducted to test the relationships between the physicochemical parameters and metal concentrations in both soils and isopods. Thus, positive correlations (0.78 < R 2 < 0.99) were obtained for Pb considering dataset from the groups 1, 2 and for Zn with data of groups 2 and 3. Finally, results showed that P. laevis could be used as a bio-indicator for monitoring and reducing the impact of pollution in terrestrial ecosystems. 相似文献
394.
Projections of a drier, warmer climate in the U.S. Southwest would complicate management of the Colorado River system—yet these projections, often based on coarse resolution global climate models, are quite uncertain. We present an approach to understanding future Colorado River discharge based on land surface characterizations that map the Colorado River basin’s hydrologic sensitivities (e.g., changes in streamflow magnitude) to annual and seasonal temperature and precipitation changes. The approach uses a process-based macroscale land surface model (LSM; in this case, the Variable Infiltration Capacity hydrologic model, although methods are applicable to any LSM) to develop sensitivity maps (equivalent to a simple empirical model), and uses these maps to evaluate long-term annual streamflow responses to future precipitation and temperature change. We show that global climate model projections combined with estimates of hydrologic sensitivities, estimated for different seasons and at different change increments, can provide a basis for approximating cumulative distribution functions of streamflow changes similar to more common, computationally intensive full-simulation approaches that force the hydrologic model with downscaled future climate scenarios. For purposes of assessing risk, we argue that the sensitivity-based approach produces viable first-order estimates that can be easily applied to newly released climate information to assess underlying drivers of change and bound, at least approximately, the range of future streamflow uncertainties for water resource planners. 相似文献
395.
Ben?E.?MandlerEmail author Julie?M.?Donnelly-Nolan Timothy?L.?Grove 《Contributions to Mineralogy and Petrology》2014,168(4):1066
Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H2O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na2O content of 6.5 wt% at 67 wt% SiO2. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H2O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H2O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H2O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H2O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H2O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H2O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H2O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H2O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H2O, which may be used to look for “damp” fractionation sequences elsewhere. 相似文献
396.
Julie D. Stopar G. Jeffrey Taylor Victoria E. Hamilton Lauren Browning 《Geochimica et cosmochimica acta》2006,70(24):6136-6152
If water was ever present on Mars, as suggested by geomorphological features, then much of the surface and subsurface may have experienced chemical weathering. Among those materials most readily altered is olivine, which has been identified on the Martian surface with IR spectroscopy and Mossbauer techniques and occurs in Martian meteorites. We use geochemical models of olivine dissolution kinetics to constrain the residence time of olivine on the surface of Mars in the presence of liquid water. From these models, we have calculated maximum dissolution rates and minimum residence times for olivine as a function of temperature, pH, Fe-composition, and particle size. In general, the most favorable conditions for olivine dissolution are fayalite-rich compositions, small particle sizes, high temperatures, and acidic solutions that are far from equilibrium. The least favorable conditions for olivine dissolution are forsterite-rich compositions, large particle sizes, ultra-low temperatures, and a neutral pH solution near equilibrium. By using kinetic models of olivine dissolution to bound dissolution rates and residence times, we can make inferences about the temporal extent of aqueous alteration on the surface of Mars. Under favorable conditions (pH 2, 5 °C, and far from equilibrium) a relatively large 0.1 cm (radius) particle of Fo65 composition can completely dissolve in 370 years. Particles may last 102–104 times longer under less favorable conditions. However, residence times of a few million years or less are small compared to the age of most of the Martian surface. The survival of olivine on the surface of Mars, especially in older terrains, implies that contact with aqueous solutions has been limited and wet periods on Mars have been short-lived. 相似文献
397.
Darcy F. Morey George M. Crothers Julie K. Stein James P. Fenton Nicholas P. Herrmann 《Geoarchaeology》2002,17(6):521-553
Indian Knoll is the largest Archaic shell midden excavated by WPA archaeologists in Kentucky. Situated in a large alluvial valley, the site is not associated with a known river shoal as might be expected, making its fluvial and geomorphic setting of interest. Based on sediment cores and auger samples, undisturbed portions of the site remain despite extensive excavations. In undisturbed portions, a shell‐bearing layer is overlain by a shell‐free midden layer. Profiles of organic matter and calcium carbonate content for both layers are similar to those of other Green River shell middens. New radiocarbon determinations date the shell deposit at 5590–4530 cal yr B.P. Analysis of mussel species collected from the Indian Knoll indicates that shell fishing took place in a swiftly flowing, shallow to moderately deep setting of the main river channel. Overall, the prehistoric river setting adjacent to Indian Knoll was characterized by deeper water on average with variable but finer‐grained substrate compared to other Green River shell midden sites. © 2002 Wiley Periodicals, Inc. 相似文献
398.
Julie K. Stein 《Geoarchaeology》1991,6(2):192-192