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Understanding biogenic silica (bSi) dissolution kinetics in margin environments is important in assessing the global silicon cycle, a cycle closely linked to the global carbon cycle. This understanding is also essential to answer the question of whether bSi content in marine sediment is a valid indicator of productivity in the overlying surface ocean. In this study, plankton tow, sediment trap, and sediment samples were collected at sites in three Southern California borderland basins. Batch dissolution experiments with plankton tow and sediment trap materials (conducted in the laboratory at 22 °C) showed linear dissolution kinetics, from which mean dissolution rate constants of 0.05 d? 1 for plankton tow samples and 0.07 d? 1 for sediment trap samples could be calculated. The dissolution rate constants for both types of samples showed seasonal variability but not the same seasonal patterns. Faster dissolution was observed with sediment trap samples collected at 800 m than at 550 m. With sediment multi-core samples, non-linear dissolution kinetics was observed, which complicates the direct comparison of dissolution rates. Nonetheless, dissolution appeared to be slower for the sediments samples than for samples collected from the water column and to decrease with depth in the sediments. Rate constants for surface sediment (0–0.5 cm) were at least 3–5 times less, and sediments at depths > 2 cm had rate constants at least 6–13 times less than those for material sinking to the sediment surface at these sites. Dissolution experiments conducted with Santa Barbara Basin surface sediment samples amended with dissolved aluminum (Al) and San Pedro Basin trap samples amended with enriched detrital materials (obtained by leaching bSi from sediment samples) suggested that dissolution was inhibited by Al and that the sediments from the different basins varied in the extent of Al release.  相似文献   
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A series of recent peat samples from tropical, sub-tropical and temperate locations were investigated using coal petrographical and organic geochemical techniques. Maceral and biomarker compositions were examined to see if they correlated or diverged from the known environment of deposition of the peats.The proportion of humotelinite was related to the water depth and the frequency of dry periods. The pH and trophic level have a modifying influence on the preservation of tissues. Furthermore, the proportions of sclerotinite in the peats seem to reflected the oxygen status in the mire, which was also mirrored in the abundance of humic and fulvic acids.A facies diagram was calculated for the peat samples to see how maceral indices would fare in interpreting their known environments of deposition. The results proved to be generally unsuccessful as the herbaceous and woody peats did not always give the predicted high humodetrinite and humotelinite compositions, respectively.The aliphatic hydrocarbon fraction from the extracts of the peat samples show that mainly odd-numbered n-alkanes are present. Diterpenoids, triterpenoids and hopanoids were found in most samples and were mainly unsaturated in nature. The relative proportion of hopanoids in pears appears to be related to bacterial activity in acidic environments. De-A-triterpenoids were also detected in the peat stage of diagenesis.In an angiosperm dominated peat type (Mariscus), polycyclic aromatic triterpenoids were present in the aromatic fraction of the extract. Their occurrence and proportion appears to be related to the degree of humification. In a Taxodium peat, mono-, di-and triaromatic diterpenoids were detected. The presence of these aromatic biomarkers indicate the rapid formation of these compounds in the peat stage, mainly mediated by microbial activity.  相似文献   
76.
In southern Florida, a vast network of canals and water control structures mediate freshwater discharge into the coastal zone. Management protocol for one such canal network (C-111) is being modified in part to try to improve habitat for estuarine fish and wading birds in northeastern Florida Bay, an estuarine part of Everglades National Park. Changes in canal management could alter the spatial and temporal salinity regime in the estuary. To better predict the effect of such changes on estuarine habitat, abundances of submersed vegetation and benthic animals were sampled repeatedly at 12 stations that differed in salinity. A variety of other parameters were also measured (nutrients, light, temperature, oxygen, sediment characteristics, and others). Mean salinity among stations ranged from 11.4‰ to 33.1‰. Densities of benthic plants and animals differed among stations by several orders of magnitude. The standard deviation of salinity was the best environmental correlate with mean plant biomass and benthic animal density: less biota occurred at stations with greater fluctuations in salinity. The two stations with the least plant biomass also had the highest mean water temperatures. In a stepwise multiple regression analysis, standard deviation of salinity accounted for 59% of the variation in the logarithm of mean plant biomass among stations. For every 3‰ increase in the standard deviation, total benthic plant biomass decreased by an order of magnitude. Mean water temperature accounted for only 14% of the variation, and mean salinity was not included for lack of significance. At stations with widely fluctuating salinities, not only was biomass low, but species dominance also frequently changed. Severe fluctuation in salinity may have prevented abundant benthos by causing physiological stress that reduced growth and survival. Salinity may not have remained within the range of tolerance of any one plant species for long enough to allow the development of a substantially vegetated benthic community. Hence, gaining control over salinity fluctuation may be the key to estuarine habitat improvement through canal management in southern Florida.  相似文献   
77.
Thermodynamic properties of 32 dissolved thorium species and 9 thorium-bearing solid phases have been collected from the literature, critically evaluated and estimated where necessary for 25°C and 1 atm pressure. Although the data are incomplete, especially for thorium minerals and organic complexes, some tentative conclusions can be drawn. Dissolved thorium is almost invariably complexed in natural waters. For example, based on ligand concentrations typical of ground water (ΣCl = 10 ppm, ΣF = 0.3 ppm, ΣSO4 = 100 ppm, andΣPO4 = 0.1 ppm), the predominant thorium species are Th(SO4)02, ThF2+2, and Th(HPO4)20below pH ≈ 4.5; Th(HPO4)2?3 from about pH 4.5 to 7.5; and Th(OH)04 above pH 7.5. Based on stability constants for thorium citrate, oxalate and EDTA complexes, it seems likely that organic complexes predominate over inorganic complexes of thorium in organic-rich stream waters, swamp waters, soil horizons, and waterlogged recent sediments. The thorium dissolved in seawater is probably present in organic complexes and as Th(OH)04. The tendency for thorium to form strong complexes enhances its potential for transport in natural waters by many orders of magnitude below pH 7 in the case of inorganic complexing, and below about pH 8 when organic complexing is important. The existence of complexes in addition to those formed with hydroxyl, is apparent from the fact that measured dissolved thorium in fresh surface waters (pH values generally 5–8) usually ranges from about 0.01 to 1 ppb and in surface seawater (pH = 8.1) is about 0.00064 ppb. This may be contrasted with the computed solubility of thorianite in pure water which is only 0.00001 ppb Th as Th(OH)04 above pH 5. Although complexing increases the solubility of thorium-bearing heavy minerals below pH 8, maximum thorium concentrations in natural waters are probably limited in general by the paucity and slow solution rate of these minerals and by sorption processes, rather than by mineral-solution equilibria.  相似文献   
78.
In recent years glacial geologists have become increasingly interested in the thermal regime at the bases of polar ice sheets, and have attempted to interpret certain Pleistocene glacial features in terms of this thermal regime. This effort has suffered from a lack of a reasonably comprehensive but qualitative review of factors controlling the temperature at the base of a polar glacier; this paper attempts to fill this void. Using the Laurentide Ice Sheet as an example, it is shown that under cold climatic conditions basal melting is likely to occur only beneath the lower part of the ablation area. If the climate is somewhat warmer, or if the accumulation rate is sufficiently low, basal melting may also occur beneath the accumulation area. The zone of greatest basal melt-water production is probably beneath the lower part of the ablation area.  相似文献   
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The reliability of habitat maps that have been generated using Geographic Information Systems (GIS) and image processing of remotely sensed data can be overestimated. Habitat suitability and spatially explicit population viability models are often based on these products without explicit knowledge of the effects of these mapping errors on model results. While research has considered errors in population modeling assumptions, there is no standardized method for measuring the effects of inaccuracies resulting from errors in landscape classification. Using landscape‐scale maps of existing vegetation developed for the USDA Forest Service in southern California from Landsat Thematic Mapper satellite data and GIS modeling, we performed a sensitivity analysis to estimate how mapping errors in vegetation type, forest canopy cover, and tree crown size might affect delineation of suitable habitat for the California spotted owl (Strix occidentalis occidentalis). The resulting simulated uncertainty maps showed an increase in the estimated area of suitable habitat types. Further analysis measuring the fragmentation of the additional patches showed that they were too small to be useful as habitat areas.  相似文献   
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