Shale weathering profiles show Hg sequestration along a New York–Tennessee transect
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For the first time in the history of the researches on nutrition of benthic organisms in the Northern Caspian Sea, the assessment of statistical regularities between the content of organic matter in soils and benthos is made. Based on the ascertained statistical connections, there were defined 2 groups of benthic organisms: the biomass and number of the 1 st group are negatively correlated with the level of accumulation of organic carbon, while the biomass and number of the 2 nd group are positively dependent on the content of organic carbon. The concentration of organic carbon in bottom sediments of the Northern Caspian Sea for the period 1994-2011 changed from 0.01 to 2.82 %. Territorial distribution was heterogeneous. The greatest accumulation of organic carbon was fixed in sludgy soils. The destruction of organic matter in bottom sediments is caused by the abundance and biomass of infauna ( Chironomidae, Hypaniola kowalevskyi, Hediste diversicolor ). This group of organisms reduces a quantity of organic matter enhancing oxygen flow due to mechanical effect on the ground during the migration period. The coefficient of correlation between organic carbon and quantitative indicators of this group of organisms changed from -0.51 to -0.89. The most significant contact was fixed between organic carbon and the number of Chironomidae. The accumulation of organic carbon in the bottom sediments influences the abundance and the biomass of some representatives of nektobenthos ( Mysidae ), endobios ( Didacna protracta ), collectors ( Abra ovata ) and filter feeders ( Hypaniа angusticostata ). A positive correlation indicates an important role of organic matter in the nutrition of these hydrobionts. The coefficient of correlation between organic carbon and the number and biomass of the above-listed organisms changed from +0.67 to +0.93. The highest values of the correlation coefficient are fixed between the organic carbon content and the number of Paramysis ullskyi ( Mysidae ).
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To understand the processes controlling production, accumulation, and preservation of organic matter in the Lower Oxford Clay (LOC), we determined the hydrogen index (HI), the oxygen index (OI), the Tmax (from Rock-Eval), the content of total organic carbon (TOC), total carbon and total sulfur, and the carbon isotopic composition of bulk organic matter from 160 samples collected from 6 different quarries and one continuous core. With concentrations of TOC varying between 0.5% and 16.6%, the LOC is an organic-rich shale. For samples dominated by organic matter of phytoplanktonic origin, the hydrogen and oxygen indices and the Tmax (~418°) indicate low levels of maturity, and, thus, the shallow burial of the LOC through geologic time. Two main sources of organic matter can be distinguished: a major phytoplanktonic source with high HI and low OI and a minor terrestrial source with low HI and high OI. A third group, represented by samples with low HI and low OI, consists mainly of altered materials from the Middle Oxford Clay and the LOC. Selection of samples for chemical analysis was based on the macrofaunal assemblages defined by Duff (1975). These various biofacies are characterized by specific organic geochemical features indicating the relationship between conditions affecting faunal assemblages and those controlling accumulation and preservation of organic matter. For example, Duff's ‘deposit feeder shales', which are dominated by epifaunal bivalves and are depleted in infaunal organisms, exhibit the highest concentration and best preservation of marine organic matter, with an average TOC of 6.8% for 56 samples analyzed. The preservation of such organic matter requires a dysaerobic water column and a high sedimentation rate. Carbon isotopic compositions within the ‘deposit feeder shale’ biofacies (−27.6 to −23.2±) appear to have been controlled by the intensity of primary productivity. The highest-TOC, marine-dominated, 13 C-rich samples reflect photosynthetic drawdown of dissolved-CO 2 level, and, thus, originated in highly productive environments. On the other hand, variations in the carbon isotopic composition of organic matter in shell beds (−27.5 to −26±) probably reflect heterotrophic reworking of the organic matter, winnowing of the sediments, and mixing with a source of organic matter enriched in 13 C, such as wood (δ 13 C from −25 to −23±). Such mixing phenomena may also explain the high variability of the carbon isotopic compositions of TOC-depleted and altered samples from the Middle and Upper Oxford Clay. The environment of deposition of the LOC would be characterized by the alternation of two major conditions: 1) periods of high productivity, dysoxic water column and high sedimentation rate leading to the development of organic-rich shales dominated by phytoplanktonic organic matter, and 2) periods of low productivity, oxic water column and high current activity implying winnowing and alteration of organic matter, and leading to the formation of shell beds where marine and terrestrial organic matter are mixed.
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