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Some aspects of the geochemistry of sulphur and iodine in marine humic substances and transition metal enrichment in anoxic sediments François, Roger


The evolution of the sulphur content of humic substances extracted from a near-shore sediment core was investigated. Special attention was taken to avoid S contamination of the humic materials during sample handling and extraction. The S/C ratios increased continuously with depth to values which strongly suggest S addition to the humic matrix during early diagenesis by reactions between organic matter and H₂S or its oxidation products. The light isotopic composition of this organic sulphur supports this view; however, subsequent isotopic exchange has obscured the mechanism initially involved. Since a large fraction of the enrichment occurred above the sulphidic zone, redox boundaries, such as the interface of anoxic microniches within the more oxidized zones, or the sulphidic/suboxic boundary of the sediment column, must have been important sites for S addition. The influence of sulphur enrichment on the complexing capacity of humic materials was also investigated, and it was shown that S-addition increases significantly the number of sites on which Cu is irreversibly bound. Iodine is characteristically enriched at the surface of hemipelagic and nearshore sediments deposited under oxygenated conditions. In such sediments, bulk I/Corg ratios usually decrease with depth to values which are characteristic of anoxic sediments, reflecting a preferential release of iodine during early diagenesis. There is some debate as to whether sedimentary iodine is associated with the iron oxyhydroxide phase or with the organic fraction, and whether the decrease in I/Corg with depth is due to the dissolution of the iron oxyhydroxides or the decomposition of labile organic matter. In this study, it is shown that in a surficial hemipelagic sediment sample and in a nearshore sediment core iodine is mainly associated with the organic fraction and, moreover, that humic substances are involved in the surficial iodine enrichment. Laboratory experiments on the uptake and release of iodine by and from sedimentary humic substances also suggest a mechanism whereby humic materials reduce iodate at the sediment/water interface to an electrophilic iodine species which further reacts with the organic matter to produce iodinated organic molecules. During burial, this excess iodine could be displaced from the organic matrix by nucleophiles such as sulphide ions or thiosulphate, thus providing a possible explanation for the decrease in I/corg ratio with depth observed in many nearshore and hemipelagic sediments. Bulk metal concentrations were measured in the sediments of Saanich Inlet in an attempt to establish the occurrence of trace metal enrichments in the anoxic central basin. Ba, Ni, V, Cr, Zn, Pb, Cu, and Mo were found to be enriched in the anoxic ooze over the possible contributions from lithogenous sources. Spatial and seasonal variations in the chemical composition of the settling particulates collected with interceptor traps gave further indications of the mode of incorporation of these metals. Biogenic Ba and Cr appeared to be associated with opaline silica, although alternative explanations are also possible, particularly for Ba. Zinc seemed to be added to the sediment essentially in association with planktonic materials, while Cu required an additional source directly linked to the anoxic environment. Similarly, Ni, V, and Mo were added to the anoxic sediments by reactions occurring at the sediment-water interface. In the nearshore environment studied here, these metals were not associated to any significant extent with planktonic materials, particularly Ni and Mo. Of all the elements analyzed, Mo showed the largest enrichment in the anoxic sediments of Saanich Inlet.

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