The influence of humic substances on the geochemistry of iodine in nearshore and hemipelagic marine sediments

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Iodine is characteristically enriched at the surface of hemipelagic and nearshore sediments deposited under oxygenated conditions. In such sediments, bulk I/C org ratios usually decrease with depth to values which are characteristic of anoxic sediments, reflecting a preferential release of I during early diagenesis. There is some debate as to whether sedimentary I is associated with the iron oxyhydroxide phase or with the organic fraction, and whether the decrease in I/C org with depth is due to the dissolution of the iron oxyhydroxides or the decomposition of labile organic matter. It is shown that in a surficial hemipelagic sediment sample and in a nearshore sediment core I 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 I by and from sedimentary humic substances also suggest a mechanism whereby humic materials reduce iodate at the sediment/water interface to an electrophilic I species which further reacts with the organic matter to produce iodinated organic molecules. During burial, this excess I 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/C org ratio with depth observed in many nearshore and hemipelagic sediments.

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