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This study suggests that varying concentration of dissolved oxygen of overlying bottom water influences geochemical fractionation, speciation, and oxidation state of sedimentary Cr in a marine system. The nature of sedimentary organic matter (labile or nonlabile) and Fe speciation also controls the geochemical fractionation and oxidation state of Cr in the sediment from the continental margin across the oxygen minimum zone of the Arabian Sea. Increasing concentration of sedimentary organic matter (under hypoxic conditions) increased association of Cr with sedimentary organic binding phases. The association of Cr with Fe‐Mn‐oxyhydroxide phase gradually decreased with decreasing dissolved oxygen concentration due to the reduction and dissolution of Fe (III). The sedimentary organic matter was found to be the major hosting phase for Cr (VI) in the shelf sediments. The decrease in sedimentary Fe (III) concentration under hypoxic conditions also prevented Cr (VI) reduction. This study suggests that the type of sedimentary organic matter and Fe (III) concentration may influence benthic exchange fluxes of Cr under variable redox conditions in many coastal/margin systems. [ABSTRACT FROM AUTHOR]

Sedimentary organic matter (SOM) plays an important role in hosting and reducing Hg II in marine/estuarine sediment. This study provides a better understanding on the influence of nature of SOM, in regulating sedimentary mercury (Hg) and elemental mercury (Hg 0 ) distribution, and speciation in the Zuari and Mandovi Estuaries that are representative of monsoon fed tropical estuaries, located in the central west coast of India. Salinity of the overlying water column controlled the physical and chemical characteristics of SOM in the estuarine systems. The high molecular weight (MW) SOM dominated at the mid and upstream (low salinity region) of the estuaries, whereas, the low MW SOM prevailed at the downstream (high salinity region). Sediment Hg showed more affinity towards the SOM of high MW. Increasing MW of SOM increased total sedimentary Hg T in both the estuaries. SOM with low MW in the estuarine sediment displayed a negative relationship with the sediment Hg concentration. Distribution of Hg 0 concentration in the estuarine sediment suggests that reduction of Hg II in presence low MW SOM was a dominant process. It was also found that distribution and speciation of Hg 0 in the estuarine sediment depends on the quantity, quality of the SOM, and the total sediment Hg loading. This study demonstrated that the competition between Hg-SOM complexation and Hg II reduction by SOM controls Hg II /Hg 0 distribution in tropical estuarine sediment systems., (Copyright © 2019 Elsevier B.V. All rights reserved.)