Element cycling in organic rich sediments on the Pakistan margin

Law, Gareth (2009). Element cycling in organic rich sediments on the Pakistan margin. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001008e

Abstract

Benthic biogeochemical processes in ocean margin sediments are of global significance, yet they remain poorly defined. Here, the distribution of major, minor and trace elements in porewaters and sediments, together with measurements of microbially mediated organic carbon oxidation reactions were determined at sites above, within, and below the oxygen minimum zone (OMZ) on the Pakistan margin of the Arabian Sea. Sampling was conducted before and during the late-to-post southwest monsoon, with the goal of assessing how low bottom-water O2 concentrations and high organic matter depositional fluxes affect sediment processes.

Element analysis revealed that the lithogenic fraction of Pakistan margin sediments was largely dominated by fluvial material, which was derived from the nearby River Indus. In comparison, aeolian deposition on the Pakistan margins appeared to be negligible. Element analysis also revealed that Fe and Mn were actively cycled at all of the Pakistan margin study sites; however, Mn supply to the OMZ sediments was limited by Mn reduction within the OMZ water-column. In turn, Mn was effectively transported out of the OMZ and accumulated in underlying oxic sediments. In contrast, Fe supply to the sediments appeared to be unaffected by the OMZ; however, benthic porewater Fe fluxes from the sediments to the water-column were documented at the OMZ sites. A range of other redox sensitive elements (e.g. Ba, Mo, and U) were also cycled in the Pakistan margin sediments; however in comparison to sediments deposited on other upwelling affected continental margins, the degree of trace element cycling on the Pakistan margin is low.

Sulphur cycling, was also investigated at the Pakistan margin sites. Here direct measurements of sulphate reduction rates (³⁵S-SO₄^2- incubation) revealed an apparent inhibition of sulphate reduction in Arabian Sea sediments. A number of factors were investigated as possible inhibiting mechanisms. The dominance of unreactive marine OM in the Pakistan margin sediments and microbial co-competition between sulphate reducing and iron reducing bacteria were suggested as potential limiting factors. Finally, the measured rates of Mn, Fe, and S-cycling at the Pakistan margin sites were combined with commensurate rates of O2 uptake and denitrification to examine if low-O2 concentrations within the OMZ suppressed microbially mediated C-oxidation reactions. Considering both seasons, C-oxidation rates at the Pakistan margin sites were found to range from 0.73 to 4.86 mmol C m^-2 d^-1. Generally, sites within the OMZ core and those at the lower OMZ transition had lower carbon oxidation rates (0.73-2.90 mmol C m^-2 d^-1) than those located below the OMZ (3.13-4.86 mmol C m^-2 d^-1) which lie under oxygenated waters. This suggests that low bottom-water O2 concentrations may suppress overall rates of OM decomposition.