Isotope evidence for the enrichment mechanism of molybdenum in methane-seep sediments: Implications for past seepage intensity.

Methane release from marine sediments strongly influences the local seafloor environment and ecosystems, and may impact Earth's climate system. Recent studies revealed anomalous molybdenum (Mo) enrichment in seep sediments, which was linked to methane release events. Marine seep sediments are a potential sink of Mo for the global ocean, while the mechanisms leading to local Mo enrichment are not fully understood. The sediments from a gas hydrate-bearing area of the South China Sea analyzed herein reveal authigenic Mo (Mo auth) contents ranging from 0 and 31.4 µg/g and δ98Mo auth values ranging from 0.18 ‰ to 3.31 ‰. The range of δ98Mo auth values of seep sediments is therefore similar to values of modern iron-rich sediments with low concentrations of dissolved porewater hydrogen sulfide and sediments deposited under weakly euxinic sediments. Among the obtained South China Sea data, the more positive δ98Mo auth values (>ca. 1.5 ‰) are interpreted to reflect diffusion of seawater Mo into the sediment at moderate seepage rates and Mo isotope fractionation during the formation of thiomolybdates in the sulfidic seep environment. The lower δ98Mo auth values (<ca. 1.5 ‰), coinciding with high Fe/Al and Mn/Al ratios of sediments, are interpreted to represent the release of Mo upon reductive dissolution of iron and manganese oxyhydroxides, following the delivery of the oxyhydroxides to the sediment during prominent particulate shuttle processes at high seepage rates. Our study suggests a prominent role of particulate shuttle processes in Mo sequestration at marine methane seeps, and sets the stage for the development of a new Mo isotope proxy that may help to constrain past methane seepage intensity. [ABSTRACT FROM AUTHOR]