Program

Redox-sensitive elements such as molybdenum (Mo) and uranium (U) have been studied during the past decades in the modern and ancient oceans. The geochemical behavior of these elements in marine environments is increasingly understood. At cold seeps, oxidation and re-oxidation of reduced chemicals (e.g. methane, organic matter) lead to sulfate and oxygen depletion in pore water, which induces changes in redox conditions and consequently abundance of redox-sensitive elements. However, the factors controlling geochemical cycle of these elements and its role in deep water biogeochemical cycling remain poorly constrained. Here, we characterized the Mo and U geochemistry of authigenic carbonates, pore waters, and sediments sampled from cold seeps located at the Congo Fan (CF), the Gulf of Mexico (GoM), and the South China Sea (SCS). A good correlation was observed between the consumption of sulfate and the removal of Mo in pore waters at four sites from SCS. It suggests that the anaerobic oxidation of methane (AOM) has a significantly influence on the geochemical cycle of Mo at seeps. Five intervals of strongly ¹³C-depleted carbonates with incorporated chemosynthetic bivalve shell were identified in gas hydrate-bearing sediment cores from SCS, indicating that methane seepages were intense. Paired occurrences of seep carbonates and pronounced Mo enrichments occurred in the bulk sediments. These results suggest that strongly Mo enrichments in sediments have potential to be a good indicator of intense methane seepages and possible occurrence of gas hydrate dissociation. We find that the patterns of Mo-U covariation in seep carbonates from CF and GoM, including authigenic Mo (Mo_auth) and U (U_auth) enrichments as well as (Mo/U)_auth ratios is a promising tool to constrain the formation environment of authigenic carbonate and assess past variability of redox conditions at seeps. The covariation pattern of Mo-U in the carbonates and sediments suggested that the Mo enrichments form at sulfidic condition usually restricted to pore water environment caused by AOM. The strongly depletions of Mo in the pore waters and resultant enrichments of Mo in the sediments and carbonates suggest that cold seep environments may serve as an important potential sink in the marine geochemical cycle of Mo.

Acknowledgments: NSF of China (Grants: 41606048, 41422602, and 41372012).