Biotic and a-biotic Mn and Fe cycling in deep sediments across a gradient of sulfate reduction rates along the California margin

The coupling between the biological and a-biotic processes controlling trace metals in deep marine sediments are not well understood, although the fluxes of elements and trace metals across the sediment-water interface can be a major contribution to ocean water. Four marine sediment profiles (ODP leg 167 sites 1011, 1017, 1018 and 1020)were examined to evaluate and quantify the biotic and abiotic reaction networks and fluxes that occur in deep marine sediments. We compared biogeochemical processes across a gradient of sulfate reduction (SR) rates with the objective of studying the processes that control these rates and how they affect major elements as well as trace metal redistribution. The rates of sulfate reduction, methanogenesis and anaerobic methane oxidation (AMO) were constrained using a multicomponent reactive transport model (CrunchFlow). Constraints for the model include: sediment and pore water concentrations, as well as %CaCO3, %biogenic silica, wt% carbon and δ¹³C of total organic carbon (TOC), particulate organic matter (POC) and mineral associated carbon (MAC). The sites are distinguished by the depth of AMO: a shallow zone is observed at sites 1018 (9 to 19 meters composite depth (mcd)) and 1017 (19 to 30 mcd), while deeper zones occur at sites 1011 (56 to 76 mcd) and 1020 (101 to 116 mcd). Sulfate reduction rates at the shallow AMO sites are on the order 1x10^-16 mol/L/yr, much faster than rates in the deeper zone sulfate reduction (1-3x10^-17 mol/L/yr), as expected. The dissolved metal ion concentrations varied between the sites, with Fe (0.01-7 μM) and Mn (0.01-57 μM) concentrations highest at Site 1020 and lowest at site 1017. The highest Fe and Mn concentrations occurred at various depths, and were not directly correlated with the rates of sulfate reduction and the maximum alkalinity values. The main processes that control cycling of Fe are the production of sulfide from sulfate reduction and the distribution of Fe-oxides. The Mn distribution in the deep sediments is controlled primarily by precipitation of MnCO3 and absorption on mineral surfaces. The Mn and Fe transformations occurring in deep sediment appear to be primarily abiotic and are controlled by mineral dissolution and internal cycling between iron manganese, and sulfide. However, the biotic processes taking place in the shallow sediments near the seawater-sediment interface determine both the extent of subsequent deeper abiotic transformations and the diffusion of Mn and Fe to sea water. The Fe cycling is complicated by reaction with sulfide, while dissolved Mn provides a more straightforward profile because it is not consumed by sulfidation reactions. Modeling of the dissolved and solid elements is used to establish the relationship between biogeochemical reactions and trace metal variations, and to better constrain the biotic and abiotic processes that affect the trace metal distributions in the sediment column.