Coupling In Situ Depth Profiles of Redox Species with Benthic Flux Measurements to Quantify Carbon Remineralization Processes in Marine Sediments

Rates of natural organic matter (NOM) degradation are difficult to quantify directly in marine sediments and are typically determined by measuring total (TOU) or diffusive (DOU) oxygen uptake fluxes assuming oxygen as the ultimate oxidant. In areas of high NOM input, dissolved oxygen is usually depleted within a few millimeters of the sediment-water interface, and nitrate, manganese and iron oxides, and sulfate are used as terminal electron acceptors at depth. To accurately quantify NOM mineralization rates using DOU or TOU fluxes, reduced metabolites produced during these processes have to be completely reoxidized by dissolved oxygen. In marine sediments receiving a substantial flux of NOM, however, DOU and TOU fluxes may underestimate NOM oxidation rates, as the chemical reduction of manganese and iron oxides by sulfides and the burial of sulfur and carbonate minerals may prevent aerobic reoxidation of these metabolites. To accurately assess carbon oxidation rates in coastal sediments, it is therefore necessary to consider individual terminal electron accepting processes and benthic fluxes simultaneously. Benthic landers carrying conventional water sampling systems were equipped with a potentiostat and micromanipulator to simultaneously acquire benthic fluxes and voltammetric depth profiles of the main redox species in pore waters (O_2(aq), Mn²⁺, Fe²⁺, ΣH₂S) of a variety of continental margin sediments. Sediment cores were also collected in parallel to characterize the depth profiles of DIC, NH₄⁺, NO₃^-, and SO₄^2-. The one dimensional reactive transport model MATSEDLAB was used to fit pore water depth profiles and estimate the burial flux of manganese, iron, and sulfur in these sediments. These burial fluxes were then combined with TOU fluxes to constrain carbon remineralization rates along a transect from estuarine to continental slope sediments.