The goal of the work funded by C-DEBI is to identify and quantify deep Fe-driven anaerobic oxidation of methane (Fe-AOM) in continental margin sediments. We analyzed samples from IODP Expedition 375 Site U1519, Holes D and E, which were drilled specifically to characterize the biogeochemistry of the Hikurangi Margin offshore New Zealand (n=43). An additional 100 samples were collected for trace metal concentrations, δ56Fe, organic chemical characterization and metal speciation analyses on a 2019 research expedition on the RV Revelle at the Hikurangi margin. At Site U1519, the concentration of Fe at the top of the core is relatively low due to of Fe-sulfide precipitation. However, 10 m below the SMTZ the Fe concentration increases to an average of 115.3μM (range 38.8 μM - 195.2μM) and elevated concentrations persist to 85 mbsf, confirming the reduction of Fe oxide minerals producing Fe2+ within the methanogenic zone. It also indicates that our glove bag processing of the samples worked and kept the sediment and porewater from becoming exposed to oxygen. The preliminary data confirms our original hypotheses that there is a broad region of unconstrained Fe reduction under the SMTZ extending to at least 85 mbsf, and that much of the reduced Fe is precipitated into authigenic carbonates. C. McKinley has been awarded a DOE Methane Hydrates Postdoctoral Fellowship administered by the National Research Council to continue this work and we will build on this data set to evaluate the drivers of Fe reduction and to constrain the fate of deep Fe.
The depth of oxygen penetration into marine sediments differs considerably from one region to another. In areas with high rates of microbial respiration, O2 penetrates only millimetres to centimetres into the sediments, but active anaerobic microbial communities are present in sediments hundreds of metres or more below the sea floor. In areas with low sedimentary respiration, O2 penetrates much deeper but the depth to which microbial communities persist was previously unknown. The sediments underlying the South Pacific Gyre exhibit extremely low areal rates of respiration. Here we show that, in this region, microbial cells and aerobic respiration persist through the entire sediment sequence to depths of at least 75 metres below sea floor. Based on the Redfield stoichiometry of dissolved O2 and nitrate, we suggest that net aerobic respiration in these sediments is coupled to oxidation of marine organic matter. We identify a relationship of O2 penetration depth to sedimentation rate and sediment thickness. Extrapolating this relationship, we suggest that oxygen and aerobic communities may occur throughout the entire sediment sequence in 15–44% of the Pacific and 9–37% of the global sea floor. Subduction of the sediment and basalt from these regions is a source of oxidized material to the mantle.