Award: OCE-1155562

Our overall goal was to better characterize carbon transformation processes occurring in the deep sub-surface of continental margin sediments, and to evaluate their significance in material exchange between sediments and the water column. Specifically, we have identified the major sinks for sulfate in the transition zone between the sedimentary regions of sulfate reduction and methanogenesis (the SMTZ), and we have also evaluated the fate of pore water dissolved organic carbon (DOC) that is produced in sub-surface sediments. We focus on pore water DOC because it is the key intermediate pool in organic matter degradation processes such as sulfate reduction and methanogenesis. We carried out our studies in the anoxic sediments of Santa Barbara Basin and Santa Monica Basin, both of which are part of the California Borderland region. Our results have verified many aspects of a model for DOC cycling in anoxic sediments that we recently proposed. Specifically, we see that most particulate organic matter is rapidly degraded (through reactive DOC intermediates) to inorganic end-products (e.g., DIC), with only a small amount of net production of refractory DOC. Our results similarly support the suggestion that the initial stages of particulate organic matter degradation (i.e., hydrolysis or oxidative cleavage of this material) represent the rate-limiting portion of the overall sediment organic matter degradation process. We have also observed that DOC produced in the deeper methanogenic sediments of Santa Barbara Basin sediments is not responsible for the observed imbalance between sulfate and DIC fluxes into the SMTZ of these sediments. Note that in Santa Barbara Basin sediments the SMTZ occurs at a sediment depth of ~125 (±20) cm. Most refractory DOC produced in Santa Barbara and Santa Monica Basin sediments escapes the sediments as a benthic flux. There is also deep source of refractory DOC to Santa Barbara Basin sediments that is essentially radiocarbon-"dead" (note that here "deep" represents sediment depths > 4.6 m, our maximum depth of sediment sampling). However, the upward flux of this material is small when compared to in situ production higher up in the sediment column of other forms of refractory DOC with more modern radiocarbon signatures. The lifetime of refractory pore water DOC in Santa Barbara Basin sediments is comparable to that estimated for refractory DOC in the marine water column. This supports arguments made elsewhere that sediment benthic fluxes may represent an important source of refractory DOC to the oceans. This benthic flux may contribute refractory and pre-aged DOC to the water column, depending on any differences in the reactivity of the different sub-components of the refractory pore water DOC pool in anoxic sediments versus the oxic marine water column. Anaerobic oxidation of methane is an important sink for sulfate in the SMTZ, and our results have shown the importance of active methanogenesis and upward fluxes of methane from deeper sources in controlling carbon dynamics in the SMTZ. These deep methane sources may include relict gas hydrates or ancient gas deposits. Furthermore, our results suggest that such deep methane sources are not unique to Santa Barbara Basin sediments but may be a common occurrence in many continental margin sediment settings. These results further indicate the effectiveness in general of the SMTZ as a barrier for the escape of this deep methane from continental margin sediments into the water column and perhaps into the atmosphere. Because methane is an important greenhouse gas, understanding these dynamics are important for addressing questions about how changes in sediment methane cycling may have affected global temperatures in the geologic past, as well as how rising bottom water temperatures and deoxygenation, two likely outcomes of anthropogenic global warming, will impact the effectiveness of this barrier in the future. Last Modi...