Carbon cycling in Santa Barbara Basin sediments: A modeling study
The primary input of organic matter to almost all marine sediments comes from deposition at the sediment surface. However, in many continental margin settings, reduced carbon can also be added to sediments from below—for example, from “deep” geologic hydrocarbon reservoirs derived from ancient source rocks or from the decomposition of deeply buried gas hydrate deposits. To examine the impact of these two differing reduced carbon inputs on sediment biogeochemistry, a modified reaction-transport model for anoxic marine sediments is described here and applied to data from sediment cores in Santa Barbara Basin to a depth of 4.6 m. Excellent model fits yield results consistent with previous studies of Santa Barbara Basin and other continental margin sediments. These results indicate that authigenic carbonate precipitation in these sediments is not centered around the sulfate-methane transition zone (SMTZ), as is seen in many other sedimentary environments but occurs at shallower depths in the sediments and over a relatively broad depth range. Sulfate profiles are linear between the surface sediments (upper ∼20 cm) and the top of the SMTZ (∼105 cm) giving the appearance of refractory particulate organic carbon (POC) burial and conservative sulfate behavior in this intermediate region. However, model results show that linear profiles may also occur when high rates of sulfate reduction occur near the sediment surface (as organoclastic sulfate reduction [oSR]) and in the SMTZ (largely as anaerobic oxidation of methane) with low, but nonzero, rates of oSR in-between. At the same time, linearity in the sulfate profile may also be related to downward pore-water advection by compaction and sedimentation plus a decrease with depth in sulfate diffusivity because of decreasing porosity. These model-determined rates of oSR and methanogenesis also result in a rate of POC loss that declines near-continuously in a logarithmic fashion over the entire sediment column studied. The results presented further here indicate the importance of a deep methane flux from below on sediment biogeochemistry in the shallower sediments, although the exact source of this methane flux is difficult to ascertain with the existing data.
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Document Type: Research Article
Publication date: May 1, 2016
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