Modeling studies of dissolved organic matter cycling in Santa Barbara Basin (CA, USA) sediments
Abstract Here we describe new reaction transport models for the cycling of dissolved organic matter (DOM, both dissolved organic carbon [DOC] and dissolved organic nitrogen [DON] in anoxic marine sediments, and apply these models to data from Santa Barbara Basin sediment cores (maximum depth of 4.6 m) Model results show that most organic carbon (and nitrogen) flow in the sediments occurs through reactive DOM intermediates that turn over rapidly to produce inorganic remineralization end products. Refractory DOM is also produced, and the vast majority of this refractory DOM is not remineralized and either escapes as a benthic flux across the sediment water interface or is buried. Except near the sediment surface, refractory DOM represents >95% of the total pore water DOM. Pore water DOM appears to be consistently depleted in nitrogen as compared to its source organic matter, which may be the result of differential production of carbon versus nitrogen containing refractory DOM during remineralization. Refractory DOC (DOCr) in Santa Barbara Basin sediment pore waters is largely produced from degradation of sediment particulate organic carbon (POC) In addition, there is an upward basal flux of DOCr that is strongly depleted in 14C 810%0) The D14C value of DOCr varies according to its source, ranging from +60%0 (a component of surface sediment POC enriched with radiocarbon from nuclear weapons testing in the 1960's) to 810%0 (the basal DOC flux) Each contributes to the DOCr benthic flux, which has a weighted average D14C value of 40%0. The model determined DOCr benthic flux is roughly half of the total DOC benthic flux, consistent with observations in the literature that sediments are a source of both labile and refractory DOC to bottom waters. These results support previous arguments that sediment benthic fluxes represent an important source of refractory DOC to the oceans. The benthic flux of refractory DOC from these sediments may also contribute pre aged DOC to the water column if the different sub components of the anoxic pore water DOCr pool with differing radiocarbon ages have differing reactivities in the oxic marine water column.