Dissolved organic carbon dynamics in anaerobic sediments of the Santa Monica Basin

Abstract

Cycling of dissolved organic carbon (DOC) was investigated in anoxic sediments of the Santa Monica Basin, California Borderland, by analyzing the concentration and isotopic signatures (Δ14C and δ13C) of pore-water DOC and dissolved inorganic carbon (DIC), and organic compound classes extracted from the bulk sediments. DOC and DIC increased across the sediment–water interface, indicating net efflux of these solutes out of the sediments. Throughout the depth interval examined (0–30cm), the Δ14C value of DOC (Δ14CDOC) was similar to, or higher than, that of bulk sedimentary particulate organic carbon (POC), indicating degradation of relatively 14C-rich components of POC. There were prominent peaks in both Δ14CDOC and Δ14CDIC in the uppermost 2 cm of the sediment column, indicating degradation and remineralization of 14C-rich, labile organic matter in the near-surface sediments. However, below these sub-surface maxima, Δ14CDOC and Δ14CDIC decreased with depth by ∼200‰ and ∼50‰, respectively. Given the diffusive time scales, these decreases were too large to be explained by 14C loss due to radioactive decay. To help explain these observations, we constructed and implemented a selective degradation model that considers bulk pore-water DOC to be the sum of three kinetically- and isotopically-distinct sub-components. Based on this model, the most reactive DOC fraction, which supported ∼60% of the DIC production, had a Δ14C value indicating the presence of bomb-14C. The intermediate fraction had a Δ14C value of ∼−60‰ and accounted for most of the pore-water DOC standing stock. The least reactive fraction was virtually non-reactive in these sediments, and had a Δ14C value of ∼−500‰. The benthic DOC flux of this 14C-depleted, poorly-reactive DOC fraction may represent a source of pre-aged, refractory DOC to the oceans.