What constitutes the refractory component of pore-water dissolved organic matter?

Marine sediments are known sources of dissolved organic matter (DOM) to the water column, but the composition and reactivity of pore-water DOM are still unclear. To address this question, we examine isotopic signatures (14C and 13C) and 1H-NMR spectra of pore-water DOM from two California Borderland basins: Santa Barbara Basin (SBB) and Santa Monica Basin (SMB). In SBB, dissolved organic carbon (DOC) concentration increases with depth, and Δ14C-DOC decreases with depth. 1H-NMR spectra show broad CRAM-like resonances along with relatively well-resolved peaks most of which appear between 1-3 ppm. These spectra show little variation throughout the depth profile. In SMB, grossly similar patterns are observed, but with greater spatial and temporal variability in DOC concentration and 1H-NMR spectra. To examine the reactivity of DOM in these sediments, pore water samples were collected from the surface sediments at both sites, inoculated with seawater from 100 m depth, and incubated for 300 days in the presence of oxygen in the dark at in situ temperature. 1H-NMR data show the persistence of CRAM-like resonances. Evolution of Δ14C-DOC during the incubation also suggest that the persistent DOC fraction in SMB pore waters is relatively depleted in 14C, while in the SBB, this fraction has 14C content that is similar to the bulk DOC pool. Implications of these results will be discussed.