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.