A Common Refractory Component in Pore-Water DOM From two Anaerobic Continental Margin Sediments With Contrasting POM Content and Composition

Degradation of sedimentary POM is an important source of DOM to the water column, but the composition and reactivity of the DOM that constitutes this benthic flux remain unclear. To help fill this knowledge gap, we examined the molecular and isotopic composition of pore-water DOM in anaerobic sediments of Santa Monica Basin (SMB) and Santa Barbara Basin (SBB), California Borderland, by applying 1H-NMR and radiocarbon analyses to unprocessed pore-water DOM samples collected as function of depth (0-1.2 m in SMB, and 0-4.5 m in SBB). Normalized 1H-NMR spectra from SMB and SBB were overall similar to 1H-NMR spectra of solid-phase-extracted seawater DOM in the literature, but with more distinct resonances in the aliphatic-H chemical shift range. Spectra from SMB and SBB showed nearly identical resonances, but at contrasting relative intensities, likely reflecting the differences in POM content and composition between the two sites. In both profiles, the baseline signal assigned to CRAM increased with depth, while more resolved resonances, such as those from protein/peptide and bound acetate, decreased. A notably uniform set of spectra was obtained from SMB samples with weak protein/peptide resonance. Because this recurring spectrum was observed throughout the SMB sediment profile, we hypothesize that it represents the refractory component of pore-water DOM. This putative refractory signal was not readily evident in the SBB profile, but incubating pore-water DOM from both SBB and SMB under aerobic conditions for 202 days rendered spectra that closely resembled this recurring spectrum. Radiocarbon signatures of the DOM also declined with incubation time. These results together point to the occurrence of a refractory and pre-aged DOM with similar molecular composition that is common to pore waters of SMB and SBB sediments.