Award: OCE-1756669

Past work we have carried out has shown that sediments are not only a source of DOC to the oceans but are also a possible source of pre-aged (radiocarbon-depleted) DOC to the oceans.This suggests a possible explanation for the anomalously old radiocarbon age of DOC in the oceans relative to ocean mixing times.However, to date, these observations have largely come from studies of anoxic marine sediments in Santa Barbara Basin (SBB) and Santa Monica Basin (SMB).As a result, global extrapolation of these results to all continental margin sediments have some degree of uncertainty. In this project, we have examined the composition and isotopic signatures of DOC from sediments at three sites on the continental margin offshore California, USA (Catalina Basin and two shelf locations near Monterey). In contrast to the laminated and anoxic sediments of SBB and SMB these sites show evidence of bioturbation and mixed redox conditions. While pore water DOC concentrations are lower by a factor of ~5-10 at these mixed-redox sites relative to SMB and SBB,14C values in pore water DOC in the mixed-redox sediments are considerably lower than that observed in SMB and SBB sediments. Furthermore,14C values of pore water dissolved inorganic carbon (DIC) from the mixed-redox sites are similar to, or greater than, the14C of DIC in SMB and SBB sediment pore waters. We attribute these observations to: (1) the occurrence of sedimentary organic carbon at the mixed-redox sites with very low14C values (as low as 14C, leading to net preferential accumulation of14C-depleted DOC in the pore waters.We suggest that redox oscillations from bioturbation not only suppresses the accumulation of pore water DOC, but also lowers its14C values. If the patterns observed here at these mixed-redox sites hold true over greater spatial scales, mixed-redox to oxic sediments with low pore water DOC concentrations could also be major sources of pre-aged DOC to the water column.Such observations greatly strengthen arguments we have made previously regarding the role of continental margin sediments as a source of pre-aged (radiocarbon-depleted) DOC to the oceans. Last Modified: 05/17/2024 Submitted by: DavidJBurdige