Organic carbon (Corg) remineralization rates are typically highest near the sediment-water interface, and decrease with depth as labile substrates and strong oxidants are consumed. However, in many ocean margin sediments, at the depth interval where sulfate (SO4=) is exhausted and CH4 concentrations begin to increase (the sulfate-methane transition; SMT), SO4= reduction rates typically show strong sub-surface maxima, indicating locally-enhanced microbial activity and carbon turnover. These hot spots for SO4= reduction are generally attributed to anaerobic oxidation of CH4 by SO4=, but a number of studies have found an excess of SO4= reduction over CH4 oxidation, indicating the presence of a major additional SO4= sink in the SMT.
In this project a research team from San Francisco State University, Florida State University, and Old Dominion University will investigate the nature of this SO4= sink by combining cutting-edge porewater compositional analyses -- del-14C and del-13C of CH4, dissolved organic and inorganic carbon (DOC and DIC), and 1H-NMR on DOC -- with numerical reactive transport modeling. They will test the hypothesis that the SMT is an oxidation front for not just CH4, but also for DOC that is produced deeper in the sediment column, and transported upward into the SMT. They will also test the idea that not all of this DOC is oxidized in the SMT, and that some reaches the surface sediments, and represents a source of 14C-depleted (pre-aged) DOC to the oceans. The premise is that DOC production from Corg is enhanced in methanogenic sediments due to an uncoupling in the anaerobic food chain between terminal metabolism and fermentation reactions involved in the overall Corg remineralization process. The work will focus on two ocean margin sites, Santa Monica Basin and Santa Barbara Basin, which despite their geographic proximity, appear to have different CH4 dynamics in the deep sediments.
Intellectual Merit: This study should result in a greater understanding of the role of sub-surface sediments in the overall benthic Corg remineralization process, and in the exchange of major elements between the sea floor and the water column. It will also allow testing of the hypothesis that marine sediments are sources of 14C-depleted, recalcitrant DOC to the overlying water column, thereby addressing a problem that has perplexed chemical oceanography for several decades: what factors control the 14C signature of DOC in the deep oceans?
Broader Impacts: This work will integrate research and education in several ways. It will allow the 3 PIs to continue with their effort to incorporate their research activities into the classes they teach, which range from advanced graduate- to 100-level undergraduate courses intended for non-science majors. Second, it will directly impact the learning experiences of 2 Ph.D. and 2 M.S. candidates, and at least 2 undergraduate students who will receive training in key aspects of the proposed work, including the field, experimental and modeling components.
