Radiocarbon Evidence for Active Turnover of Pore-Water Dissolved Organic Carbon in the Methanogenic and Sulfate-Methane-Transition Zones of Santa Barbara Basin Sediments

Monday, 15 December 2014
Tomoko Komada1, Huan Lei Li2, Abraham King Cada2, David Burdige3, Cedric Magen4, Jeffrey Chanton5 and Ashley M. Grose2, (1)San Francisco State University, Romberg Tiburon Center, San Francisco, CA, United States, (2)San Francisco State University, Romberg Tiburon Center, Tiburon, CA, United States, (3)Old Dominion University, Dept. of Ocean, Earth and Atmospheric Sciences, Norfolk, VA, United States, (4)The University of Maryland Center for Environmental Science, Cambridge, MD, United States, (5)Florida State Univ, Tallahassee, FL, United States
Diverse metabolic activities have been documented in the deep biosphere. However, how these activities affect carbon cycling in the subsurface, and how they in turn affect the marine and global cycles of carbon are still unclear. Here we present natural-abundance 14C and 13C data from the uppermost 4.5 m of the sediments of the Santa Barbara Basin, California Borderland, showing active turnover of dissolved organic carbon (DOC) within, and immediately below, the sulfate-methane transition zone (SMTZ; ~1.25 m). DOC concentrations increased with depth throughout the core, indicating net production within the sediment column. Enhanced DOC production was observed near the sediment-water interface, and also at ~30 cm below the SMTZ (~1.55 m). ∆14C values of DOC increased across the sediment-water interface, then decreased with depth, consistent with net production of modern DOC near the sediment-water interface, and input of 14C-depleted DOC from deeper horizons. An isotope mixing plot constructed with these data shows that the DOC diffusing upward at the base of the core is devoid of 14C, yet the DOC diffusing into and out of the SMTZ is relatively enriched (-460‰ and -300‰, respectively). This difference in 14C content of the DOC flux can only be reconciled if the following two are occurring within, and immediately below, the SMTZ: (1) >90% of the 14C-dead basal DOC flux is removed from the pore water (by, e.g., oxidation, fermentation, methanogenesis, precipitation), and (2) this DOC is replaced by material produced in this region at a rate that exceeds the upward basal flux. The 14C and 13C signatures suggest sedimentary organic matter to be the dominant source of DOC in process (2). Our data provide a unique insight into the active transformation of DOC and sedimentary organic matter in the subsurface.