Semi-Recalcitrant Dissolved Organic Matter Particulate Organic Matter Anaerobic Degradation Of Organic Matter In An Intertidal Sediment: Harnessing Natural Carbon Isotopes To Understand Organic Matter Transformations Tomoko Komada, Department of Chemistry and Biochemistry, and Romberg Tiburon Center for Environmental Studies, San Francisco State University We aim to better understand sediment organic matter degradation by using natural 14C abundance as a proxy for organic matter reactivity. Is net production of pore-water dissolved organic carbon supported by reactive or resistant component of the substrate? What fraction of the sedimentary organic matter supports respiration? preservation Semi-Recalcitrant Dissolved Organic Matter Labile Dissolved Organic Matter CO2, ammonium water sediment sedimentation Rich in nitrogen Depleted in 13C Bulk 14C age ~1000 yr Particulate Organic Matter Respired rapidly 96 % 4 % Respired slowly Nitrogen-poor Enriched in 13C Modern 14C age An incubation experiment using coastal surface sediment showed rapid production and consumption of labile organic matter. This fraction was 14C-rich, indicating that it was photosynthesized in the recent past. There was also a steady accumulation of semi-recalcitrant dissolved organic matter. These findings suggest that marine sediments may be net sources of not only labile, but also semi-recalcitrant dissolved organic matter to the oceans. The results also show that microbial degradation of sedimentary organic matter is highly selective, with only <5% of the degraded particulate matter accumulating in the semi-recalcitrant dissolved pool. To better understand the factors that control the rate of anaerobic organic matter degradation, we are currently testing a dry combustion method that can be used to determine the 14C content of the semi-labile dissolved organic matter fraction.