UV-Induced Serial-oxidations of Marine Dissolved Organic Carbon (DOC) and Biogeochemical Implications

Serial-oxidation of dissolved organic carbon (DOC) to CO2 via a 1200 W medium-pressure Hg lamp revealed an inverse relationship between the proportion oxidized and radiocarbon content (Δ14C). In samples from both 20 m and 2000 m depth of the NE Pacific (Station M, 34° 50' N, 123° 00' W), ~50 % of the DOC was significantly enriched relative to bulk DOC Δ14C values. In 20 m DOC, the initial 1 % oxidized was significantly depleted relative to DIC Δ14C, indicating concurrent photo-mineralization of Δ14C-depleted material. These results are consistent with molecular and isotopic heterogeneity throughout the water column, as well as preliminary evidence for a photochemical sink of Δ14C-depleted DOC. Modified procedures and a well-constrained DOC isotopic composition may improve estimates of the natural mineralization of Δ14C-depleted DOC by sunlight.