Use of Keeling plots to determine sources of dissolved organic carbon in nearshore and open ocean systems

We apply a Keeling mixing model commonly used in atmospheric sciences to oceanic dissolved organic carbon (DOC) C-14 and concentrations to demonstrate that DOC distribution across spatial transects or depth profiles can be modeled as two-component mixtures; one component consists of an old refractory pool to which a newly synthesized component pool is added. We then use the Keeling approach to determine the Delta C-14 signature of excess DOC added to background concentrations in nearshore and open ocean systems. Our analysis indicates enrichment in C-14 of excess DOC in coastal waters relative to offshore waters, particularly in spring when river discharge increases. These findings are consistent with the export of C-14-enriched terrestrial DOC to the ocean and inconsistent with the idea of selective degradation of young terrestrial DOC in estuaries, resulting in export of old refractory component to the ocean. In the middle Atlantic Bight region, during the spring, excess DOC has a C-14 signature similar to that of DOC in the major rivers, whereas during the peak primary productivity of the summer season, excess DOC has a signature more similar to that of dissolved inorganic carbon. Variability in the Delta C-14 of excess DOC in the open ocean was lower than that of the coastal ocean. Minima in Delta C-14 values of excess DOC in the open ocean were measured during periods of high particulate organic matter (POC) flux, indicating that POC can interact with DOC and selectively remove C-14-enriched components from the surface ocean. The Keeling plot provides a powerful approach for determining the isotopic composition of excess DOC from an entire DOC profile or transect.