| Summary: | Organic carbon (OC) depleted in C-13 is a widely used tracer for terrestrial organic matter (OM) in aquatic systems. Photochemical reactions can, however, change delta C-13 of dissolved organic carbon (DOC) when chromophoric, aromatic-rich terrestrial OC is selectively mineralized. We assessed the robustness of the delta C-13 signature of DOC (delta C-13(DOC)) as a tracer for terrestrial OM by estimating its change during the photobleaching of chromophoric DOM (CDOM) from 10 large rivers. These rivers cumulatively account for approximately one-third of the world's freshwater discharge to the global ocean. Photobleaching of CDOM by simulated solar radiation was associated with the photochemical mineralization of 16 to 43% of the DOC and, by preferentially removing compounds depleted in C-13, caused a 1 to 2.9 parts per thousand enrichment in delta C-13 in the residual DOC. Such solar-radiation-induced photochemical isotopic shift could bias the calculations of terrestrial OM discharge in coastal oceans towards the marine end-member. Shifts in terrestrial delta C-13(DOC) should be taken into account when constraining the terrestrial end-member in global calculation of terrestrially derived DOM in the world ocean. Peer reviewed
|
|---|