Revisiting the disappearance of terrestrial dissolved organic matter in the ocean: a δ¹³C study

Abstract. Organic carbon (OC) depleted in ¹³C is a widely used tracer for terrestrial organic matter (OM) in aquatic systems. Photochemical reactions can, however, change δ¹³C of dissolved organic carbon (DOC) when chromophoric, aromatic-rich terrestrial OC is selectively mineralized. We assessed the robustness of the δ¹³C signature of DOC (δ¹³C_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 ¹³C, caused a 1 to 2.9‰ enrichment in δ¹³C 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 δ¹³C_DOC should be taken into account when constraining the terrestrial end-member in global calculation of terrestrially derived DOM in the world ocean.