Photochemically‐induced alteration of stable carbon isotope ratios (δ¹³C) in terrigenous dissolved organic carbon

Exposure of riverine waters to natural sunlight initiated alterations in stable carbon isotope ratios (δ 13 C) of the associated dissolved organic carbon (DOC). Water samples were collected from two compositionally distinct coastal river systems in the southeastern United States‐the Satilla River which has high DOC concentrations (10–35 mg/l), and the Altamaha River which has relatively low DOC concentrations (3–9 mg/1). Approximately 21–26% of the DOC was remineralized to DIC. The δ 13 C of remineralized DIC was isotopically “light” (δ 13 C = −28.8 to −33.2‰), relative to the initial DOC (δ 13 C = −27.4‰), leaving a residual fraction of isotopically “heavy” (δ 13 C = −25.8 to −26.6‰) DOC. Photochemically‐induced fractionation of 13 C results from selective degradation of certain biochemical constituents including lignin phenols. These results are consistent with shifts in δ 13 C of DOC observed during mixing of river and marine waters, suggesting that photochemically‐induced alterations are a factor in determining these changes.