Shifts in stable carbon isotope ratios (δ13C) in the dissolved organic carbon (DOC) pool upon exposure to solar radiation have been well documented, with the δ13C of riverine DOC generally becoming isotopically heavy and more closely resembling that of marine DOC. Studies to date have measured the δ13C of DOC after long (days to weeks) UV exposures, and estimate shifts in the δ13C of photoproduced dissolved inorganic carbon (DIC) by mass balance. Here, using ultra precise measurements (sub 0.1‰), we directly monitor the time course of δ13C shifts in photochemically produced DIC during irradiation of samples from several Southeastern U.S. rivers. The δ13C of photoproduced DIC became increasingly light as solar-simulated irradiation progressed with shifts in δ13C detected within the first hour. Rates of DIC photoproduction and shifts in δ13C decreased over time (8.5 μM/h and -0.33 δ13C/h after 1 h to 1.4 μM/h and -0.12 δ13C/h after 15 h, respectively); a process not quantitatively explained by loss of colored dissolved organic matter (CDOM). This indicates that the efficiency for DIC photoproduction (i.e. apparent quantum yield spectra) decreases with continued UV exposure. Because long irradiations are required to measure either DOC isotopic changes or [DIC] changes, shifts in δ13C in photoproduced DIC can be a more sensitive proxy for initial rates of DIC photoproduction in low-DIC natural waters.

Photo-oxidation of dissolved organic carbon in natural waters: Insights from isotopic fractionation of DIC during initial stages of irradiation

251st National Meeting (2016)

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