Constraining photochemical production rates of dissolved inorganic carbon in the open ocean using the moderate dissolved inorganic carbon (DI13C) isotope enrichment (MoDIE) method

Photochemical degradation of dissolved organic carbon (DOC) is an important process in the cycling of biologically important elements in the ocean. Although dissolved inorganic carbon (DIC) is a major product of DOC photochemistry, in situ rates and the magnitude of global photochemical DIC production are poorly constrained due to the analytical challenges inherent in measuring extremely low production rates (nM h^-1 to μM h^-1) against the high DIC background in seawater (~2 mM). The moderate dissolved inorganic carbon (DI¹³C) isotope enrichment (MoDIE) method was developed to directly measure this low DIC photoproduction in seawater with minimal sample manipulation; an improvement over previous methods that stripped DIC from samples or used indirect proxies. Preliminary photochemical experiments with North Atlantic Subtropical Gyre surface water collected from the BATS station indicated that DIC photoproduction rates of these blue water samples were below the current detection limit of the MoDIE method (24 nM h^-1). Methodological modifications to optimize the MoDIE method for high precision marine measurements in photochemical experiments will be presented along with new results from additional irradiations of ¹³C enriched blue seawater samples. Sequential additions of solid phase-extracted DOC, collected from coastal and open ocean stations, back into 0.2 μm-filtered blue water are used to explore the photoreactivity of marine carbon sources and the extent to which such extracts represent the “average” photoreactivity of DOC. The utility and validity of optical extrapolations of nearshore results to blue water photochemical rate estimates for DIC production will also be explored.