Photochemical oxidation of dissolved organic carbon (DOC) is now recognized as a significant and ubiquitous process in surface waters, occurring everywhere chromophoric dissolved organic matter (CDOM) absorbs solar UV radiation. In 1995, Miller and Zepp published a first rough estimate of the degree to which photochemical production of dissolved inorganic carbon (DIC) and carbon monoxide (CO) controls the lifetime of terrestrial DOC in the ocean, concluding that “photo-oxidation of DOC by sunlight should be considered a dominant removal mechanism of organic carbon from the ocean.” Considerable effort has been spent since then to quantify this process on regional and global scales. Direct DIC photochemical rates have been virtually impossible to measure in blue waters with vanishingly low CDOM absorbance, forcing the extrapolation of coastal DIC rates and quantum efficiency curves using optics and photochemical proxies to model the global significance of DOC photooxidation. A new and novel method using moderate DIC isotope enrichment (MoDIE) has potential to directly examine the validity of these extrapolations and directly test chemical proxies. Here we report MoDIE results that directly compare DIC photoproduction using concentrated DOM isolated from coastal to blue waters by solid phase extraction, re-dissolved in in a blue water matrix to have the same starting absorbance, and irradiated in matched solar simulators. Preliminary results suggest blue water DOC isolates, collected in July ,are less efficient than those from coastal and inshore stations; and there may be a seasonality to DIC photoproduction in the open Atlntic ocean. From this, and using coincident measurements of HOOH, we examine the validity of photochemical proxies, the fundamental optical extrapolations used routinely in regional and global models for photochemical rates, and reevaluate the premise that “photo-oxidation of DOC by sunlight should be considered a dominant removal mechanism of organic carbon from the ocean.”

Photochemical production of dissolved inorganic carbon from terrestrial organic matter: Significance to the oceanic organic carbon cycle - then and now

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.