Award: OCE-2017577

The main goal of this project was to constrain the source of oceanic dissolved black carbon (DBC) using compound-specific stable carbon isotopes. To achieve this goal, we collected analyzed, and compared the stable carbon isotopic signatures of DBC in global rivers (Amazon, Congo, Northern Dvina, Kolyma, and Mississippi) and along depth profiles in the Atlantic and Pacific Oceans. For the Congo River, seasonal samples were obtained to evaluate hydrologically driven changes in DBC isotopic composition. We also conducted controlled photodegradation experiments of river waters that varied in dissolved organic carbon concentration and quality (Amazon, Columbia, Mississippi, St. Lawrence, Suwannee, and Yukon) to determine whether exposure to sunlight could fractionate, or alter, the isotopic signature of riverine DBC. Major research findings include: 1) The stable carbon isotopic composition of oceanic DBC is significantly enriched in 13C compared to DBC exported by major rivers, which indicates that oceanic DBC does not predominantly originate from rivers and instead may derive from another source with an isotopic signature similar to that of marine phytoplankton. 2) Despite large variations in Congo River discharge, the stable carbon isotopic composition of DBC remained relatively stable and depleted in 13C, therefore hydrology cannot account for the isotopic discrepancy between riverine and oceanic DBC. 3) Photodegradation resulted in some degree of 13C enrichment of riverine DBC, therefore solar irradiation likely contributes to, but does not fully explain, the isotopic discrepancy observed between rivers and the open ocean. Rivers mobilize DBC from the land. Taken together, our project findings suggest most of this terrestrial DBC does not persist in the open ocean. Although sunlight can alter riverine DBC isotopic signatures to look more marine-like, our results indicate that other, yet unconstrained, oceanic sources of condensed aromatic carbon are also likely. This work provided early career researcher and now assistant professor, Sasha Wagner, with her first experience in the successful planning, leadership, and completion of a major funded research project. Co-investigators Aron Stubbins and Jay Brandes provided invaluable mentorship and advice to Wagner during the study period. Project funds supported two postdocs, who mentored undergraduates, gained stable isotope analytical skills, and developed academic soft skills under the mentorship of project investigators. Project funds also supported two undergraduate students through the Northeastern University Cooperative Program. In collaboration with an undergraduate student at Rensselaer Polytechnic Institute, Wagner developed and published a scientific zine entitled "Molecular Tales of Marine Dissolved Organic Matter" - An illustrated story that follows four molecules who tell tales about their journey to the ocean. The zine is freely available as a downloadable PDF or e-booklet and will be used for educational outreach to introduce students and the public to black carbon and organic molecular cycling in the ocean in a fun and engaging format. Last Modified: 06/01/2022 Submitted by: Sasha Wagner