CYCLING OF DISSOLVED ORGANIC MATTER IN THE PACIFIC AND ATLANTIC OCEANS USING RADIOCARBON

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Dissolved organic carbon is the largest pool of carbon in the ocean, comparable to the total carbon content in the atmosphere. Radiocarbon, a radioactive isotope of carbon, can provide information on the source and timescale of carbon cycling in the environment. Therefore, knowing the carbon isotopic signatures of dissolved organic carbon is important for understanding the biogeochemistry and dynamics of dissolved organic carbon cycling, and is essential for the carbon cycle modeling community. The main objective of this research is to determine the radiocarbon values of dissolved organic carbon in seawater samples from areas in the Pacific and Atlantic Oceans for which little or no data is available to provide much needed, quantitative information on the timescale of dissolved organic carbon cycling in the ocean. Atmospheric carbon will end up in the ocean, and it is critical to improve our understanding of the processes that influence its long-term storage in the deep sea. Results of this research will be made available for use in models that assess present and future concentrations of atmospheric carbon dioxide and its impact on ocean biogeochemistry. This research will also support the work of one graduate student and two undergraduate students and provide hands-on experience that will prepare them for careers as scientists and educators.The average radiocarbon (14C) age of dissolved organic carbon (DOC) in the deep ocean ranges from 4000 to 6500 years. Previous research indicates that DOC 14C in the deep South Pacific is equal to that in the deep North Pacific, suggesting that there is a sink for labile DOC in the subtropical South Pacific, In addition, findings suggest that the content of post-bomb DOC has decreased in Sargasso Sea waters on decadal timescales. These findings indicate that processes other than water mass circulation controls DOC 14C in the deep sea. In this study, a researcher from the University of California, Irvine, will use seawater samples from the Pacific and Atlantic Oceans to test two hypotheses: (1) Radiocarbon of bulk DOC is similar at all latitudes of the deep Pacific basin, and values are higher in the Southern Ocean, suggesting the dual governance of DOC aging, remineralization, and young DOC injection concomitant with deep ocean circulation; (2) Deep DOC radiocarbon values in the North Atlantic Ocean reflect the presence of variable amounts of post-bomb carbon. To test these hypotheses, seawater samples will be analyzed for DOC isotopic signatures in profiles from Pacific and Atlantic Oceans to establish a quantitative understanding of the timescale of DOC cycling in the ocean. This study addresses fundamental gaps in knowledge of the global carbon cycle and the dynamic nature of DOC in the ocean.