| Summary: | 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. Water samples from six detailed profiles were collected on three research cruises to the northern North Atlantic, the southern Beaufort Sea, and the Beaufort sea and slope. The following three hypotheses were tested: (1) 14C of bulk DOC in the South Pacific Ocean is intermediate between values in the Southern Ocean and those in the North Pacific, and these 14C ages can be used to determine the timescale of DOC cycling in deep waters of the Pacific; (2) 14C levels in bulk DOC of the Atlantic Ocean are highest in the north (43°N), lower in the equatorial region (10°N) and lowest in the South Atlantic (30°S), and reflects the timescale of DOC cycling as deep water travels south in the western Atlantic; and (3) black carbon constitutes a significant amount of DOC in open ocean water, and its 14C age is greater than 20,000 14C years.
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