Sources and cycling of dissolved and particulate organic radiocarbon in the northwest Atlantic continental margin

Continental shelves and slopes are productive and dynamic ocean margin syst ems that also regulate the fluxes of terrestrial, riverine, and estuarine m aterials between the continents and oceans. In order to evaluate the ages, potential sources, and transformations of organic matter in an ocean margin system, we measured the radiocarbon (Delta C-14 and delta C-13 distributio ns of total dissolved organic carbon (DOC), suspended particulate organic c arbon (POC), and dissolved inorganic carbon (DIC) in waters of the Middle A tlantic Bight (MAB) continental shelf and slope in April-May 1994. The Delt a C-14 of DOC was greatest (as high as -39 parts per thousand) in surface w aters of the shelf, decreasing rapidly offshore and with depth, even in rel atively shallow (25-50 in depth) shelf waters. The lowest Delta C-14-DOC va lues were observed in deep slope waters, where they were significantly lowe r than values measured previously for the deep Sargasso Sea. There was a st rong inverse relationship between Delta C-14-DOC and delta C-13-DOC in all shelf and surface slope waters of the MAB, which is likely attributable to varying contributions of young, C-14-enriched organic matter of terrestrial and/or riverine origin. The more highly C-14-depleted DOC in deep : slope waters (as low as -442 parts per thousand) generally had a correspondingly lower delta C-13 (as low as -22.3 parts per thousand) component. However, t his must originate from relic terrestrial material either in the MAB itself or be discharged to the MAB from rivers and estuaries. The isotopic signat ures of POC were clearly differentiable from DOC and indicate that this poo l also contained a broad range of both old and young material of terrestria l (delta C-13 as low as -24.9 parts per thousand) and marine (delta C-13 as high as -19.9 parts per thousand) origin throughout the MAB shelf and slop e. The highest Delta C-14-POC values (up to 78 parts per thousand) were obs erved in shallow shelf waters of the southern MAR Conversely, the lowest De lta C-14-POC values (as low as -394 parts per thousand) were found in MAB d eep slope waters and were also significantly more depleted in C-14 than POC from the central north Atlantic (Sargasso Sea). A multiple-source isotopic mass balance model employing both C-14 and C-13 was used to evaluate the r elative contributions of both young and old terrigenous versus marine organ ic matter to DOC and POC in the MAR The results indicate that shelf and slo pe DOC is comprised of an old "marine" fraction (represented by offshore Sa rgasso Sea material) and either a young "terrestrial/riverine/estuarine" (T RE) component (in shelf and shallow slope waters) or a relic TRE component (in deep and some shallow slope waters). In contrast, suspended POC from th e MAB appears to originate predominantly from a mixture of recent MAB prima ry production and an old, TRE component, similar to that observed in one of the major subestuaries of the Chesapeake Bay. These results suggest that b oth young and old sources of terrestrial and riverine organic matter can co mprise a significant fraction of the DOC and POC in ocean margins. Prelimin ary calculations indicate that the export of this compositionally unique DO C and suspended POC may be significant terms in the organic carbon budgets of the MAB and other margin systems.