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 systems that also regulate the fluxes of terrestrial, riverine, and estuarine materials 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 (14)C and delta (13)C distributions of total dissolved organic carbon (DOC), suspended particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in waters of the Middle Atlantic Bight (MAB) continental shelf and slope in April-May 1994. The Delta (14)C of DOC was greatest (as high as -39 parts per thousand) in surface waters of the shelf, decreasing rapidly offshore and with depth, even in relatively shallow (25-50 in depth) shelf waters. The lowest Delta (14)C-DOC values were observed in deep slope waters, where they were significantly lower than values measured previously for the deep Sargasso Sea. There was a strong inverse relationship between Delta (14)C-DOC and delta (-13)C-DOC in all shelf and surface slope waters of the MAB, which is likely attributable to varying contributions of young, (14)C-enriched organic matter of terrestrial and/or riverine origin. The more highly (14)C-depleted DOC in deep : slope waters (as low as -442 parts per thousand) generally had a correspondingly lower delta (13)C (as low as -22.3 parts per thousand) component. However, this must originate from relic terrestrial material either in the MAB itself or be discharged to the MAB from rivers and estuaries. The isotopic signatures of POC were clearly differentiable from DOC and indicate that this pool also contained a broad range of both old and young material of terrestrial (delta (13)C as low as -24.9 parts per thousand) and marine (delta (13)C as high as -19.9 parts per thousand) origin throughout the MAB shelf and slope. The highest Delta (14)C-POC values (up to 78 parts per thousand) were observed in shallow shelf waters of the southern MAR Conversely, the lowest Delta (14)C-POC values (as low as -394 parts per thousand) were found in MAB deep slope waters and were also significantly more depleted in (14)C than POC from the central north Atlantic (Sargasso Sea). A multiple-source isotopic mass balance model employing both (14)C and (13)C was used to evaluate the relative contributions of both young and old terrigenous versus marine organic matter to DOC and POC in the MAR The results indicate that shelf and slope DOC is comprised of an old "marine" fraction (represented by offshore Sargasso Sea material) and either a young "terrestrial/riverine/estuarine" (TRE) component (in shelf and shallow slope waters) or a relic TRE component (in deep and some shallow slope waters). In contrast, suspended POC from the MAB appears to originate predominantly from a mixture of recent MAB primary production and an old, TRE component, similar to that observed in one of the major subestuaries of the Chesapeake Bay. These results suggest that both young and old sources of terrestrial and riverine organic matter can comprise a significant fraction of the DOC and POC in ocean margins. Preliminary calculations indicate that the export of this compositionally unique DOC and suspended POC may be significant terms in the organic carbon budgets of the MAB and other margin systems.