Sources and cycling of dissolved and particulate organic radiocarbon in the Northwest Atlantic Continental Margin

Abstract

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 (Δ14C) and δ13C 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 Δ14C of DOC was greatest (as high as −39‰) in surface waters of the shelf, decreasing rapidly offshore and with depth, even in relatively shallow (25‐50 m depth) shelf waters. The lowest Δ14C‐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 Δ14C‐DOC and δ13C‐DOC in all shelf and surface slope waters of the MAB, which is likely attributable to varying contributions of young, 14C‐enriched organic matter of terrestrial and/or riverine origin. The more highly 14C‐depleted DOC in deep slope waters (as low as −442‰) generally had a correspondingly lower δ13C (as low as −22.3‰) 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 (δ13C as low as − 24.9‰) and marine (δ13C as high as −19.9‰) origin throughout the MAB shelf and slope. The highest Δ14C‐POC values (up to 78‰) were observed in shallow shelf waters of the southern MAB. Conversely, the lowest Δ14C‐POC values (as low as − 394‰) were found in MAB deep slope waters and were also significantly more depleted in 14C than POC from the central north Atlantic (Sargasso Sea). A multiple‐source isotopic mass balance model employing both 14C and 13C was used to evaluate the relative contributions of both young and old terrigenous versus marine organic matter to DOC and POC in the MAB. 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.