Temporal and regional variability in sources and cycling of DOC and POC in the northwest Atlantic continental shelf and slope

The Δ14C and δ13C distributions in total dissolved and suspended particulate organic carbon (DOC and POC, respectively) and dissolved inorganic carbon (DIC) were measured throughout the Middle Atlantic Bight (MAB) continental shelf and slope in April–May 1994, March 1996, and July–August 1996. The highest Δ14C values for both DOC (up to −29‰) and POC (up to 78‰) were observed in relatively low-salinity shelf waters. The Δ14C values for DOC from the shelf and shallow (∼5 m depth) slope generally increased progressively with proximity to the coast, and along-shelf from northeast to southwest. A significant system-wide increase in Δ14C-DOC values occurred between spring and summer 1996, indicating a major flux of young DOC to the MAB over this timeframe. The lowest Δ14C-DOC values (as low as −476‰) were found in deep (≳300 m depth) slope waters, and they were lower than values measured previously at similar depths in the open North Atlantic. A significant inverse relationship was found between Δ14C-DOC and δ13C-DOC for shelf and shallow slope waters, which we speculate may be due to variable contributions of young, 14C-enriched organic matter of terrestrial and/or riverine origin. The suspended POC of shelf and shallow slope waters was isotopically distinct from DOC, and exhibited little of the characteristic areal variability of DOC concentrations and Δ14C across the MAB. In deep slope waters, suspended POC had the greatest ages ever observed for water column POC in any marine system. These 14C-depleted POC values correlated positively and significantly with δ13C-POC values, which ranged from −31‰ to −23‰ in deep slope waters. A multiple-source dual isotopic mass balance model was applied to the observed Δ14C and δ13C distributions to evaluate the potential sources of DOC and POC to shelf and slope waters of the MAB. With the exception of the highly 14C- and 13C-depleted POC from the deep slope, all other DOC and POC values could be constrained by the isotopic signatures of various potential source materials measured for the MAB region. These include varying contributions from potential sources of open ocean, terrestrial and riverine, and bottom nepheloid layer colloidal organic matter, as well as from recent shelf and slope primary production.