Abstract
The D 14 C and d 13 C 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 D 14 C values for both DOC (up to � 29%) and POC (up to 78%) were observed in relatively low-salinity shelf waters. The D 14 C values for DOC from the shelf and shallow (B5 m depth) slope generally increased progressively with proximity to the coast, and along-shelf from northeast to southwest. A significant system-wide increase in D 14 C-DOC values occurred between spring and summer 1996, indicating a major flux of young DOC to the MAB over this timeframe. The lowest D 14 CDOC 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 D 14 C-DOC and d 13 C-DOC for shelf and shallow slope waters, which we speculate may be due to variable contributions of young, 14 C-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 D 14 C across the MAB. In deep slope waters, suspended POC had the greatest ages ever observed for water column POC in any marine system. These 14 C-depleted POC values correlated positively and significantly with d 13 C-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 D 14 C and d 13 C distributions to evaluate the potential sources of DOC and POC to shelf and slope waters of the MAB. With the exception of the highly 14 C- and 13 C-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. r 2002 Elsevier Science Ltd. All rights reserved.
Highlights
Continental shelves and slopes comprise only B15–20% of the ocean’s surface area, yet may account for up to half of the total oceanic primary production and new production (Eppley and Peterson, 1979; Eppley, 1989)
Earlier studies on carbon export from ocean margins focused primarily on particulate organic carbon (POC, typically measured as the organic carbon collected by filtration of seawater through B0.7-mm glass fiber filters), and mostly evaluated the larger sinking fraction of POC
The upper value clearly represents the presence of bomb-derived 14C, and as we will show, only those samples with D14C-DOC values \À150% contain discernible amounts of freshly derived modern material derived from Middle Atlantic Bight (MAB) primary production
Summary
Continental shelves and slopes comprise only B15–20% of the ocean’s surface area, yet may account for up to half of the total oceanic primary production and new production (Eppley and Peterson, 1979; Eppley, 1989) These ocean margin systems are important regions where terrestrial and riverine organic matter is exported and transformed en route to the open ocean (Mantoura and Woodward, 1983; Mulholland and Watts, 1982; Moran et al, 1991; Hedges, 1992; Hedges et al, 1997; Keil et al, 1997; Bauer et al, 2001). The fluxes of DOC and suspended POC vs. sinking POC may be similar in magnitude similar to the differences in horizontal vs. vertical eddy diffusivity (Ledwell et al, 1993)
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