Distributions of dissolved organic and inorganic carbon and radiocarbon in the eastern North Pacific continental margin

Temporal variations in the natural radiocarbon ((14)C) signatures of dissolved organic and inorganic carbon (DOC and DIG, respectively) in seawater have been studied previously (Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S., Wolgast, D.M., 1996. Seasonal variability of radiocarbon in particulate organic carbon in the northeast Pacific. J. Geophys. Res. 101, 20 543-20 552; Bauer, J.E., Druffel, E.R.M., Williams, P.M., Wolgast, D.M., Griffin, S., 1998. Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean. J. Geophys. Res. 103, 2867-2882) at a long-term time-series station (Sta. M: 32 degrees N, 123W) in the eastern North Pacific located at the eastern edge of the North Pacific abyssal plain. In June 1995 a transect was made from Sta. M inshore to approximately 500 m depth in order to evaluate the distributions of (14)C in DOC and DIC from the abyssal plain to the upper continental slope. Concentrations and Delta(14)C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Delta(14)C values were in general similar at all three sites. Differences in DOC concentrations and Delta(14)C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Delta(14)C along constant density (sigma(t)) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Delta(14)C-DOC at mesopelagic ( similar to 700 m) and near-bottom ( similar to 3600-3900 m) depths of the continental rise. Comparable findings are reported for DIG, where sigma(t)-normalized concentrations and Delta(14)C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Delta(14)C values in near-bottom waters of the rise. These observations indicate that both DOC and DIC in continental rise and slope surface waters of the eastern North Pacific Ocean margin are comprised of a component of actively upwelled material derived from deeper offshore waters and a component of recent surface-derived material. Our data suggest that contributions of both (14)C-enriched and (14)C-depleted DOC and DIC to localized regions of the water column occur in waters of the continental rise and slope. The sources of material having anomalous Delta(14)C values are not certain, but may include organic matter derived from off-shelf and slope transport, particulate organic carbon solubilization, and sediment porewaters. We also examine the relationships between Delta(14)C-DOC and both Delta(14)C-DIC and Delta(14)C of suspended POC measured in a companion study as a means of evaluating similarities or differences in the mechanisms responsible for the distributions of each at all three sites. (C) 1998 Elsevier Science Ltd. All rights reserved.