Department of Earth System Science

Temporal variations in the natural radiocarbon (¹⁴C) signatures of dissolved organic and inorganic carbon (DOC and DIC, 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°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 ¹⁴C in DOC and DIC from the abyssal plain to the upper continental slope. Concentrations and Δ¹⁴C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Δ¹⁴C values were in general similar at all three sites. Differences in DOC concentrations and Δ¹⁴C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Δ¹⁴C along constant density (σ _t) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Δ¹⁴C-DOC at mesopelagic (∼700 m) and near-bottom (∼3600– 3900 m) depths of the continental rise. Comparable findings are reported for DIC, where σ _t-normalized concentrations and Δ¹⁴C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Δ¹⁴C 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 ¹⁴C-enriched and ¹⁴C-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 Δ¹⁴C 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 Δ¹⁴C-DOC and both Δ¹⁴C-DIC and Δ¹⁴C 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.