Abstract
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 Δ 14C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Δ 14C values were in general similar at all three sites. Differences in DOC concentrations and Δ 14C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Δ 14C along constant density ( σ t) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Δ 14C-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 Δ 14C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Δ 14C 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 Δ 14C 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 Δ 14C-DOC and both Δ 14C-DIC and Δ 14C 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.
Highlights
Radiocarbon (C) natural abundances have been used in studies of marine carbon cycling to determine the sources, residence times, transformations and interactions of both organic and inorganic carbon reservoirs (Williams and Druffel, 1987; Bauer et al, 1992; Druffel et al, 1992)
One region where sources of DOC could have unique C signatures is ocean margins, where material derived from terrestrial, riverine, coastal, sedimentary and even petroleum sources may be transported to the open ocean (Walsh et al, 1988; Bauer et al, 1990; Moran et al, 1991; Bauer et al, 1992; Guo et al, 1996; Opsahl and Benner, 1997)
When the entire C-DOC and C-dissolved inorganic carbon (DIC) data sets are transformed to C concentrations in order to normalize for disparities in DOC and DIC pool sizes (Fig. 10B), we find that below the mixed layer there exists a linear relationship between [DOC] and
Summary
Radiocarbon (C) natural abundances have been used in studies of marine carbon cycling to determine the sources, residence times, transformations and interactions of both organic and inorganic carbon reservoirs (Williams and Druffel, 1987; Bauer et al, 1992; Druffel et al, 1992). The natural abundance of C in dissolved inorganic carbon (DIC, or total CO) has been used as an indicator of both the rate of deepwater mass formation and of the circulation times of the deep-water masses between the major ocean basins (Stuiver et al, 1983; Broecker et al, 1996) In both the dissolved and particulate organic carbon pools (DOC and POC, respectively), C provides information on the aging of modern, surface-derived organic matter in the oceans as well as on the sources of carbon to the DOC and POC pools in cases where the sources have distinct C signatures (Williams et al, 1969; Druffel et al, 1986; Druffel and Williams, 1990; Williams et al, 1992). Organic matter in coastal regions may have C signatures that reflect the magnitude of terrestrial and riverine source(s) (Aller et al, 1991), but the greater dynamic range of organic C (ca. !1000 to #200 vs &!30 to !12 for organic C, excluding methane) allows it to be used as a more sensitive source indicator (in addition to it being an age indicator) than C for studies of marine DOC and POC cycling (Trumbore and Druffel, 1995)
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