Radiocarbon was measured in dissolved inorganic carbon (DIC) and living organisms collected off the west coast of Baja California in October 1980. Samples from three locations were examined. O14C of DIC at the southernmost station was higher than those further north, which reflects reduced upwelling in the southern region. Crabs and anchovies had a14C values significantly lower than surface DIC L14C, indicating the incorporation of `older', sediment-derived carbon sources from their diets. Comparisons are made between our DIC L 4C measurements and those obtained during other cruises and at a coastal site, from 1959 through 1987. Two distinct time histories of DIC i14C are apparent for the post-bomb period; 1) a lower L14C curve for sites close to the coast influenced by enhanced coastal upwelling; and 2) a higher &4C curve for sites further offshore within the California Current. 

The origin of dissolved organic carbon (DOC) in the ocean has been long debated. Whereas Mantoura and Woodward¹ have used the conservative nature of DOC in a British estuary to conclude that ≥50% of DOC in the oceans could be river-derived, recent lignin results in the equatorial Pacific² have indicated that ≤10% of the DOC is potentially of terrestrial origin. In addition, the δ¹³C signature (relative to the PDB standard) of DOC ranges from –20 to –24‰ (refs 3,4), indicating that the primary source of DOC is from marine-derived organic carbon. Here we present the first detailed profile of radiocarbon measured in DOC and dissolved inorganic carbon (DIC) in the oligotrophic gyre of the central North Pacific. δ¹⁴C (per mil deviation from the activity of 19th-century wood) of DOC ranged from –150‰ (1,310 yr BP) in surface waters to –540‰ (6,240 yr BP) at 5,710 m, 40 m off the bottom, where these 'apparent ages' or 'residence times' are mean values for the combined constituents of the DOC. The surprising similarity in the shapes of the profiles of δ¹⁴C in the DOC and DIC pools suggests that similar processes are controlling the radiocarbon distribution in each of the two reservoirs and that bomb-produced radiocarbon has penetrated the DOC + DIC pools to a depth of ˜900 m. The depletion of the δ¹⁴C _DOC values by 300‰ with respect to the δ¹⁴C _DIC values suggests that a certain fraction of the DOC is recycled within the ocean on longer time-scales than DIC.

We present evidence suggesting that only a portion of the dissolved organic carbon (DOC) in the seawater analyzed previously by Williams and Druffel (1987) was oxidized by the UV-radiation method. High temperature catalytic (HTC) methods (Sugimura and Suzuki, 1988) used to reoxidize the central North Pacific gyre water samples reveal that the total DOC (DOC_HTC) is about twice that of the UV-oxidizable DOC (DOC_uv). Indications from the original study suggest that this additional DOC contains higher concentrations of radiocarbon than in the DOC_uv(Williams and Druffel, 1987). This evidence implies that DOC is older and thus more refractory with respect to biological utilization, yet is more chemically reactive with respect to photooxidation, than the fraction resistant to UV (DOC_res).

We report accelerator mass spectrometry (AMS) Δ¹⁴C measurements of humic, fulvic and hydrophilic acid fractions isolated from water collected at 180 m in the North Pacific (19°N, 158°W) using XAD macroreticular resins. Δ¹⁴C values of the humic material are less than those of DOC_uv from a similar depth 1200 km further north (Williams and Druffel, 1987) indicating that these humic substances are part of the ‘old’ recycled DOC_uv in the ocean.

Radiocarbon (Δ¹⁴C), δ¹³C, bulk carbon and organic constituent concentration measurements are presented for dissolved and particulate carbon pools from the North Central Pacific Ocean (NCP) and the Sargasso Sea (SS). We operationally define three overlapping pools of dissolved organic carbon (DOC): (1) DOC that is oxidizable by UV radiation (DOC_uv); (2) “extra” DOC measured by Co/CoO flow-through high-temperature catalytic oxidation (DOC_Ft-htc), which also has low Δ¹⁴C values like DOC_uv (Bauer et al., 1992a); and (3) a potential residual DOC fraction that is the difference between DOC measured by discrete-injection high-temperature catalytic oxidation (DOC_htc) and DOC_Ft-htc, and which has unknown Δ¹⁴C signature. The distribution of a large fraction of DOC appears to be controlled by circulation of deep ocean waters between major oceans. The DOC in the SS is slightly younger than would be expected if circulation was the sole process controlling DOC cycling. We propose that there is more bomb ¹⁴C in the deep SS DOC to account for this difference. The Δ¹⁴C values of suspended, and to a lesser extent sinking particulate organic carbon (POC), decrease with depth, with the suspended POC displaying a much steeper gradient in the SS than in the NCP. These data reflect the incorporation of low-activity organic matter into the POC pool, possibly through incorporation of DOC by physical adsorption and/or biological heterotrophy.