Radiocarbon in dissolved organic matter in the central North Pacific Ocean

Abstract

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.