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.