14C activity of dissolved organic carbon fractions in the north-central Pacific and Sargasso Sea

Abstract

RADIOCARBON measurements of dissolved organic carbon (DOC) oxidizable by ultraviolet irradiation (DOCuv) yielded apparent ages of ∼6,000 yr in the deep waters of the oligotrophic north-central Pacific gyre1. Recent reports of a potentially larger pool of DOC as measured by high-temperature catalytic combustion (DOChtc) using discrete injections of sea water2,3 have led to speculation that 'younger', more recently produced DOC could contribute significantly to overall oceanic organic carbon fluxes, owing to its suspected greater biological lability4–6. Here we present a comparison ofΔ14C (the deviation in parts per thousand from the 14C activity of nineteenth-century wood)7 of the DOChtc, DOCuv and humic substances in profiles from the oligotrophic north-central Pacific and Sargasso Sea. For each ocean, the Δ14C values of all three fractions are remarkably similar, yielding no evidence for a component of DOC that is cycled through the system on timescales shorter than several thousands of years. We observe an age difference between the two oceans of ∼ 2,000 yr for the deepest DOC, which can largely be accounted for by differences in the Δ14C of the DOC sources to the deep basins, and by the different deep-water circulation patterns and transit times in the two oceans.