Identification of a deep marine source of particulate organic carbon using bomb 14C

Abstract

THE influx of bomb radiocarbon (14C) into the oceanic food chain has been evaluated by radiocarbon dating of pelagic organisms in the North Pacific1,2. These studies found a significant gradient with depth of Δ14C (the per mil deviation from the Standard' activity of nineteenth century wood). Such a gradient is not expected according to long-standing assumptions about carbon cycling in the water column3–5; instead, one could expect the Δ14C of organisms throughout the water column to have become equal to that in surface-water dissolved inorganic carbon (DIG) by about 1970 (10–20 years after the production of bomb radiocarbon). Here we present Δ14C values measured in the profiles of suspended and sinking particulate organic carbon (POC) from an open-ocean site and a coastal basin. The 14C activity of suspended POC decreases significantly with depth, as is observed in organisms, whereas that of sinking POC is only slightly lower than that in surface DIC and surface suspended POC. All POC Δ14C results, however, are greater than the corresponding pre-bomb, surface-derived DIC values, and therefore contain bomb 14C. This decrease in 14C activity requires a deep source (or sources) of carbon to sub-surface POC pools. Adsorptive processes involving low-14C-activity dissolved organic carbon (DOC) may provide a mechanism for lowering Δ14C values in suspended and (to a lesser extent) sinking POC.