Department of Earth System Science

We report profiles of concentrations and radiocarbon contents of suspended particulate organic carbon (POC_susp) and sedimentary organic carbon from an abyssal site (Stn M) in the northeast (NE) Pacific collected in September 1994 (a period of very high flux of particulate carbon in the deep sea) and June 1995, as well as from stations on the continental rise and slope off the coast of California in June 1995. We show that during a period of anomalously high sinking POC flux to the deep sea (September 1994), Δ¹⁴C of suspended POC did not decrease detectably between 85 and 1600 m depth. This is in contrast to depth profiles during low and moderate fluxes of sinking POC at this station where Δ¹⁴C-POC_susp decreased 50–60‰ in this depth range. One explanation for the constant Δ¹⁴C values of POC_susp between 85 and 1600 m is that large quantities of sinking POC could continuously release labile, ¹⁴C-enriched POC_susp during biological and chemical alteration of the sinking POC. The radiocarbon evidence further suggests that resuspension of organic carbon from the sediment surface, either locally or laterally transported from the slope to the deep sea, is likely, but is probably limited to depths within a few hundred meters of the bottom. Sorption of ‘old’ DOC by suspended particulate matter in the water column is also possible, especially at shallower depths (<3500 m), though proof of this mechanism cannot be demonstrated at this time.