Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean

Abstract

The factors regulating the steady state inventories and residence times of dissolved organic carbon (DOC) in the deep ocean are not well established. Previous studies of DOC have been limited to single time‐point profiles that provide general information on the potential role of vertical advective‐diffusive processes in controlling DOC distributions and mean apparent ages. We present results from a 2‐year time series station in the eastern North Pacific (station M) where short‐term (months) changes in inventories and Δ14C signatures of DOC as measured in deep profiles were examined in conjunction with changes in particulate organic carbon (POC) pools. Significant long‐term (i.e., months to years) changes in both DOC concentrations and Δ14C values were observed. These changes were especially evident at mesopelagic (∼450 and 700 m) depths, close to the oxygen minimum. Both within the mixed layer and at mesopelagic depths, positive relationships were found between DOC Δ14C values and concentrations of station M, primarily reflecting diminishing vertical inputs of “recent” DOC throughout the main thermocline. At abyssal depths (≥1600 m), however, Δ14C was inversely correlated with DOC concentration. The Δ14C signature of the less abundant suspended and sinking POC pools has been observed to fluctuate over seasonal timescales at station M, presumably due in part to sorption of DOC to POC [Druffel et al., 1996]. However, the Δ14C values and concentrations of the correspondingly much larger DOC pool do not appear to be related to seasonal changes in either sinking POC fluxes or suspended POC abundances. Significantly elevated concentrations of DOC were observed at station M when compared with a previously occupied site in the north central Pacific (NCP) in all regions of the water column except mesopelagic depths, where concentrations were lower. The corresponding Δ14C values of DOC at all depths at station M were lower than in the NCP. We speculate that dissimilarities in the size and Δ14C signature of the DOC pools at seasonally productive station M and the oligotrophic NCP result from differences in DOC sources and sinks between the two regions, as well as from the magnitude of interaction between DOC and POC at these sites.