Inorganic Radiocarbon in Time-Series Sediment Trap Samples: Implication of Seasonal Variation of 14C in the Upper Ocean

Abstract

In order to verify sediment trap samples as indicators of upper ocean 14C concentrations, particulate inorganic radiocarbon (PIC-Delta-14C) collected by time-series sediment traps in the Sea of Okhotsk and the Bering Sea was measured by accelerator mass spectrometry (AMS). All of the PIC-Delta-14C measurements were <0%, in contrast to GEOSECS 14C data in the upper ocean from the northwestern North Pacific. This difference is attributed to the upwelling of deepwater that contains low Delta-14C of dissolved inorganic carbon (DICAI4C) and to the decrease over time of surface DIC-Delta-14C owing to the decrease of atmospheric Delta-14C values. In addition, PIC-Delta-14C values showed significant seasonal variability: PIC-Delta-14C collected in the fall was the greatest (-22 per mil on average), whereas PIC-Delta-14C collected in winter showed an average minimum of -48 per mil. It is likely that this difference was caused by changes in mixed layer thickness. Although some uncertainties remain, further study on PIC-Delta-14C will enable us to estimate seasonal variability in DIC-Delta-14C and air-sea CO2 exchange rate.