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 (PICA14C) 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 PICA14C measurements were $lt;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 A14C of dissolved inorganic carbon (DICAI4C) and to the decrease over time of surface DICAI4C owing to the decrease of atmospheric A14C values. In addition, PICAI4C values showed significant seasonal variability: PICAI4C collected in the fall was the greatest (-22%o on average), whereas PICA14C collected in winter showed an average minimum of -48%o. It is likely that this difference was caused by changes in mixed layer thickness. Although some uncertainties remain, further study on
PICA14C will enable us to estimate seasonal variability in DICAI4C and air-sea CO2 exchange rate.