Variability of surface ocean radiocarbon and stable isotopes in the southwestern Pacific

Abstract

We present high‐precision radiocarbon (Δ14C) results and stable isotope (δ18O and δ13C) records for a coral from Heron Island (23°S, 152°E) and new stable isotope (δ18O and δ13C) records for annual coral bands from Abraham Reef (22°S, 153°E) in the southern Great Barrier Reef studied earlier [Druffel and Griffin, 1993]. These tracers provide unique information on the regional water mass history, and together these data allow us to constrain the variability of circulation in the upper Pacific over the past four centuries. First, we observe decreases in δ18O of coral from Abraham Reef and Heron Island, indicating an increase in sea surface temperature and/or a decrease in surface salinity since 1850. Second, the small Suess effect value (Δ14C decrease from 1880 to 1955, due mostly to fossil fuel CO2) observed previously at Abraham Reef[Druffel and Griffin, 1993] is confirmed in the measurements reported here from the Heron Island coral. This value is low compared to those observed in other areas of the ocean [Druffel, 1997; Druffel and Linick, 1978; Nozaki et al., 1978] between 1880 and 1955. Third, we report alterations in the correlation between El Niño events and the occurrence of low Δ14C, which is indicative of long‐term change(s) in circulation in the SW Pacific. The Δ14C shifts reported here are not large, but even small temporal changes in prebomb Δ14C suggest that important changes in the large‐scale state of the ocean have occurred, such as a temporal change in circulation.