Department of Earth System Science

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.