Department of Earth System Science

Radiocarbon (Δ¹⁴C) and stable isotope (δ¹⁸O and δ¹³C) records are presented for biannual samples from a 323-year banded coral series collected from the southern Great Barrier Reef, Australia. The high-precision Δ¹⁴C record contains variations on an interannual timescale, that are particularly large between A.D. 1680 and 1730. By comparison with tree ring Δ¹⁴C records [Stuiver and Quay, 1980; M. Stuiver, personal communication, 1992), it is clear that these shifts were not caused by changes in the Δ¹⁴C of atmospheric CO₂. Changes in vertical mixing and large scale advective changes involving source waters to the western Coral Sea region are likely processes that could account for these large Δ¹⁴C variations. Most low Δ¹⁴C values for the period A.D. 1635-1875 coincide with El Niño/Southern Oscillation (ENSO) events as reported by Quinn et al. [1987] for the eastern tropical Pacific. However, ENSO does not explain all of the variations, especially during 1875–1920 when Δ¹⁴C values remained high. Cross-spectral analysis of the early half of the Δ¹⁴C and δ¹⁸O records (A.D. 1635–1795) reveals that the 6-year period is coherent; this coherency is not present in the latter half (A.D. 1797–1957) of the isotope records. These data support the concept of century timescale changes in the nature of ENSO, as it is manifest in the southwestern Pacific. Our coral record shows no evidence of a Suess effect, the lowering of Δ¹⁴C from late 1800s through 1955 due mainly to CO₂ input from fossil fuel burning. This is coincident with the change we observe in the nature of ENSO and is further evidence that a long-term change in mixing of upper waters occurred in this region.