Large variations of surface ocean radiocarbon: Evidence of circulation changes in the southwestern Pacific

Radiocarbon (Δ 14 C) and stable isotope (δ 18 O and δ 13 C) records are presented for biannual samples from a 323‐year banded coral series collected from the southern Great Barrier Reef, Australia. The high‐precision Δ 14 C record contains variations on an interannual timescale, that are particularly large between A.D. 1680 and 1730. By comparison with tree ring Δ 14 C records [ Stuiver and Quay , 1980; M. Stuiver, personal communication, 1992), it is clear that these shifts were not caused by changes in the Δ 14 C of atmospheric CO 2 . 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 Δ 14 C variations. Most low Δ 14 C 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 Δ 14 C values remained high. Cross‐spectral analysis of the early half of the Δ 14 C and δ 18 O 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 Δ 14 C from late 1800s through 1955 due mainly to CO 2 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.