A MONTHLY-RESOLVED OXYGEN ISOTOPIC TIME SERIES FROM A PRISTINE FOSSIL CARIBBEAN CORAL SUPPORTS MODERN ENSO CONDITIONS AT THE MIOCENE/PLIOCENE BOUNDARY

Abstract:

The Pliocene warm period (PWP) (5.3-2.6 Ma) is a common analog for a future, warmer world. The nature of El Niño Sothern Oscillation (ENSO) during the PWP is the subject of debate, with conflicting models and centennial-scale marine core proxy data suggesting either a (1) persistent El Niño-like state (Wara et al., 2005, Science v.309, p.758) or (2) persistent La Niña-like state (Rickaby and Halloran, 2005, Science v.307, p.1948). Alternatively, a sub-annually-resolved oxygen isotopic time series of a pristine fossil coral from the western Pacific identified PWP sea surface temperature variability consistent with modern ENSO conditions (Watanabe et al., 2011, Nature v.471, p.209). No similar analysis has been performed on pristine PWP corals from the eastern Pacific. Because prior to ~2.7 Ma the Central American Seaway (CAS) allowed Pacific waters to flow into the Caribbean Sea, ENSO signals could have been propagated directly into the Caribbean.

Pristine corals from the latest Miocene now found in the Dominican Republic have been previously dated using U-Pb techniques (5.5±0.1 Ma) and analyzed for oxygen and carbon isotope values (Denniston et al., 2008, Geology v.36, p.151). We have micromilled at ~15 samples/year adjacent sections of the previously analyzed corallite, thereby extending this stable isotope record to 27 years in length. Following the methods used by Watanabe et al. (2011), the seasonal cycle in these oxygen isotopes was deconvolved and positive and negative departures were identified. This record suggests that anomalous winter sea surface temperatures occurred at intervals consistent with modern ENSO behavior. These findings should be integrated into paleoceanographic models of the CAS at 5.5 Ma to better understand their connection to ENSO. Future research will lengthen this record, further clarifying the state of PWP ENSO.