Late 20th Century Freshening in the Central Tropical Pacific Inferred from Modern Coral Records

Modern coral oxygen isotopic records from the central tropical Pacific (Cobb et al., 2001; Evans et al., 1999) are characterized by a trend towards lighter oxygen isotopic (d18O) values in the late 20th century. It is important to identify the physical changes that underlie this trend: warming sea-surface temperatures (SST) and/or decreasing salinity would both lead to more negative coral d18O. Here, we present new monthly Sr/Ca records from Palmyra (6ºN, 162ºW) and Christmas (2ºN, 157ºW) for the period 1950-1998 (~50 years) and 1970-1998 (~30 years), respectively. Sr/Ca data from Palmyra Island exhibit interannual anomalies of ±0.15mmol/mol about a mean of 8.96mmol/mol, with no resolvable trend (given analytical error of ±0.027mmol/mol (±0.3%)(1σ)). We combine our new Sr/Ca records with published d18O records to reconstruct the history of d18O of seawater (d18Osw) in the central tropical Pacific over the last decades. The d18Osw records imply that significant freshening took place in the central tropical Pacific during the late 20th century. We investigate whether the freshening is a result of 1) enhanced convection associated with a reduction in the tropical Pacific zonal SST gradient, or 2) a change in the intensity or/and location of the Intertropical Convergence Zone, through a comparison of the Christmas coral, the Palmyra coral, and available instrumental data. Reference: Evans, M.N., R.G. Fairbanks, and J.L. Rubenstone. 1999. The thermal oceanographic signal of ENSO reconstructed from a Kiritimati Island coral. J. Geophys. Res. 104: 13,409-13,421.