The Georgia Tech Southern Line Islands coral cruise, October 2008

The primary objectives of the cruise (partially funded by PRF funding) was to survey corals in the southern Line Islands group (Caroline, Malden and Flint Islands; see Figure 1) and retrieve living coral cores to reconstruct 20^th century tropical Pacific climate. Samples collected from the cruise complement our ongoing PRF-funded work from the northern Line Islands group (Palmyra, Fanning and Christmas Islands) in investigating the role of the tropical Pacific climate in the current greenhouse forcing (Nurhati et al., submitted). Along the cruise track (Figure 1), we conducted seawater sampling every 1û latitude and continuous underway sea-surface temperature logging.

Long temperature and hydrology records from northern Line Island corals:  Sr/Ca and d¹⁸O_sw

            Our previous work (Nurhati et al., submitted) demonstrates the power of combining the two most robust coral climate proxies, namely Sr/Ca ratios and oxygen isotopic values (d¹⁸O), to reconstruct the temperature and hydrological history of the central tropical Pacific over the last thirty years. Our approach emphasizes regional reproducibility (by constructing coral geochemical records from three neighboring islands) and site reproducibility (by constructing multiple coral geochemical records from the same site). Our results indicate that the tropical Pacific has undergone a shift towards "El Nino-like" conditions over the last decades – a trend that we believe is related to anthropogenic greenhouse forcing and thus will continue in coming decades.

            Ongoing work with corals from the same northern Line Islands sites that were used in Nurhti et al. (submitted) focus on extending the coral climate proxy records to the early 20^th century. This research will allow us to better interpret the climatic signals uncovered in the short, 30yr records already completed – allowing for the clear identification of any anthropogenic-related signals (which presumably could be detected by comparing the early and late 20^th century portions of the coral geochemical timeseries). Preliminary data from Palmyra Island (6N, 162W) is shown in Figure 2. By comparing the coral d¹⁸O and Sr/Ca records over the 20^th century, we can gain insights into the history of warming (Sr/Ca) versus freshening (d¹⁸O-Sr/Ca) over this time period. The Sr/Ca record indicates that late 20^th century warming is not unprecedented in the Palmyra coral record (a result that will be tested with additional coral Sr/Ca records from southern neighbors Fanning and Christmas Islands) (Figure 2). However, the d¹⁸O record indicates that late 20^th century freshening is unprecedented, and likely reflects a shift in the Inter-tropical Convergence Zone that has occurred since ~1970AD.