A review of paleoenvironmental studies on Sr/Ca ratios and oxygen isotopes from fossil corals

To elucidate the "Quaternary" history of climate and ocean variability necessitates the use of paleoclimate proxy records from the tropical/subtropical regions that extend far beyond instrumental data currently obtainable. Fossil corals are one of the best archives for reconstructing thermal and hydrologic changes at the sea surface from the past. Massive Porites corals, forming annually-banded aragonite skeletons and growing rapidly, provide chronological control and allow high-resolution sampling. In particular, coupled determinations of coral Sr/Ca ratio and oxygen isotope can yield independent timeseries of sea surface temperature and salinity, showing seasonal-to-decadal climatic variability such as El Niño/Southern Oscillation. This paper is a review of previous studies on paleoenvironmental analyses using fossil corals and suggests what needs to be addressed to develop coral paleoclimatology. Some previous studies indicated that estimates of sea surface temperature and salinity from coral Sr/Ca ratio and oxygen isotope records were biased to some degree due to seawater-temperature calibrations, changes in chemical composition of "Quaternary" seawater, diagenetic alteration, and/or vital effects. Further accurate evaluations of potential influences affecting coral geochemistry will enable more robust and high-fidelity reconstruction of tropical/subtropical sea surface conditions from the past.