Seasonal changes in sea surface temperature and salinity during the Little Ice Age in the Caribbean Sea deduced from Mg/Ca and 18O/16O ratios in corals

The oxygen isotopic composition (delta O-18) of coral skeletons reflects a combination of sea surface temperature (SST) and the delta O-18 of seawater, which is related to sea surface salinity (SSS). In contrast, the magnesium/calcium (Mg/Ca) ratio of a coral skeleton reflects SST independent of salinity. By using the relationships among coral Mg/Ca ratios, coral delta O-18, seawater delta O-18 and SST, it is possible to determine past SST and SSS uniquely. Such determinations were made and calibrated using the Mg/Ca ratio and the delta O-18 of the modern part of a 3 m long coral core (Montastrea faveolata), collected from the southwest coast of Puerto Rico in the Caribbean Sea where both SST and SSS changes seasonally and the seawater delta O-18 measured at the coral site, Our results yielded three relationships (coral Mg/Ca-SST, delta O-18(coral)-delta O-18(water)-SST. and delta O-18(water)-SSS). With these calibration equations seasonal changes in SST and SSS during the little ice age (LIA) in the Caribbean Sea were reconstructed. The delta O-18 and Mg/Ca ratio of the coral skeleton between 1699 and 1703 suggests that the SST during the LIA was approximately 2 degreesC cooler than present with the SSS showing greater seasonal changes as well. These results are in good agreement with climate-based reconstruction from corals based on oxygen isotopes, although the possibility of some uncertainty remain in our estimation including long-term decadal scale trends in climate. (C) 2001 Elsevier Science B.V. All rights reserved.