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

摘要

The oxygen isotopic composition (#delta#~(18)O) of coral skeletons reflects a combination of sea surface temperature (SST) and the #delta#~(18)O 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#~(18)O, seawater #delta#~(18)O 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#~(18)O 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#~(18)O measured at the coral site. Our results yielded three relationships (coral Mg/Ca-SST, #delta#~(18)O_(coral)- #delta#~(18)O_(water)- SST, and #delta#~(18)O_(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#~(18)O and Mg/Ca ratio of the coral skeleton between 1699 and 1703 suggests that the SST during the LIA was approximately 2 deg C 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.