EVALUATION OF OXYGEN ISOTOPE AND SR/CA RATIOS FROM A MALDIVIAN SCLERACTINIAN CORAL FOR RECONSTRUCTION OF CLIMATE VARIABILITY IN THE NORTHWESTERN INDIAN OCEAN

Abstract

Corals are important marine archives for high-resolution reconstructions of low-latitude climate variability in preinstrumental and historical periods. Herein, we present monthly-resolved records of delta O-18 and Sr/Ca for the 20th century (1917-2007) from a Porites lutea colony from the Maldives (northwestern Indian Ocean). Previous studies of annual mean extension rates of this coral revealed a distinct dependency of coral growth on variations of sea surface temperature (SST) and hydraulic energy, driven by El Nino-Southern Oscillation (ENSO) and southwestern Indian monsoon forcing. This enables the investigation of coral delta O-18 and Sr/Ca ratios from the Maldives as archives of historical ENSO and Indian monsoon variability. Unlike other locations in the Indian Ocean, correlation of delta O-18 and SST is weak (r = -0.42; p < 0.001), suggesting interferences of SST and seawater delta O-18 at the location. These interferences probably caused significantly weaker interannual ENSO signatures in delta O-18 as evident in the extension rates of our coral and in geochemical proxies measured at other sites in the northwestern Indian Ocean. Sr/Ca ratios show bias by nontemperature effects, and were not used to estimate seawater delta O-18 and salinity. Strong decadal variability (10-14 years) in delta O-18, indicative of a Pacific ENSO signal, is not found in instrumental SST, and could be explained by variations in salinity. Interannual and decadal monsoon variability (6-7 yrs and 18-19 yrs), found in the extension rates, was not found in delta O-18, suggesting, in agreement with other studies, that geochemical coral proxy records from the northwestern Indian Ocean do not capture temporal variations of the Indian monsoon strength.