Coral Bleaching And Enso Anomaly : Was The Past Similar To The Present

Abstract

Bleaching of reef corals is due to partial or total loss of symbiotic micro-algae as a consequence of environmental stress such as an oceanic warming linked to the ocean-scale hydroclimatic anomaly termed ENSO (El Nino Southern Oscillation). In the Indian ocean, ENSO episodes modify the monsoon regime and are accompanied by large changes in rainfall, wind oceanic current and heat distribution. As growth and calcification in corals are controlled by light and temperature, their skeletal record can be used to reconstruct past hydroclimatic regimes, such as ENSO. Long term (i.e., 100s of years) skeletal records can be obtained by removing drill core samples form massive reef corals such as Porites, which can live for many centuries and attain sizes of several meters in diameter. The oxygen and carbon isotopic composition of cores taken form corals correlates well with sea surface temperature and salinity. Changes in isotopic composition can reveal growth hiatus due to bleaching events Indian reef corals also show sensitivity to hydroclimatic fluctuations : depletion in 18-0 and 13-C reflect high oceanic temperature combined with enhanced precipitation and cloud cover. Other paleoclimatic records can be extracted from the aragonite skeletons of corals, such as the ratios of sr/Ca, U/Ca and Mg/Ca. These ratios are also temperature-dependent and can be used to construct a paleothermometer with an accuracy of ± 0.5 °C. Reef corals in the Arabian Gulf can be used in the same way to compare present and past patterns of growth in relation to global climate change.