Abstract
The tropical oceans drive climatic phenomena such as the El Niño-southern oscillation (ENSO) and the Asian–Australian monsoon, which have global scale impacts. In order to understand future climatic developments, it is essential to understand how the tropical climate has developed in the past, on both short and longer timescales. However, good instrumental records are limited to the last few decades. The oxygen isotopic (δ 18 O) composition and strontium/calcium (Sr/Ca) ratio of massive corals have been widely used as proxies for past changes in sea surface temperature (SST) of the tropical and subtropical oceans, because the geochemistry of the skeleton is believed to vary as a function of several environmental parameters (such as seawater temperature, salinity, light, …). However, recent microanalytical studies have revealed large amplitude variations in Sr/Ca and oxygen isotopic composition in coral skeletons; variations that cannot be ascribed to changes in SST or in salinity. Such micro- and nanometer scale studies of geochemical variations in coral skeletons are still few and somewhat scattered in terms of the species studied and the problems addressed. But collectively they show the great potential for determining chemical variations at length scales of direct relevance to the biomineralization process. For example, it is now possible to measure geochemical variations within the two basic, micrometer-sized building blocks of the coral skeleton: Early mineralization zones (EMZ) and aragonite fibres. Such micro- and nanometer scale observations, in combination with controlled laboratory culturing of corals, hold the promise of yielding important new insights into the various biomineralization processes that may affect the chemical and isotopic composition of the skeletons. One aim of these efforts is to better understand the elemental and isotopic fractionation mechanisms in order to improve the conversion of the geochemical variability into environmental changes.
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