Strontium heterogeneity and speciation in coral aragonite: Implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/C a ratios of coral aragonite. However, microanalytical studies have indicate d that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr s ubstituted for Ca in aragonite and as separate SrCO3 (strontianite) domains . Variations in the size, quantity, or both of these domains may account fo r small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 mum scale. Var iations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS ) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) ar e absent or in minor abundance. Variations in strontianite abundance are no t responsible for the Sr/Ca fluctuations observed in this sample. The Sr mi crodistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect none quilibrium calcification processes. Nonequilibrium incorporation of Sr comp licates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXA FS technique may prove to be valuable, allowing the selection of coral micr ovolumes for Sr/Ca measurement where strontium is incorporated in a known s tructural environment. Copyright (C) 2001 Elsevier Science Ltd.