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Dolomitization in the Bahamas as Constrained Using the Clumped Isotope Method


We have measured the ?47 in dolomites formed less than ∼150,000 years ago in Pliocene aged sediments from the island of San Salvador in the Bahamas. By virtue of their formation age and the depth at which they were found, it was proposed by previous workers that these dolomites formed at relatively low temperatures (20-25oC) and from fluids with a salinity close to that of normal seawater, possessing a d18O value of between 0 and + 1‰ SMOW. These dolomites therefore offer the opportunity to verify the equation proposed to link ?47 to temperature in dolomites. Based on the ?47 analyses of these samples we propose a modified equation, adjusted for the theoretical difference between calcite and dolomite. Considering the proposed mechanism of dolomitization, the present burial depth, and temperature of the waters at that depth, this adjustment provides more realistic formation temperatures for the San Salvador dolomites than the equation used in previous studies on dolomites. Utilizing the temperatures calculated in this manner, we have assessed the suitability of seven different equations which link the fractionation of oxygen isotopes between dolomite and water as a function of temperature. Finally, we have applied these new findings to the previous studies of dolomites which have employed the ?47 method and present a modified model of dolomite formation for San Salvador in which an initial dolomitization event involving slightly evaporated seawater occurred soon after deposition and was followed by a later episode in which the fluids were normal seawater.