Clumped Isotopes in Bahamian Dolomites: A Rosetta Stone?

Abstract:

Low temperature dolomite formation continues to be an enigmatic process. However, with the advent of the clumped isotope technique, there is an opportunity to determine the temperature of formation as well as the δ¹⁸O of the fluid (δ¹⁸O_w) from which it formed. By using samples with a well constrained geologic and thermal history, we have attempted to accurately develop a technique for the application of clumped isotopes to varying dolomite systems.

Samples for this study were taken from two cores, one from the island of San Salvador and one on Great Bahama Bank (known as Clino), located on the eastern and western edges respectively of the Bahamian Archipelago. Both cores penetrate through Pleistocene to Miocene aged carbonates. The San Salvador core has a 110m section of pure, near stoichiometric dolomite, while the Clino core is of a mixed carbonate composition with varying abundances (0% - 50%) of calcian dolomite (42-46 mol % MgCO₃). The water temperature profile of the Bahamas can be assumed over time due to the stable geology and no influence of higher temperature waters. Because of its location and the present burial depth, the largest influence on dolomite formation has been changes in sea level.

As the dolomites from San Salvador are 100% dolomite, the Δ₄₇ was determined directly. The Clino dolomites however were only partially dolomitized and so were treated with buffered acetic acid to remove non-dolomite carbonates. This was carried out in stages, using X-ray diffraction to determine composition, followed by the measurement of Δ₄₇ after each leaching episode. Because the dolomite formation temperature and δ¹⁸O_w can be constrained, it becomes possible to evaluate the applicability of the multitude of clumped isotope correction schemes that have been applied to various dolomite samples. Also tested were several different equations which link temperature to the δ¹⁸O of the dolomite allowing the δ¹⁸O of the water to be calculated. This is a necessary exercise for the study of dolomite because of the inability to properly standardize low temperature dolomite formation as well as the novelty of the clumped isotope technique. In doing so, we hope to establish accurate protocols in how to best handle low temperature dolomite clumped isotope data.