Analytical development of the use of the isotopic composition of gypsum hydration water as a paleoclimate tool

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One of the fundamental problems in oxygen isotope paleoclimatology is the temperature equation is often under constrained. The d18O of calcite or aragonite can be measured on fossil shell material but a unique temperature solution is not possible without knowing the d18Owater from which the carbonate precipitated. ToC = 16 - 4.14 (d18Ocalcite - d18Owater) + 0.13 (d18Ocalcite - d18Owater)2 Or for aragonite: ToC = 21.8 - 4.69 (d18Oaragonite - d18Owater) Gypsum (CaSO4 2H2O) is a hydrated mineral that contains 20.9% crystallization water by weight. The 18O/16O and D/H of the hydration water record the isotopic ratio of the mother water from which the gypsum crystallized. Provided the fractionation factors are known and no isotopic exchange has occurred post-crystallization, the d18O of gypsum hydration water can be used to calculate the d18O of the water. Thus, tandem d18O measurements of both carbonate and gypsum hydration water from the same samples potentially offers a power paleoclimate tool for determining past temperature. We propose to measure the d18O and dD of gypsum hydration water (CaSO4 2H2O) in controlled experiments and natural samples to test the potential of this method and its applicability as a paleoclimate tool. Specifically we will: 1. Determine the isotopic fractionation factors between gypsum hydration water and mother water to high precision; 2. Experimentally evaluate the potential for post-crystallization isotope exchange of gypsum hydration water and ambient environmental water; 3. Test the application of tandem measurements of d18O of calcite and gypsum hydration water to reconstruct glacial tropical temperature using sediment cores from Lake Peten-Itza, Guatemala. Successful demonstration of the method by this pilot study would constitute a breakthrough for terrestrial paleothermometry studies and offer the opportunity for widespread application.