Quantitative estimates of paleo-aridity using stable isotopes

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Quantitative estimates of paleo-aridity using stable isotopesThure CerlingUniversity of UtahEAR-0617010This study was co-funded by EAR's Geobiology and Low Temperature Geochemistry and Sedimentary Geology and Paleobiology programs and NSF's Office of International Science and Engineering.ABSTRACTThe objective of this study is to determine the paleo-aridity of past environments using a water-deficit approach where water deficit the amount of excess potential evaporation compared to precipitation. During evaporation the lighter isotope of oxygen, 16-O, preferentially escapes to the atmosphere leaving the heavier isotope, 18-O, enriched in the remaining pool of water. Leaves represent such a situation and thus are highly enriched in 18-O compared to the source water. This project will use this difference in evaporative concentration to determine the amount of water deficit by comparing materials that preserve both the evaporated versus the non-evaporated water bodies. Preliminary studies have shown that for Africa, the hippopotamus and the giraffe represent two such end-members: the hippopotamus represents a non-evaporated source and the giraffe represents a highly evaporated water signal because of its dependence on leaves as a major source of water in its daily activities. Tooth enamel preserves well in the fossil record and thus should be able to be used to evaluate the water-deficit in the geological past. Other mammals will be identified that serve as indicators of the non-evaporated and the evaporated signals. This study is easiest to carry out in equatorial regions because of the low seasonal amplitude in the stable isotopic composition of water. If this method proves successful in tropical regions its application will be able to be extended to higher latitudes where the isotope cycle in water is more complex. Furthermore, it may lead to other methods of determining water deficit by demonstrating a method that uses the relative isotope enrichment resulting from evaporation from a water source.