Quantitative estimates of paleo-aridity using stable isotopes

本研究由美国地质科学院地球生物学、低温地球化学、沉积地质学和古生物学项目和美国国家科学基金会国际科学与工程办公室共同资助。亏缺法，即与降水量相比，水分亏缺多余的潜在蒸发量。 在蒸发过程中，较轻的氧同位素16-O优先逃逸到大气中，留下较重的同位素18-O，富集在剩余的水池中。 叶子代表了这样的情况，因此与源水相比，叶子高度富集18-O。 本项目将利用蒸发浓度的差异，通过比较保存蒸发水体和非蒸发水体的材料来确定缺水量。 初步研究表明，在非洲，河马和长颈鹿代表了两个这样的终端成员：河马代表了一个非蒸发源，长颈鹿代表了一个高度蒸发的水信号，因为它依赖于树叶作为其日常活动的主要水源。 牙釉质在化石记录中保存得很好，因此应该能够用来评估地质学过去的缺水情况。 其他哺乳动物将被识别为非蒸发和蒸发信号的指示器。这项研究最容易在赤道地区进行，因为水的稳定同位素组成的季节幅度较低。 如果这种方法在热带地区证明是成功的，它的应用将能够扩展到水的同位素循环更复杂的高纬度地区。 此外，它可能会导致其他方法来确定水的赤字，通过演示一种方法，使用相对同位素富集导致蒸发从水源。

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.