Comparing and refining two novel methods for paleothermometry in speleothems - carbonate clumped isotopes vs. fluid inclusion noble gas concentrations

洞穴方解石越来越多地被用作古气候档案。然而，它们的碳氧同位素受许多参数的影响，因此不能用于直接的古温度重建。创新的方法已经开发出来，以确定绝对古温度独立于方解石δ18O。流体包裹体稀有气体浓度和碳酸盐团块同位素测温是两种最有前途的方法。块状同位素温度计是基于碳酸盐晶格中13C-18O键的温度依赖丰度，并已成功应用于各种海洋生物的碳酸盐壳，反映方解石形成过程中的温度。稀有气体方法是基于气体在水中的温度依赖性溶解度，是地下水中传统稀有气体方法的一种变体，适用于岩洞流体包裹体。两种方法的精度均可达±1℃。这些方法分别在以色列和德国的洞穴中进行了初步测试，结果在不同时期之间产生了合理的温度差异，尽管在这两种方法中都观察到现代系统中的温度偏移。两种方法都受到不同约束条件的限制；在块状同位素的情况下，不太清楚的非平衡分馏和在惰性气体测温的情况下，在一些石笋中检测到的剩余成分。拟议的研究将通过测量现代洞穴样品和稀有气体温度已经确定的样品中的团块同位素来比较两种测温方法。这将使我们能够更好地了解在这两种方法中观察到的温度偏移的来源，并表征这些方法可以应用的洞穴形成条件。

Speleothem calcite is increasingly used as paleoclimate archive. However, their carbon and oxygen isotopes are affected by many parameters and therefore cannot be used for direct paleotemperature reconstruction. Innovative methods have been developed to determine absolute paleotemperatures independently of calcite δ18O. Fluid inclusion noble gases concentrations and carbonate clumped isotope thermometry are two of the most promising approaches. The clumped isotope thermometer is based on the temperature dependent abundance of 13C-18O bonds in the carbonate lattice and has been successfully applied in carbonate shells from a variety of marine organisms, reflecting the temperature during calcite formation. The noble gas method is based on the temperature dependent solubility of gases in water and is a variant of the traditional noble gas approach in groundwater, applied to speleothem fluid inclusions. The precision of both methods is up to ± 1°C. Preliminary tests of these methods in speleothems from Israel and Germany, respectively, resulted in reasonable temperature differences between distinct time periods though in both methods a temperature offset in the modern system was observed. Both methods are limited by different constraints; the not-well understood non-equilibrium fractionation in the case of clumped isotopes and a surplus component detected in some stalagmites in the case of the noble gas thermometry. The proposed research will compare the two thermometry methods by measuring clumped isotopes in modern speleothem samples and in samples where noble gas temperatures have been already determined. This will enable us to better understand the source of temperature offsets observed in both methods and to characterize the speleothem formation conditions in which these methods can be applied.