Understanding the processes affecting 'clumped isotopes' composition in speleothems and their implication to paleo-temperature reconstruction

Yale University, New Haven CT

Investigators

Abstract

The aim of the proposed research is to study the mechanism that controls "clumped isotope" values in speleothems, in an attempt to apply the newly developed carbonate "clumped isotopes" thermometry to paleo-temperature reconstruction using carbonate cave deposits. The oxygen isotope composition of carbonate minerals is widely used as a proxy for climatic variations, based on the temperature dependency of the isotope exchange equilibrium between the carbonate mineral and the water in which it was formed. It is increasingly applied in carbonate cave deposits, speleothems, that provide an excellent source material for efforts to reconstruct past climatic conditions on land. However, using carbonate oxygen isotopes suffers a major disadvantage in that their values reflect a combination of variations in temperatures and in the isotope composition of the water in which the carbonate is formed. This can be overcome by the newly developed "clumped isotopes" thermometry. "Clumped isotope" values are based on mass 47 in CO2 extracted from carbonates and reflect a temperature dependent preference of 13C and 18O to create a bond with each other. Being an internal characteristic of the carbonate mineral, it is independent of the isotopic composition of the water in which the carbonate is formed and can therefore be used to independently determine carbonate growth temperatures. This approach has been applied to growth temperature reconstruction of a wide range of marine organisms, in which the observed values in modern carbonate shells followed a "clumped isotopes" temperature calibration that was produced by equilibrium precipitation of synthetic calcite. Modern speleothems, however, deviate from this calibration. This deviation is consistent with a theoretical description of a kinetic isotope effect associate with degassing of CO2 from a bicarbonate solution. The proposed research will explore the differences in "clumped isotopes" between carbonate precipitation in bulk solution (as in marine organisms) vs. precipitation in a thin film of solution, as in speleothem formation. Further, it will examine the effect of precipitation conditions (such as temperature, film thickness, and rate of CO2 degassing) on the extent of deviation from the equilibrium. It will test the hypothesis, based on preliminary work in speleothems from Soreq cave (Israel), that the deviation is not sensitive to precipitation conditions so that a simple correction can be performed by comparison to modern speleothems, to allow paleo-climate reconstruction using "clumped isotopes" in speleothems. This will be performed by (1) "clumped isotopes" analyses of different types of modern speleothems in Soreq cave, precipitated at varying film thickness (from stalagmites formed under slow water drips that result in a very thin film, to carbonate precipitating in water pools, equivalent to precipitation in bulk solution) in order to identify the cause of the deviation from equilibrium and test the hypothesis that it is related to thin film precipitation;(2 examining "clumped isotopes" in laboratory carbonates, precipitated synthetically from a thin film solution, at a variety of controlled conditions, to test the sensitivity to precipitation conditions; (3) obtaining a speleothem specific temperature calibration by analyzing modern speleothem samples from caves of a wide range of modern-day temperatures. Intellectual merit. "Clumped isotopes" thermometry is a powerful new technique in paelo-climate research. Its combination with speleothems, that provide a well-dated archive material, would significantly improve the ability to reconstruct past climatic conditions on land. However, the application of the "clumped isotopes" technique in speleothems is hindered by not yet understood kinetic isotope effects. The proposed research will clarify these effects, adapting the ?clumped isotopes? thermometry to speleothems, and will thus enable future efforts of continental paleo-temperature reconstruction. Broader impact. The proposed project will provide educational training for a postdoctoral scholar and a graduate student as well as a potential undergraduate student, providing them the opportunity to learn a novel and complex geochemical technique. This project will establish a "clumped isotopes" laboratory beyond the one existing facility, providing an opportunity for testing the methodology and developing standardization. It will thus significantly extend the accessibility of "clumped isotopes" analyses to potential collaborators within the paleo-climate community.