Mass-47 Paleothermometry in Deep Geologic Time: A Test for Episodes of Extreme Warmth in the Phanerozoic

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The role of atmospheric CO2 as a greenhouse gas, and its impact on today?s global temperature, is well established. Based on models and geochemical proxies, researchers propose that during Earth?s history, CO2 concentrations may have been orders of magnitude higher than today?s levels. This coupled with measurements of d18O carbonate minerals formed in shallow marine settings representing 100?s of millions of years of Earth history, has been used as prime evidence for an exceedingly hot Earth with ocean temperatures in excess of 50 °C. A fundamental assumption of these studies, however, is the constancy of the d18O composition of the seawater. Rather than requiring an Earth with unrealistically high surface temperatures,an alternative hypothesis is that the composition of seawater has varied through the span of geologic time (billions to millions of years) and that the d18O record of carbonate reflects such changes. The purpose of this study is to test this hypothesis: Is the d18O marine carbonate record primarily controlled by temperature, or alternatively, by first-order shifts in marine water composition. The analytical approach will utilize a newly developed paleothermometer which is based on the abundance of uniquely bounded isotopes of 13C and 18O (clumped isotopes) that are present in carbonate minerals precipitated from water. Unlike the conventional d18O paleothermometer, the clumped isotope measurement of temperature is independent of the water composition, and thus, allows simultaneous calculation of both water temperature and composition. A unique approach of this study is the use of crystalline carbonate that forms beneath the freshwater table, and as such, records the mean annual air temperature of a region. These carbonates are chemically and mineralogically stable and should preserve a consistent and reliable record which can be used to reconstruct past temperatures over the broad spectrum of geologic time. If the findings of our research indicate that exceeding high CO2 concentrations in the atmosphere are not directly coupled to Earth surface temperature, this will have a significant impact by modifying the reference frame of how we predict changes in Earth?s future temperature based on computer simulations. Other researchers have suggested such a decoupling of CO2 and temperature in deep geologic time and this study provides an empirical test of this hypothesis. Resetting of this reference frame for climate modelers would directly affect their estimates of the magnitude of future climate change in response to the rising concentration of atmospheric greenhouse gases caused by the continuing use of fossil fuel energy sources.