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

大气中二氧化碳作为温室气体的作用及其对当今的影响？的全球温度，是公认的。 基于模型和地球化学代理，研究人员提出，在地球？在历史上，二氧化碳浓度可能比今天高出几个数量级？的水平。 这再加上测量d18O碳酸盐矿物形成的浅海设置代表100？在数百万年的地球历史中，它被用来作为海洋温度超过50 °C的极热地球的主要证据。 然而，这些研究的一个基本假设是海水的d18O组成是恒定的。 与其要求地球具有不切实际的高表面温度，另一种假设是海水的成分在地质时期（数十亿至数百万年）发生了变化，碳酸盐的d18O记录反映了这种变化。 本研究的目的是测试这一假设：是d18O海洋碳酸盐记录主要由温度控制，或者，由一阶位移海水成分。 分析方法将利用新开发的古温度计，该温度计基于从水中沉淀的碳酸盐矿物中存在的独特结合的13 C和18 O同位素（聚集同位素）的丰度。 与传统的d18O古温度计不同，温度的聚集同位素测量与水的成分无关，因此，可以同时计算水温和成分。 这项研究的一个独特方法是使用在淡水表下形成的结晶碳酸盐，因此记录了一个地区的年平均气温。 这些碳酸盐在化学和矿物学上是稳定的，应该能保存一致和可靠的记录，可用于重建地质时期的过去温度。如果我们的研究结果表明，大气中过高的二氧化碳浓度与地球表面温度没有直接联系，这将通过修改我们预测地球变化的参考框架产生重大影响。的未来温度基于计算机模拟。 其他研究人员已经提出了这样一个脱钩的二氧化碳和温度在地质年代深处，这项研究提供了一个实证检验这一假设。 为气候建模者重新设定这一参照基准将直接影响他们对未来气候变化幅度的估计，因为持续使用化石燃料能源造成大气温室气体浓度上升。

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.