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

大气二氧化碳作为温室气体的作用，以及它对今天的影响？S，全球温度，是众所周知的。基于模型和地球化学指标，研究人员提出，在地球S的历史上，二氧化碳浓度可能比今天的S水平高出几个数量级。再加上代表地球几百万年历史的百年S的浅海环境中形成的d18O碳酸盐矿物的测量，已被用作海洋温度超过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中d18O成分的稳定性。另一种假设是，海水的成分在地质时间跨度(数十亿年到数百万年)中有所不同，而不是要求地球拥有不切实际的高表面温度，而碳酸盐的d18O记录反映了这种变化。这项研究的目的是检验这一假设：D18O海相碳酸盐记录主要受温度控制，还是受海水组成的一阶变化控制。分析方法将利用一种新开发的古温度计，该温度计基于从水中沉淀的碳酸盐矿物中存在的13C和18O(团状同位素)的独特束缚同位素的丰度。与传统的d18O古温度计不同，块状同位素温度测量与水的组成无关，因此可以同时计算水温和水的组成。这项研究的一个独特方法是使用在淡水地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，应该保持一致和可靠的记录，可以用来重建广泛地质时间范围内的过去温度。如果我们的研究结果表明，大气中二氧化碳浓度过高与地球表面温度没有直接耦合，这将通过修改我们如何预测地球未来温度变化的参照系而产生重大影响？S，基于计算机模拟。其他研究人员提出了深部地质时期二氧化碳与温度的脱钩现象，这项研究对这一假设进行了实证检验。气候模型者重新设定这一参考框架将直接影响他们对未来气候变化幅度的估计，以应对因继续使用化石燃料能源而导致的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.