A tandem Mg/Ca-clumped isotope approach for palaeotemperature estimation of past climate

对全球温度变化幅度的估计是预测未来气候的核心。然而，大多数数值气候模式研究都集中在仪器周期上，它只覆盖相对较短的时间（<100年）。为了更好地了解未来的气候变化并测试这些模型的稳健性，重建整个地球历史上过去的温度变化是很重要的。然而，这不是一个微不足道的问题，因为我们必须依靠温度变化的代理，而这些代理往往产生模棱两可的结果。在过去的几十年里，研究人员开发了几种过去海洋温度的代用指标，包括古生态传递函数、Mg/Ca和生物标志物（Uk'37, TEX86）。古温学的最新进展之一是基于碳酸盐中13C-18O界丰度的“团块同位素”的应用。该方法的优点是13C18O16O2的形成与温度有关，但与溶解的无机碳和海水的d18O无关。因此，同时测量“团块同位素”和传统氧同位素为碳酸盐古温度方程提供了一个独特的解决方案，同时限制了水的温度和d18O。有孔虫方解石的Mg/Ca比值也同样显示出温度依赖性。因此，对来自同一有孔虫标本的Mg/Ca和氧同位素进行配对测量，可以重建海水的氧同位素。该提案的主要目的是将同一有孔虫试验的Mg/Ca和团块同位素测量结果结合起来，以产生对温度的独立估计。结合Mg/Ca和团块同位素古温度方程，可以预测Mg/Ca和团块同位素之间的预期关系。对化石有孔虫的预期关系的遵守将提供对古温度估计的信心，或者，如果它们偏离预期关系，将确定一个或两个代理的问题。首先，我们将使用现代有孔虫来测试预测的关系，这些有孔虫覆盖的温度范围从大约。6 ~ 28度验证和校准方法。接下来，我们将使用北大西洋一个年代确定的沉积物岩心样带来测试该方法在末次冰期的应用。在最近一次冰期消退期间，气候突变事件的规模在科学界仍存在争议。例如，在这些极端事件期间，根据海洋（~2-4度）和陆地（~6-10度）档案推断的热带降温估计存在很大差异。这种差异可能是温度代用品季节偏差的结果。然而，热带地区的平均季节性温差的温度范围比预期的要大得多。这就提出了这样一个问题：这些差异是真实的，还是由特定代理的敏感性或可靠性差异引起的。通过对Mg/Ca和团块同位素的串联测量，将有可能确定末次冰期气候突变事件期间温度变化的幅度并估计其空间范围。此外，确定年代的晚第四纪岩心为我们的串联Mg/ ca块状同位素古温计提供了一个理想的测试案例，其成功的开发和应用将允许研究许多有关地球气候历史的开放性问题。

The estimation of the magnitude of global temperature change is central for predicting future climate. However, most numerical climate modeling studies have focused on the instrumental period, which covers only a relatively small amount of time (<100 years). To better understand future climate change and to test the robustness of these models, it is important to reconstruct past temperature changes throughout Earth history. This is not a trivial problem, however, because we must rely on proxies of temperature change that often yield ambiguous results. Over the past decades several proxies of past ocean temperature have been developed including palaeoecological transfer functions, Mg/Ca, and biomarkers (Uk'37, TEX86). Among the newest advances in palaeothermometry is the application of "clumped isotopes" which is based on the abundance of 13C-18O bounds in carbonates. The advantage of this method is that the formation of 13C18O16O2 is temperature dependent but independent of the dissolved inorganic carbon and the d18O of seawater. Therefore, the simultaneous measurement of "clumped isotopes" and traditional oxygen isotopes provides a unique solution to the carbonate palaeotemperature equation by constraining both temperature and d18O of water. The Mg/Ca ratio of foraminiferal calcite has been similarly shown to be temperature dependent. Hence, paired measurements of Mg/Ca and oxygen isotopes from the same foraminiferal specimens permit reconstruction of the oxygen isotopes of the sea water. The main aim of the proposal is to combine Mg/Ca and clumped isotope measurements of the same foraminiferal tests to produce independent estimates of temperature. By combining the Mg/Ca and clumped isotopes palaeotemperature equations, the expected relationship between Mg/Ca and clumped isotopes can be predicted. Adherence to the expected relationship in fossil foraminifera will provide confidence in the palaeotemperature estimates or, if they diverge from the expected relationship, will identify problems with either one or both proxies. Initially, we will test the predicted relationship using modern foraminifera which cover a temperature range from approx. 6 to 28 degree to verify and calibrate the method. Next, we will test the application of the method during the last glacial period using a transect of well-dated sediment cores in the north Atlantic. The magnitude of abrupt climate change events during the last deglacial is still under debate in the scientific community. For example, large discrepancies exist in estimates of tropical cooling inferred from marine (~2-4 degree) and terrestrial (~6-10 degree) archives during these extreme events. This discrepancy may be the result of seasonal bias of the temperature proxies. However, the temperature range is much larger than expected for average seasonal temperature differences in the tropics. This raises the question whether these discrepancies are real or caused by differences in the sensitivity or reliability of specific proxies. With tandem measurements of Mg/Ca and clumped isotopes it will be possible to determine the magnitude of temperature change and estimate its spatial extent during abrupt climate change events of the last glacial. Furthermore, the well-dated cores of the late quaternary provide an ideal test case for our tandem Mg/Ca-clumped isotope palaeothermometer whose successful development and application will permit to investigate many relevant open questions about earth's climate history.