Carbonate clumped-isotopic constraints on marine temperatures during the Cretaceous

关于地球过去的气候，我们可以问的一个基本问题是，地表温度是如何对外部强迫做出反应的？有证据表明，在白垩纪时期（约1.46亿至6500万年前），地球的气候比现在更温暖、更稳定——这一解释是基于热生理（不耐冷）动物、植物和地球化学指标的分布。白垩纪的温暖与大气中高浓度的二氧化碳有关。尽管大多数证据表明地球明显变暖，极地地区变暖（包括碳酸盐块状同位素数据，一种重建温度的新工具），但一些研究表明，在这一时期，温度和二氧化碳之间存在负相关关系（或脱钩）。虽然中生代不是未来温室变暖的直接模拟，但地球历史上的这些间隔为了解气候系统的运行过程提供了重要的见解。因此，当我们试图将当前气候置于预测的未来情景的背景下时，大气中二氧化碳和温度之间的关系就显得更加重要了。白垩纪温暖的极地温度确实挑战了我们对海洋-大气系统如何运作的理解，因为它们比一般环流模型所能重现的要暖和得多。这对预测未来气候具有重要意义，因为它意味着我们可能低估了这些地区未来的气候变化。尽管对白垩纪海洋温度进行了深入的研究，但白垩纪温室世界的赤道到极点温度分布仍然很有限。问题源于这样一个事实，即传统的古代用物，如氧同位素，需要对海水的同位素组成进行估计——这在高纬度地区尤其难以限制。最近的一种方法是使用TEX86古温度计（一种温度代理）来回避这个问题，尽管相对于白垩纪，这种技术受到适当保存的沉积物分布的限制，特别是在高纬度地区。本研究旨在通过使用新型的块状同位素古温计，对从软体动物化石中获得的早白垩世（瓦兰吉尼亚-豪特里维）海洋温度进行首次定量和系统的研究，以解决这一重大知识空白。这些数据将与其他技术产生的新温度数据相结合。除了生成白垩纪热带、温带和极地地区的海洋温度外，我们还将能够限制海水的同位素组成，因此也将能够为关于什么是最可能增加热量从赤道向两极转移的机制的辩论提供数据。我们的新块状同位素衍生的温度也有望通过提供广泛的新数据来测试模型输出，从而对更广泛的古海洋学家和气候建模者有益。

A fundamental question we can ask about the Earth's past climate is how does surface temperature respond to external forcing? Evidence suggests that the Earth's climate was warmer and more equable during the Cretaceous Period (circa 146 to 65 million years ago) than it is today - an interpretation based on the distribution of thermophylic (cold intolerant) animals, plants and geochemical proxies.This warmth during the Cretaceous has been linked to high atmospheric CO2 concentrations. Although most evidence indicates a significantly warmer Earth, with warm polar regions (including carbonate clumped-isotope data, a new tool for reconstructing temperatures), some research has suggested a negative relationship (or decoupling) between temperature and CO2 during this period. Although the Mesozoic is not a direct analogue for future greenhouse warming such intervals in Earth history provide important insights into processes operating in the climate system. Hence the relationship between atmospheric CO2 and temperature has added significance as we try to place the present climate in the context of predicted future scenarios. Warm polar temperatures during the Cretaceous indeed challenge our understanding of how the ocean-atmosphere system operated as they are significantly warmer than General Circulation Models can reproduce. This has important implications for the prediction of future climates as it implies we may be underestimating future climate change in such regions. Despite the intensive study of Cretaceous marine temperatures an equator-to-pole temperature profile for the Cretaceous greenhouse world remains poorly constrained. Problems stem from the fact that traditional palaeoproxies like oxygen isotopes requires an estimate of the isotopic composition of seawater - particularly difficult to constrain at high latitudes. A more recent approach has been to sidestep this issue by using the TEX86 palaeothermometer (a temperature proxy), although with respect to the Cretaceous this technique is limited by the distribution of suitably preserved sediments, particularly at high latitudes.This proposal aims to address this significant gap in knowledge by undertaking the first quantitative and systematic study of early Cretaceous (Valanginian-Hauterivian) marine temperatures obtained from fossil molluscs using the novel clumped-isotope palaeothermometer. These data will be integrated with new temperature data arising from other techniques. In addition to generating marine temperatures for the Cretaceous tropics, temperate and polar regions we will be able to constrain the isotopic composition of seawater and hence will also be able to provide data concerning the debate about what is the most likely mechanisms to increase the transfer of heat from the equator towards the poles. Our novel clumped-isotope derived temperatures are also expected to be of benefit to the wider community of palaeoceanographers and climate modellers by providing extensive, new data against which to test model outputs.