Middle Miocene greenhouse and cooling: the deep North Atlantic Ocean record

我们社会上几乎每个人都意识到，人类活动增加了大气中二氧化碳的浓度，导致了气候变暖。不为人所知的是，上一次地球的二氧化碳水平与近期预测的一样高是在大约1600万年前，中新世中期的气候最佳时期。这一时期是全球发生重大变化的时期，包括广泛的火山活动导致二氧化碳含量高，“最佳气候”变暖，随后二氧化碳含量显著下降，导致气候变冷。因此，研究中新世中期的气候将提供必要的基线信息，以了解预期的未来二氧化碳驱动的全球变暖的影响。这些信息反过来又可以帮助决策者了解减缓和适应气候变化的方法。我的研究将应用化石中的“块状同位素”来确定中新世中期大西洋的深海和海面温度。这是一种直接的古温度计：与其他可用的选择不同，我们可以从古代化石中测量到的“团块”同位素组成直接受化石最初生长的温度控制。块状同位素古温度测量也使我们能够估计深水氧同位素组成，而不需要做出假设，这是替代替代方法所必需的。这些估算是估算全球冰量的关键组成部分，使我们能够了解当时气候变暖对地球冰盖的影响程度。由于我们目前没有全球覆盖中新世中期的深海氧同位素组成，我们目前无法破译迄今为止观测到的信号是否是真正的全球冰体积特征，或者是否有其他过程在起作用。最后，由于“团块”特征不受海水碳酸盐离子浓度变化的影响，因此团块同位素方法避免了唯一替代深水温度代理（Mg/Ca比）的已知问题；这一基本性质随着大气CO2的变化而显著变化，并在中新世中期被观察到影响Mg/Ca比值。在Exp 395期间，我们将回收沉积物的位置对其他对气候有重大影响的基本地球系统机制至关重要。北大西洋的海洋环流是当今气候系统的一个关键组成部分；不幸的是，在这一地区很少发现早于中新世中期的深海沉积物，因此不可能检验在气候变化的关键时期北大西洋环流是否有所不同。IODP 395探险队将首次从北大西洋的这一地区恢复适当的古老和连续的沉积物，为我们提供了一个令人兴奋的机会来发现，比较和对比北大西洋与其他地方发生的变化。这些信息对于将温度和深海氧同位素组成变化置于当前气候范式的背景下至关重要。

Nearly everyone in our society is aware that human activity has increased the concentration of carbon dioxide in the atmosphere, leading to warming. What is not common knowledge is that the last time Earth's CO2 levels were as high as predicted for the near future was about 16 million years ago, during the middle Miocene Climatic Optimum. This period was a time of significant global change, including extensive volcanism leading to the high CO2, warmer 'climate optimum', later followed by significant drops in CO2 linked with cooling. Consequently, studying the climate of the middle Miocene will provide baseline information required to understand the effects of the expected future CO2-driven global warming. This information can in turn help inform policy makers on ways to mitigate and adapt to climate change. My research will apply a proxy known as 'clumped isotopes' in fossils to determine deep sea and sea surface temperatures in the Atlantic Ocean during the middle Miocene. This proxy is a direct palaeothermometer: unlike other available options, the "clumped" isotope composition we can measure from ancient fossils is directly controlled by the temperature the fossils initially grew in. Clumped isotope palaeotemperature measurements also allow us to estimate deep water oxygen isotope compositions without the need to make assumptions, as would be necessary with alternative proxies. Such estimates are a key component for estimating global ice volume, enabling our understanding of how much Earth's ice sheets were impacted by warming climate at that time. Since we do not currently have global coverage for deep sea oxygen isotope compositions during the middle Miocene, we are currently unable to decipher whether the signal observed so far is a true global ice volume signature, or whether other processes are at play. Finally, the clumped isotope method avoids known issues with the only alternative deep water temperature proxy (Mg/Ca ratios), as the "clumping" signature is not impacted by changes in the carbonate ion concentration of seawater; this fundamental property changes significantly with changing atmospheric CO2, and has been observed to impact Mg/Ca ratios during the middle Miocene.The location where we will recover sediment during Exp 395 is crucial for other fundamental earth system mechanisms which exert large influences on climate. Ocean circulation in the North Atlantic is a key component of the climate system today; unfortunately, deep sea sediments as old as the middle Miocene have rarely been recovered in this area, so it has been impossible to examine whether North Atlantic circulation was different during that critical period of climate change. IODP Expedition 395 will recover suitably old and continuous sediments from this region of the North Atlantic for the first time, providing us with an exciting opportunity to discover, compare and contrast the changes that took place in the North Atlantic with elsewhere. This information is critical for placing the temperature and deep sea oxygen isotope composition changes into context within our current climate paradigm.