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

我们社会中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，导致气候变暖。不同寻常的是，上一次地球的二氧化碳水平在不久的将来达到预测的水平是在大约1600万年前，也就是中新世中期气候最佳时期。这段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了高二氧化碳，更温暖的“气候最佳”，随后与降温相关的二氧化碳显著下降。因此，研究中新世中期的气候将提供所需的基线信息，以了解预期的未来二氧化碳驱动的全球变暖的影响。这些信息反过来可以帮助政策制定者了解缓解和适应气候变化的方法。我的研究将在化石中应用一种被称为“块状同位素”的替代物，来确定中新世中期大西洋的深海和海面温度。这种替代物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“块状”同位素成分直接受化石最初生长的温度控制。聚集的同位素古温度测量还使我们能够估计深水氧同位素组成，而不需要做出假设，就像使用替代指标所必需的那样。这样的估计是估计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。由于我们目前没有中新世中期深海氧同位素组成的全球覆盖，我们目前无法破译到目前为止观察到的信号是否是真正的全球冰量特征，或者是否有其他过程在起作用。最后，块状同位素方法避免了唯一的替代深水温度指标(镁/钙比)的已知问题，因为“块状”特征不受海水碳酸盐离子浓度变化的影响；这一基本性质随着大气二氧化碳的变化而显著变化，并已被观察到在中新世影响镁/钙比。在Exp 395期间，我们将在哪里回收沉积物对于其他对气候产生重大影响的地球系统基本机制来说是至关重要的。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像中中新世那样古老的深海沉积物，因此无法研究北大西洋环流在气候变化的关键时期是否有所不同。IODP探险395号将首次从北大西洋的这一区域采集到合适的古老和连续的沉积物，为我们提供了一个发现、比较和对比北大西洋与其他地区发生的变化的令人兴奋的机会。这一信息对于将温度和深海氧同位素组成的变化纳入我们当前气候模式的背景至关重要。

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.