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

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.