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Climate change across Cenozoic cooling steps reconstructed with clumped isotope thermometry

Periodic Reporting for period 2 - C4T (Climate change across Cenozoic cooling steps reconstructed with clumped isotope thermometry)

Reporting period: 2017-02-01 to 2018-07-31

The Earth’s climate system is highly complex and future changes are difficult to predict. Fortunately, the geological record holds a wealth of information about how the system behaved in the past, when boundary conditions such as greenhouse gas concentrations were different or changed rapidly. The challenge is that past climate change cannot be measured directly, but has to be reconstructed based on indirect evidence from so-called climate proxies, such as changes in the chemical composition of fossil shells. The problem is that all proxies we use to reconstruct past climate depend on assumptions that are increasingly uncertain back in time.

A new kind of temperature proxy, the carbonate ‘clumped isotope’ thermometer, has great potential to overcome these obstacles. This method relies on the temperature-dependence of the distribution of different versions (isotopes) of carbon and oxygen atoms within molecules – a built-in thermometer that is largely independent of assumptions. If applied to the carbonate minerals making up the minute shells preserved in ocean sediments, this proxy would allow robust reconstructions of past temperatures in the surface and deep ocean, as well as global ice volume, far back in time. However, the technique has in the past required very large amounts of sample, limiting widespread application to reconstruct past temperatures in the ocean.

In this project, we set out to significantly reduce the sample amount requirements of the clumped isotope technique, building on recent successful modifications of the method and ideas for further analytical improvements. Afterwards, we have tested the proxy for application to marine fossil shells and are now applying it to reconstruct past climate change across major climate transitions in the past, where this new method can immediately contribute to solving long-standing questions about the climate system.
During the first half of the project period, we spent a long time setting up the method, due to undesired alterations of the signals in the instrument that is used for the analyses. After this problem had been solved, we tested the clumped isotope proxy for our intended application by measuring on modern shells from the top layers of the seafloor, where growth temperatures are known. We could show that shells from all the different species we tested behave the same way (with the clumped isotope signal reflecting growth temperature), meaning that we can use any of them, or a mixture, for the reconstructions. This will make it much easier to obtain sufficient material for the analyses from ocean sediments. Furthermore, we assessed how much the clumped isotope signal is altered when the shells are stored in the sediments over many millions of years. We could show that only in some locations, the addition of additional carbonate at the seafloor affects the proxy (in a similar way to other proxies based on shell material), but no other modifications were observed. This finding adds much confidence to the results from clumped isotope thermometry from ancient sediments. Most recently, we have started applying the method to sediments dating to known major climatic transitions, where information from other proxies has been limited.
With the reconstruction of ocean temperature changes across major climatic events in Earth’s history, we expect to add new knowledge about how the climate system behaved during these transitions. In the first preliminary data we are obtaining, we are already beginning to see variations that have not been observed before. Altogether, the project will
1) provide the climate community with an improved tool to reconstruct past changes in the ocean
2) yield new reliable data that can be used to test climate models used for future projections
3) further our understanding about the Earth’s climate system by deciphering changes that happened during climatic turnovers in the past.