At the beginning of the project, we first had to set up the laboratory, which included some unexpected delays, but was very successful. We furthermore spent some time experimenting with analytical settings that allow us to measure smallest possible samples. Here we achieved a reduction in sample size by more than half compared to before, which is even better than what we had hoped for! We then tested the clumped isotope proxy for our intended application for reconstructing ocean temperatures by measuring 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, together with the analytical improvements, 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.
In the second half of the project we have been applying the method to reconstruct changes in ocean temperature across the most pronounced climate events of the last 50 million years. During all three intervals we studied (35 million years ago, 14 million years ago, and 3 million years ago), large reorganizations happened in the climate system, including the establishment of major ice sheets, but the exact nature of the climate changes are not well understood. Our results in some cases confirmed previous interpretations, which adds confidence to those. We have also seen some surprises, for example with deep ocean temperatures varying more than previously appreciated. Several of these latest findings are still being prepared for publication, and they might change our view on the role of the ocean in these global climate events. Overall, this project has greatly helped establishing clumped isotope thermometry as a reliable tool for reconstructing ocean temperatures. Reconstructions from this project and future work improve our understanding of the climate system under conditions different than those experienced by humans so far.
