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Climate change across Cenozoic cooling steps reconstructed with clumped isotope thermometry

Periodic Reporting for period 4 - C4T (Climate change across Cenozoic cooling steps reconstructed with clumped isotope thermometry)

Reporting period: 2020-02-01 to 2021-01-31

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. Unfortunately all proxies we use to reconstruct past climate depend on assumptions that are increasingly uncertain back in time.

In this project, we focused a new kind of temperature proxy, the carbonate ‘clumped isotope’ thermometer, that can overcome these obstacles, because it is largely independent of assumptions.
We used the method on the carbonate minerals making up the minute shells preserved in ocean sediments, to derive 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 first focused on improving the method for reconstructions of ocean temperature. We were able to significantly decrease required sample amounts, and could show that clumped isotope thermometry in microfossil shells reliably records ocean temperature, and that the signal is preserved while the shells are stored in ocean sediments over many million of years. We have then applied the method and derived new deep and surface ocean temperature records covering three periods of major climate change, including the establishment of ice sheets in the southern and northern hemispheres. Our data show that during these periods, the deep ocean functioned differently from today, as we often found unexpectedly warm and variable temperatures. These results already fill some important knowledge gaps and will pave the way for future applications of this proxy to more locations and time periods, to derive a more complete picture of past climate changes that allows us to better understand the climate system and predict its behavior in the future.
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.
We have now established clumped isotope thermometry as a reliable and highly valuable tool for reconstructing changes in ocean temperature far back in time. This allows us to find out with much more certainty how greenhouse gases, ocean circulation, and ice sheet interacted during major climate events and transitions in the past. From our results, we have already learned that the temperature in the Southern Ocean around Antarctica and the Antarctic ice sheet were closely coupled during a time when that ice sheet greatly expanded, around 14 million years ago. We also know that the ice sheet expanded despite much warmer deep water temperatures compared to today, which probably means that the ocean circulation around the ice sheet was different. Further back in time, we see unexpected changes in deep ocean temperature that previous reconstructions had not revealed, and which we are still trying to understand. Overall, this project was the starting point for many more reconstructions of ocean temperature, allowing for exciting discoveries and new knowledge about the climate system and how its components interact.
Nele Meckler preparing gas samples for clumped isotope analysis
Microfossils that will be analysed to determine past ocean temperature