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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Tropical Temperature Reconstruction Across 0.5 million years from Cave formations

Alternative title: Tropiske temperaturendringer gjennom de siste 0.5 million år rekonstruert fra dryppestein

Awarded: NOK 9.9 mill.

Future climate change in tropical regions is among the most uncertain aspects of climate projections under rising levels of greenhouse gases yet will affect a large fraction of the global population. Fortunately, we can improve our understanding of tropical climate by studying changes in the past when global climate was different than today. The T-TRAC project has used dripstones (stalagmites) from caves in Borneo to reconstruct how climate in the tropics has changed over the last half million years. Stalagmites grow continuously by precipitation of calcite from drip water, recording information about the environment they grew in. The age of individual layers can be determined precisely, making them extremely valuable archives. We have previously been using the chemical (isotopic) composition of these stalagmites as an indicator for past variations in rainfall. With T-TRAC, we turned to another hidden archive in stalagmites: Tiny inclusions of ancient drip water trapped within the rocks. We can determine the density of the water in these inclusions to reconstruct the temperature at which the water was closed off from the environment. For this we use a method called liquid-vapor homogenization. The homogenization method is now established in Bergen, the only place worldwide where this method is applied to stalagmites. We analyzed samples of different age from Borneo. The results yielded detailed temperature reconstructions for several important time intervals, including major climate transitions out of and into ice ages. The temperature data are exceptionally reliable because the homogenization method requires much fewer assumptions than previous approaches based for example on microfossils in ocean sediments. In addition, stalagmites have a built-in "clock": We can date the age of the layers based on the decay of Uranium that is incorporated when the stalagmites grow. This means that we now have precise knowledge on how much and also when tropical temperatures changed in the past. Our results show that tropical temperature changed significantly between ice ages and warm intervals and that it closely followed changes in the concentration of atmospheric CO2. This result is noteworthy as there have been repeated periods in the past characterized by opposing temperature changes in different regions of the world, due to changes in heat distribution by ocean currents, and it is important to understand how the tropics behaved during these times. We found that during the time periods we studied, temperature in Borneo always followed CO2 and Southern Hemisphere temperature, even during times when the Northern Hemisphere cooled strongly, and ice sheets expanded in the North. In other words, tropical temperature appears insensitive to changes in heat redistribution by ocean currents that can strongly affect the high Northern latitudes. In addition to the homogenization method, we used other methods on the same samples and found that those can yield reliable temperatures as well, although with larger uncertainties. One of these approaches uses the isotopic composition of fluid inclusion water. The analytical system we established to do these measurements allowed us to not only determine past temperatures but also yields more direct information on the composition of ancient rainwater and thereby changes in the water cycle. These results showed that in contrast to temperature, tropical rainfall responds strongly to Northern Hemisphere cooling events and associated changes in ocean and atmospheric circulation patterns. While the homogenization method yielded reliable results in several samples, we also found one stalagmite where the method did not work as expected. Based on this finding, we were able to acquire new funding from the European Research Council for the next five years to further improve the method and apply it in other regions of the world. In spring 2018 we had a big field campaign in Borneo, together with colleagues from the US, Austria and Great Britain, as well as local guides from Gunung Mulu National Park. We set out instruments to monitor variations in cave temperature today, collected many samples of stalagmites and drip water, and made some chemical analyses on the water. Pictures from our expedition were posted on Instagram (@muluclimatescience) and we were accompanied by a team from the German magazine GEO who published an article about our work in January 2019. While in the park we talked a lot with the local guides, who can teach the many park visitors about the climate archives that can be found in the caves. The stalagmite samples we took in Borneo have been dated with the help of our American collaborators. It turned out that we were lucky and one of the samples covers exactly the last interval we so far did not have any data for. This means that in the future we will have a complete tropical climate record over the last half million years.

With the T-TRAC project, we have set up two methods for reconstructing past climate from stalagmites in Bergen, which will be available to the research community. One of them, the laser-aided liquid-vapor homogenisation method is currently the only setup of its kind worldwide in speleothem research and has already attracted international attention. We expect that the user group of this new facility will increase over the next years, which will also further enhance international collaboration. This facility can furthermore be of interest in many other applications where classical liquid-vapour homogenisation thermometry is hindered by metastability, such as minerals formed under low temperatures or in salty fluids. Our robust temperatures reconstructed from tropical speleothems provide long-sought constraints and showcase the large potential of speleothem data as climate model benchmarks.

Future climate change in tropical regions is among the most uncertain aspects of climate projections under rising levels of greenhouse gases. We can improve our understanding of climate in this crucial region by reconstructing tropical climate under varying boundary conditions in the past. However, precisely dated paleoclimate data from tropical regions that cover sufficiently long time intervals to disentangle various influences are still rare. T-TRAC aims to provide the first temperature record for the tropical West Pacific covering the last half million years from well-dated stalagmites from Borneo. The project will be embedded in an international collaborative network and can build on previous achievements of this group. We will use a highly innovative method based on physical properties of fluid inclusions trapped in the calcite matrix, called liquid-vapor homogenization. In a recent proxy comparison study we found this technique to yield the most reliable and precise temperature estimates. We will complement our previous ground-truthing work with extensive cave monitoring and further measurements on modern samples grown at known cave temperatures. In addition, we will apply two other methods, namely fluid inclusion isotope and clumped isotope thermometry, affected by isotopic disequilibria during speleothem formation. Applying these proxies alongside our primary method and on modern speleothems will shed new light on speleothem formation and lead to a better understanding of these new methods. We will furthermore try to close the last remaining gap of 50 kyr within our 570 kyr long speleothem record. With T-TRAC, we will be able to address several long-standing questions, such as the amplitude of glacial-interglacial tropical temperature changes, relative importance of various influences on tropical precipitation on multiple timescales, and tropical climate sensitivity to past changes in greenhouse forcing.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek