Final Report Summary - PACEMAKER (Past Continental Climate Change: Temperatures from marine and lacustrine archives)
Global climate change is a topic of major interest as it has a large impact on human societies. Computer models used to predict directions of future climate change are validated by means of retrospective analysis of past climate changes. Detailed reconstruction of past climate, especially temperature, is, therefore, of considerable importance. Several tools (proxies) are available to reconstruct absolute sea surface temperatures. Continental temperature reconstructions, however, are hampered by a lack of quantitative temperature proxies and, consequently, are often qualitative rather than quantitative. The goal of the PACEMAKER project was to evaluate in depth a newly quantitative continental temperature proxy that was discovered by our laboratory. This proxy is based on the distribution of membrane lipids (so called brGDGTs; branched glycerol dialkyl glycerol tetraethers) of soil bacteria found in marine sediments that are believed to be transported by rivers to the ocean and deposited in marine sediments. Since the composition of brGDGTs in soil is a function of annual mean air temperature (MAT), determination of the brGDGT distribution in sediment cores from river fans can, thus, potentially be used to reconstruct continental, river basin-integrated, temperatures from a marine record in front of large river outflows.
We studied the biological origin of brGDGTs in soil and found that that they are likely produced by Acidobacteria, a dominant member of the microbial community in soil. For four major river systems, the Amazon river in Brazil, the Yenisei in Siberia, the Tagus river in Spain/Portugal, and the Rhône river in France, brGDGT were investigated from source to sink, by extensive sampling campaigns. We discovered, by using new methodologies, that in-situ production of brGDGT in the river effects the distribution of the brGDGTs that are delivered by soil erosion. This varies from one river system to another. In the Yenisei, for example, in-situ production plays an important role while this is less so in the Amazon and Rhone. A further complication in the use of brGDGT is that they can also be produced in-situ in marine sediments, which indicates that brGDGTs in marine sediments can only be used to reconstruct continental climate when the core location is close to a river mouth. We constructed improved temperature calibrations for brGDGTs in soil. We also discovered that the analytical methodology for the analysis of brGDGTs could be improved, resulting in the identification of a new group of brGDGTs that helped to improve the soil calibration even further. We have also investigated the potential of lake sediments as archives of continental climate change using the palaeothermometer based on brGDGTs. Extensive field work on various lakes was performed to unravel the sources of brGDGTs and environmental information contained in their distributions. We discovered that the distribution of brGDGTs does vary with temperature and pH but in a different way than for soils. In contrast to what was thought earlier, brGDGTs in most lakes are predominantly produced in the lake itself. Consequently, application of brGDGT to reconstruct climate of the past is more complicated than previously thought but still holds great promise. During the project also important improvements (i.e. calibration, recognition of deep water Thaumarchaeota) for TEX86 palaeothermometry (the other GDGT-based palaeothermometer) were made. Using these two palaeothermometers, reconstructions of past climate condition have been performed, assisting in our understanding of the climate evolution of the Earth.
We studied the biological origin of brGDGTs in soil and found that that they are likely produced by Acidobacteria, a dominant member of the microbial community in soil. For four major river systems, the Amazon river in Brazil, the Yenisei in Siberia, the Tagus river in Spain/Portugal, and the Rhône river in France, brGDGT were investigated from source to sink, by extensive sampling campaigns. We discovered, by using new methodologies, that in-situ production of brGDGT in the river effects the distribution of the brGDGTs that are delivered by soil erosion. This varies from one river system to another. In the Yenisei, for example, in-situ production plays an important role while this is less so in the Amazon and Rhone. A further complication in the use of brGDGT is that they can also be produced in-situ in marine sediments, which indicates that brGDGTs in marine sediments can only be used to reconstruct continental climate when the core location is close to a river mouth. We constructed improved temperature calibrations for brGDGTs in soil. We also discovered that the analytical methodology for the analysis of brGDGTs could be improved, resulting in the identification of a new group of brGDGTs that helped to improve the soil calibration even further. We have also investigated the potential of lake sediments as archives of continental climate change using the palaeothermometer based on brGDGTs. Extensive field work on various lakes was performed to unravel the sources of brGDGTs and environmental information contained in their distributions. We discovered that the distribution of brGDGTs does vary with temperature and pH but in a different way than for soils. In contrast to what was thought earlier, brGDGTs in most lakes are predominantly produced in the lake itself. Consequently, application of brGDGT to reconstruct climate of the past is more complicated than previously thought but still holds great promise. During the project also important improvements (i.e. calibration, recognition of deep water Thaumarchaeota) for TEX86 palaeothermometry (the other GDGT-based palaeothermometer) were made. Using these two palaeothermometers, reconstructions of past climate condition have been performed, assisting in our understanding of the climate evolution of the Earth.