To improve climate models that predict the sensitivity of our climate system, we need reconstructions of past climates. To quantitatively reconstruct temperature changes in the past, chemical fossils are a recent addition to the climate scientist toolbox. Chemical fossils are molecular components that carry information on the environment in which they were produced, sometimes millions of years ago. The distribution of a group of 15 membrane lipids in soils, for instance, depends on the temperature and pH of the soils in which these organisms are living. They are conserved in several types of geological archives (marine sediments, lake sediments, soils) and their distribution has been used to reconstruct changes in the temperature of the past. However, even the most recent temperature calibration is not accurate enough to reconstruct absolute temperatures. In this research project we aim to improve this thermometer of the past, with geochemical and microbiological research. The thermometer is based on bacterial membrane lipids, but at the moment it is still unclear which bacteria produce them. Previous studies have indicated that Acidobacteria are probably the source organisms of this set of branched tetraether lipids, but only 1 of the 15 compounds that are frequently encountered in soils, has been recovered from an Acidobacterial culture. To shed light on the abundance and variability of the source organisms in soils, the branched tetraether distribution and the bacterial diversity along local environmental gradients (ForHot soils, REPEAT soils, Ossekampen soils) will be analyzed. This research will allow to gain a better insight in the environmental factors that influence the branched tetraethers and their bacterial source organisms.
The research performed in the WISLAS project has resulted in exciting new insights. Because of the dominant effect of the bacterial community composition on the brGDGT lipids, the dependency of these compounds with the environment is not linear. Instead, the linear dependency with temperature observed on a global scale, on which the calibration was based, contains several environmental threshold values. With this knowledge a new type of model can be designed to reconstruct past changes in soil temperature. This research has resulted in 3 manuscripts (2 submitted, 1 draft), and also in three oral presentations at international conferences (IMOG 2017, Goldschmidt 2017, AGU 2018 (invited)).
