Computer models used for predicting the impacts of climate change are validated using analyses of past climate events. Therefore, detailed reconstructions of past climates, particularly temperature, are of major importance. Several tools, referred to as proxies, are available to reconstruct absolute sea surface temperatures. However, reconstructions of continental temperatures are hindered by a lack of quantitative temperature proxies. The goal of the PACEMAKER (Past continental climate change: Temperatures from marine and lacustrine archives) project was to conduct a detailed evaluation of a new quantitative temperature proxy based on the distribution of membrane lipids. Branched glycerol dialkyl glycerol tetraether (brGDGT) lipids are found in bacteria that live in marine sediments, which were transported by rivers to the ocean. Since the composition of brGDGTs in soil is affected by annual mean air temperature (MAT), determination of brGDGT distribution in sediment cores from river fans can be used to reconstruct past continental temperatures. Researchers studied the biological origins of brGDGTs in soil and discovered that they were most likely produced by Acidobacteria, a common type of soil bacteria. They also investigated how soil bacterial membrane lipids were transported to the sea by four major river systems: the Amazon in Brazil, the Yenisei in Siberia, the Tagus in Spain/Portugal and the Rhone in France. Scientists found that in situ production of brGDGTs in the river influenced the distribution of the brGDGTs that are delivered by soil erosion. This varied from one river system to another. In the Yenisei, for example, in situ production played an important role, but this was less significant in the Amazon and Rhone. One drawback to the use of brGDGTs is that they can also be produced in situ in marine sediments. Thus, brGDGTs in marine sediments can only be used to reconstruct continental climate when the core samples are located close to a river mouth. The identification of a new group of brGDGTs in soil enabled the scientists to carry out improved temperature calibrations. In addition, they investigated the potential of lake sediments as archives of continental climate change using brGDGTs. Fieldwork was used to identify the sources of brGDGTs and the environmental information contained in their distribution. PACEMAKER enabled reconstructions of past climates to be performed, thereby assisting in the understanding of climate evolution on Earth. This has opened up new areas of palaeoclimatological research and contributed to the improvement of current climate models.
Climate change, continental temperatures, temperature proxies, PACEMAKER, brGDGT