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Molecular Fossils as Climate Witnesses – Lake Sediments Reveal Climate Changes

Taking samples in Cameroon. Picture: Dirk Sachse

Taking samples in Cameroon. Picture: Dirk Sachse

Climate change and its possible consequences are widely discussed among experts and the lay public. Dr. Dirk Sachse, a geologist, addresses a specific aspect of this issue: as leader of the Emmy Noether Research Group Paleohydrology he and his team examine past changes in the hydrologic cycle and its influence on climate. They analyze molecular plant remnants from lake sediments. These so-called “molecular fossils” tell researchers about the climate history in the study region.

When Dirk Sachse and his team head out for field expeditions, they look for traces of past climate. Their research field is paleohydrology, aiming at unraveling past changes in the hydrologic cycle as an important part of past climate variability. Specific chemical or sedimentological data, so-called proxies, from lake sediment cores provide information about changes in a region’s precipitation – the main interest of Sachse and his research team. They examine paleohydrological changes, for instance in the Meerfelder Maar, a volcanic lake in the Eifel. “There are few direct proxies of changes in the hydrologic cycle. Most are somewhat indirect,” Sachse says. Vegetation, for example, changes with fluctuations in precipitation. Scientists can reconstruct such changes in plant growth by analyzing pollen in the sediment. However, it may take decades before vegetation responds to climate changes.

To assess past changes in the hydrologic cycle directly, the research team analyzes samples from lake sediment cores with novel molecular methods. The geologists extract molecular plant remnants from those lake sediment and analyze the abundances of the stable hydrogen isotopes in these molecules. Through this, they can reconstruct changes in the isotopic composition of ancient precipitation water and draw conclusions about changes in precipitation and dry periods. Nevertheless, systematic collaboration with plant physiologists and examinations of modern plants grown in a greenhouse are necessary to interpret the sedimentary record. “In a greenhouse we can influence and measure the climatic conditions and measure which hydrogen isotopes accumulate in the plants and their molecular components,” Sachse explains the procedure. The geologists cooperate closely with plant physiologists at the University of Basel to understand the behavior of hydrogen isotopes during plant biosynthesis. They also collaborate with the Institute of Biochemistry and Biology of the University of Potsdam, for instance with Prof. Ralph Tiedemann, on other projects.

In some lakes, like in the investigated Eifel region, sediment layers form annually consisting mainly of mineral particles and dead organic matter like dead zooplankton remnants and algae. Counting the individual layers, just like tree rings, enables scientists to date the sediment precisely. Each sediment layer contains remnants of the lacustrine ecosystem – for instance shells and pollen. Researchers use these microfossils as an “archive” of climate history. Sachse’s working group is specifically interested in organic matter, like lipids. These compounds remain in the sediment for extremely long periods, sometimes for millions of years, and hardly change. “This is a great advantage and also the reason we are investigating these socalled ‘molecular fossils’ or biomarkers,” says Sachse. Biomarkers are organic substances in sediments that are indicative for their biological origin. Researchers can identify whether they were produced by a land plant or by algae. The substances contain mostly carbon and hydrogen. The hydrogen comes from the water taken up by plants and algae. The isotopic composition of hydrogen in water records various processes in the hydrological cycle. Thus they are direct indicators for hydrological changes. Changes in the composition of hydrogen isotopes in the biomarker molecules enable the researchers to map droughts as well as fluctuations in humidity and precipitation. This analytical method used by the Potsdam scientists is relatively new.

Sachse and his colleagues use their findings to reconstruct hydrological changes during past climate changes, which is relevant for a better understanding of the effects future climate change may have on us. Climate models predict future changes in temperature as a result of manmade climate change, and there is no doubt that the average global temperature will rise. While this will lead to a higher percentage of water vapor in the atmosphere, it does not necessarily mean that temperature equally rises everywhere. Climate models are less precise when it comes to predicting changes in precipitation. This raises the question of the effect of temperature changes on the hydrological cycle in specific regions. A better understanding of past climate changes, and their effect on the water cycle, will help to improve the predictions by these models.

Sachse and his team do not only work within Germany but also worldwide. Their research takes them to Mexico, Africa and the Himalayas. “Tropical regions are important for us because there is a lot of water in the atmosphere, and we expect stronger changes in the hydrological cycle there.”

Dirk Sachse studied geology in Jena. Although he was interested in natural sciences rather early, he did not have a clear idea about his professional development as a child. His “appetite” for it came only while “eating”. “I had no idea when I began my studies that there was something like what I am doing now.” Traveling to the USA and Latin America, visiting national parks and the phenomenon of volcanos fascinated him so much that he turned to geology. During his studies, he was fascinated how climatic processes shape the earth’s surface and how sediments from the past offer clues about the climate of that time. He wanted to understand climate changes. He worked for his PhD at the Max-Planck-Institute for Biogeochemistry in Jena. “There were always people who gave me the opportunity to work on completely new topics,” Sachse says. Born in Halle, he definitely wanted to go abroad after his PhD. His scientific work since, has also taken him to Boston and Seattle.

Since 2009 Sachse has led the Emmy Noether Research Group Paleohydrology at the Institute for Earth and Environmental Sciences, which is funded by the Deutsche Forschungsgemeinschaft. The geologists in his team study past climate changes on the continents throughout the more recent geological history – with a special focus on the Holocene (roughly the last 11,000 years). Sachse decided to work at Potsdam because of the terrestrial working groups there as well as good analytical facilities because he needs rather complex laboratories and analytical instrumentation for his analyses. “I do not know of many other places in Germany where I could work in such an environment,” he says.

The Scientist

Dirk Sachse studied geology in Jena and Granada, Spain. He received his PhD from the University of Jena in 2005. In Potsdam he is leading the Emmy-Noether Research Group Paleohydrology at the Institute for Earth and Environmental Sciences. His scientific interests are the interactions of geospheres, biospheres, and hydrospheres and their influence on the climate – and vice versa.

Contact

Universität Potsdam
Institut für Erd- und Umweltwissenschaften
Karl-Liebknecht-Str. 24-25
14476 Potsdam
dsachse@geo.uni-potsdam.nomorespam.de

This research is linked to the research initiative NEXUS: Earth Surface Dynamics, which clusters approaches from various scientific disciplines in the Berlin-Brandenburg area within the overarching theme of Earth surface dynamics. The University of Potsdam, along with its partnering institutions the Helmholtz-Centre Potsdam - German Research Centre for Geosciences (GFZ), the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) and partners from the Potsdam Institute for Climate Impact Research (PIK), the Museum für Naturkunde - Leibniz Institute for Evolution and Biodiversity Science (MfN) and the Technische Universität Berlin (TUB) therefore combines the outstanding expertise from geo-, bio-, climate and data sciences.

Text: Dr. Barbara Eckardt, Online-Editing. Agnes Bressa, Translation: Susanne Voigt