A new archive for climate science
Under extreme climatic conditions only few “witnesses” of past environmental conditions endure. Pollen, for example, serving as indicators for the composition of the vegetation of ancient times, are only conserved under very specific conditions. Under an extremely dry climate such as prevailing in the Kalahari in South Africa, climate archives comprising pollen or other climate “witnesses” are especially scarce. Scientists from the GFZ sections Geomicrobiology and Organic Geochemistry, together with a colleague from the Carl von Ossietzky University in Oldenburg, Germany, were searching for previously not exploited climate archives.
The Kalahari in southwestern Africa is a xeric shrubland, also referred to as desert due to the predominance of sand. Climate science was so far investigating the climatic past of this region mainly based on calcareous deposits like tufa (cemented volcanic rock) or stromatholites (deposits from metabolic residues of bacteria), and speleothems. However, these climate archives are scarce and the reconstruction of Kalaharian climates of the past is therefore only fragmented.
Pan depressions are an abundant landscape feature in South Africa. During dry phases these troughs are formed by wind erosion, during wet periods sediment is accumulated by surface water run-off, further sediment accumulates by wind transport. In a new study now published in the scientific journal Organic Geochemistry, the scientists investigated the microbial abundance and activity in a pan depression. Steffi Genderjahn, first author of the study and joint PhD student in the GFZ sections Geomicrobiology and Organic Geochemistry: “Water is a basic requirement for microbial life. Thus, if we find microbial signals, we can conclude that water must have been present during their time of deposition. Therefore, our hypothesis was that we are able to reconstruct ancient climatic conditions based on the abundance and composition of microbial signals in pan sediments.”
Fossil molecules in pan sediments
Since microbes of the past are dead for a long time, the scientists investigated their molecular remnants so-called biomarkers. These are molecular remains of lipids in this case originating from microbial cell membranes. The characteristic structure of the biomarkers still allows to trace back to the microorganisms that once produced them. In Witpan, a pan depression in the southern Kalahari, the team took sediment material from the upper1.20 meters. Together with investigations of changes in its sediment properties, like the amount of organic carbon, the biomarkers found in the sediment were used as a key to the past.
The scientists found biomarkers of microorganisms that point to saline and nutrient-poor conditions indicating a well-adapted microbial community. Furthermore, strong variations in the abundance and composition of the microbial communities in time – the further down the sediment core, the further back in time – displayed the climate picture of the past.
The core sediments cover a time span of 20,000 years, from the Last Glacial Maximum (LGM) up to today. The climate “witnesses” from the pan sediments indicate that the Kalahari was comparatively wet during the LGM. During the subsequent warm period only very few traces of life are found and those detected point to dry and nutrient-poor conditions as well as only a sparse vegetation cover. In the pan surface layers there are, however, significant traces of life. Hence, in the recent past water must have been at least temporarily available, probably due to seasonal rainfall. The scientists did apply their method on another pan, located further to the northwest, were they complemented it with DNA-based analyses. The results from that site are close to be published and seem to confirm the current study. Hence, it has been proven that in fact pan depressions can serve as climate archives in regions that are otherwise poor in climate information. Thus, the study opened new archives for climate science in dry regions such as the Kalahari.