A sharper look into the past for archaeology and climate research

The radiocarbon method for dating organic and calcareous materials uses the known decay rates of the radioactive isotope ¹⁴C, which is formed in very small amounts in the upper atmosphere by cosmic rays. Since the formation of ¹⁴C is affected by Earth's magnetic field and solar activity and is therefore not constant, this relative time scale is has no absolute timestamp in calendar years. The timescale developed through the measured decay rates must thus be calibrated to indicate the age in calendar years. This works best with a parallel count of annual layers in lake sediments or tree rings. A calibration reaching very far into the past was now possible with data from the Suigetsu Lake, located in Mikata near the Sea of Japan. Here, a drill core could be retrieved from the sediments whose seasonal resolution dates back to over 50,000 years ago.

These new data are very important for both archaeological and paleoclimatic research. "With such information, one can not only improve the understanding of regional impacts of climate change, but also find the triggering mechanisms ," explains Achim Brauer, who is one of the initiators of the project and responsible for creating the time scale in calendar years of the Suigetsu sediment profile at the GFZ German Research Centre for Geosciences. "It allows us to synchronize paleoclimatic key profiles from different regions, such as the Arctic, East Asia and Europe more accurately, in order to determine whether abrupt climate changes occurred worldwide at the same time, or whether changes in some regions can be identified as sooner than others. The new calibration also allows a more accurate determination of the extinction times of the Neanderthals and the temporal spread of modern humans in Europe. "

The Suigetsu Lake is ideal for using both dating methods, measurement of ¹⁴C and counting of annual layers, because deciduous trees grew on its shores during the last ice age, the leaves of which were preserved in large numbers in the sediments and are ideal for ¹⁴C dating. At the same time, this lake is one of the rare cases in which annual layers have been preserved in the sediment.

Due to the long experience of Achim Brauer's working group with creating precise calendar time scales from lake sediments, the GFZ scientists were entrusted with this task. Using special microscope techniques, it was possible to decipher the detailed structure of the finest, thousands of years old layers in the Suigetsu sediments. The scientists identified springtime layers which were formed by the melting of snow, summer layers of organic material or algae residues, fall layers of a special iron carbonate and winter layers of fine clay. The knowledge of this seasonal rhythm of sedimentation was the basis for the exact annual layer timescale. The high quality of the new Suigetsu chronology for the period from 12500 to 52800 years before present is shown by the fact that it was selected as the basis for the next iteration of the IntCal compilation, an internationally valid composite record of radiocarbon calibration.