19.10.2012 | Potsdam: By using a new series of measurements of radiocarbon dates on seasonally laminated sediments from Lake Suigetsu in Japan, a more precise calibration of radiocarbon dating will be possible. In combination with an accurate count of the seasonal layered deposits in the lake, the study resulted in an unprecedented precision of the known 14C method with which it is now possible to date older objects of climate research and archeology more precisely than previously achievable. This is the result published by an international team of geoscientists led by Prof. Christopher Bronk Ramsey (University of Oxford) in the latest edition of the journal Science.
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.
Christopher Bronk Ramsey et al.,“A Complete Terrestrial Radiocarbon Record for 11.2 – 52.8 kyr BP,” Science, 338, (6105), 370-374, 10.1126/science.1226660
This study was funded within the framework of coordinated programs of the DFG and NERC.