October 20th, 2012 by James Ayre
Researchers have just discovered a new, very accurate, benchmark for the dating of organic materials. Sediment cores that have remained ‘undisturbed’ for the last tens of thousands of years in Japan’s Lake Suigetsu. The cores will be especially valuable for the dating of older objects, providing much more precise estimates for objects between 11,000-53,000-old.
The researchers obtained the sediment cores, beautifully preserved layers of organic material like leafs and twigs, from under the Japanese lake where they had been undisturbed for tens of thousands of years.
These findings are very significant across a wide variety of fields, because of their ability to greatly improve the precision of radiocarbon dating of organic material. The researchers say, as an example, that it will now be easier to estimate the timing of the disappearance of Neanderthals and the spread of different human populations.
“Radiocarbon is continuously produced in the upper atmosphere. These roughly constant levels of radiocarbon from the atmosphere are then incorporated into all living organisms. Once the organisms die, the radioactive isotope decays at a known rate, so by measuring the radiocarbon levels remaining in samples today scientists can work out how old things are. However, the complication in the calculation is that the initial amounts of radiocarbon in the environment, which are in turn incorporated into growing organisms, vary slightly from year to year and between different parts of the global carbon cycle.”
“The radiocarbon in the leaf fossils preserved in the sediment of Lake Suigetsu comes directly from the atmosphere and, as such, is not affected by the processes that can slightly change the radiocarbon levels found in marine sediments or cave formations. Before the publication of this new research, the longest and most important radiocarbon dating records came from such marine sediments or cave formations, but these needed to be corrected. At last, the cores from Lake Suigetsu provide a more complete, direct record of radiocarbon from the atmosphere without the need for further correction.”
These sediment cores are very unique: they display layers for every year, creating a very simple way for researchers to look back in time. “These counts are compared with over 800 radiocarbon dates from the preserved fossil leaves.The only other direct record of atmospheric carbon comes from tree rings, but this only goes back to 12,593 years ago. The Lake Suigetsu record extends much further to 52,800 years ago, increasing the direct radiocarbon record by more than 40,000 years.”
It’s often been the case that radiocarbon levels taken from marine records or other ways, haven’t been very accurate. So finally having a very accurate measuring tool will allow the comparison of atmospheric and marine carbon levels, potentially allowing further insight into the carbon cycle.
“To construct a radiocarbon record from Lake Suigetsu, Professor Ramsey and his colleagues measured radiocarbon from terrestrial plant fragments spaced throughout the core. The research team also counted the light and dark layers throughout the glacial period to place the radiocarbon measurements in time. Many of the layers were too fine to be distinguished by the naked eye, so the researchers used a microscope, as well as a method called X-ray fluorescence that identifies chemical changes along the core.”
‘This record will not result in major revisions of dates. But, for example in prehistoric archaeology, there will be small shifts in chronology in the order of hundreds of years,’ said Professor Ramsey. ‘Such changes can be very significant when you are trying to examine human responses to climate that are often dated by other methods, such as through layer counting from the Greenland ice cores. For the first time we have a more accurate calibrated time-scale, which will allow us to answer questions in archaeology that we have not had the resolution to address before.’
Some background on the lake:
“Lake Suigetsu is a lake in Honshu Island, Japan located near the towns of Mikata and close to the coast of Sea of Japan. Since 1993, it has been attracting the attention of scientists because of the undisturbed nature of the water for many thousands of years. It is possible to identify the annual deposits of silt in a similar manner that tree rings are identified.”
“The only inflow to Lake Suigestu is through a shallow channel from the neighbouring Lake Mikata and there is little outflow. Consequently only the finest sediment comes into the lake. The water is anoxic (deoxygenated) preventing the growth of organisms and due to seasonal variations it is usually but not always possible to distinguish the annual deposits visually. It has taken almost twenty years to overcome the consequent problems, using multiple cores and new detection techniques to complete the sequence.”
The research was just published in the journal Science.
Image Credits: Chiku
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