Sediments at the bottom of Lake Suigetsu in Japan are being used to improve the calibration of radiocarbon records.
In the latest issue of the journal Science, an international team of experts including University of Groningen researcher Hans van der Plicht explains how this discovery makes it possible to date more ancient materials (more than 25,000 years old) with greater accuracy.
Carbon dating has been used in archaeology and climate research for decades. The Centre for Isotope Research (CIO) in Groningen is one of the world’s leading laboratories in this field. CIO researcher Prof. Hans van der Plicht has been working on calibrating and improving the technique for years.
Calibration is necessary because the results obtained by carbon dating are not absolute. Plants extract carbon-14 from the atmosphere, whereas other organisms absorb it via their food. Carbon-14 is a heavier isotope of carbon than the ‘regular’ carbon-12 and, moreover, it is radioactive. When organisms die they stop absorbing carbon-14, after which the concentration of this isotope starts to decline through radioactive decay. The amount of carbon-14 halves every 5730 years.
To be able to date an object, you need to know how much carbon-14 was in it to start with. The concentration in plants and animals is almost identical to the concentration in the atmosphere. However, the concentration of carbon-14 in the atmosphere varies due to the activity of the sun and the strength of the earth’s magnetic field.
Radiocarbon records can be calibrated by measuring the amount of carbon-14 in materials of which the age is known. This is achieved by means of wood samples, the age of which can be calculated on the basis of the tree rings. There is now an archive of tree ring samples dating back to more than 12,000 years ago. Materials of greater age cannot be calibrated as accurately and so it is harder to date older materials precisely.
In 1998, Van der Plicht reported in Science that a carbon archive of more than 50,000 years old had been found in Lake Suigetsu in Japan. Algae and cyanobacteria grow in this lake in the spring and die off in the autumn, after which they sink to the bottom, thus creating distinguishable annual sedimentary layers (known as varves).
The layers can be counted in a core sample taken from the bottom of the lake, just as the growth rings from a tree sample. The concentration of carbon-14 in the atmosphere can be derived from the amount measured in the organic material from the same period.
Despite this new information, gaps still remained in the carbon archive. ‘Japan suffers from regular earthquakes, when portions of the soil can slide apart,’ explains Van der Plicht. It is impossible to create an accurate calibration curve based on the varves if you do not know precisely how big the gaps in the soil are.
To solve this problem, a British-Japanese team has extracted three new core samples from the bottom of the lake. By comparing the stratification in each of the samples, the scientists have been able to fill nearly all the gaps in the archive. By counting the varves they are now able to travel back in time to about 52,000 years ago. They have been able to obtain detailed information on the variations in the atmospheric concentration of carbon-14 over the same period by carrying out a few hundred carbon-14 measurements in these layers. This means they can now date prehistoric materials with more accuracy.
‘Furthermore, we have compared the samples taken from the lake with other calibration curves made in recent years based on measurements of stalactites, stalagmites and corals,’ explains Van der Plicht. All these methods have their limitations, but by combining the information they have been able to create an extremely accurate ‘super calibration curve’. ‘The margin of error of samples dating back to about 30,000 years ago is a few centuries, that of 50,000-year-old samples is about 1500 years.’ Anything older than 50,000 years has so little carbon-14 left in it that this cannot be measured with any accuracy.
This greater precision is required, for example, to be able to chart the history of modern man with more accuracy. The Neanderthals disappeared from Europe about 30,000 years ago and their place was taken by modern man. ‘It is suggested that climate change was involved in this succession, which can be derived from ice core samples taken in Greenland. But you have to be able to demonstrate the correlation between climate change and the rise of modern man to prove the theory. Up until now there was too much uncertainty about the absolute dates of the remains of our forefathers and the Neanderthals.’
The varves in Lake Suigetsu have made the carbon-14 technique much more accurate, but even more accuracy is possible. ‘In New Zealand, submerged forests of Kauri trees of thirty thousand years old have been found,’ explains Van der Plicht. ‘This should make it possible to build an archive of tree ring samples going back to maybe 50,000 years ago. That will enable us to achieve maximum accuracy.’
Prof. J. van der Plicht.
Hans van der Plicht is researcher with the Centre for Isotope Research (CIO) and professor by special appointment of Isotope Archaeology at Leiden University.
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