18 October 2012
Sediment built up over thousands of years on the floor of a Japanese lake has enabled scientists to make significant improvements to the precision and accuracy of radiocarbon dating. This work means that radiocarbon dating will now be accurate up to nearly 54,000 years.
Widely used to measure the ages of archaeological sites, past climate events and changes to the environment, radiocarbon dating was invented in the 1950s.
Until now it has provided an accurate method for dating back as far as 12,000 years or so, but beyond this point, major changes in the climate at the end of the last ice age have made it much harder to get accurate measurements.
However, a series of cores – sections of mud – taken from the floor of Lake Suigetsu in Japan, and analysed by UK researchers, including scientists at Aberystwyth University, has provided an exquisitely preserved record of environmental change that extends back 70,000 years.
Their findings are reported in Science, the journal of the American Association for the Advancement of Science (AAAS), on Friday 19 October.
Previous studies of Lake Suigetsu had revealed that the sediment on the floor was made up of fine layers which were formed annually. By counting the layers, it is possible to establish a precise age scale with annual resolution.
Lake Suigetsu is in an area which was never covered by glaciers. Even during the last ice age, it was surrounded by trees, and leaves from those trees were falling into the lake.
These leaves were eventually preserved in the sediment and are ideal for working out past atmospheric radiocarbon content, the key indicator for radiocarbon dating.
Dr Henry Lamb from the Institute of Geography and Earth Sciences at Aberystwyth University is one of four Principal Investigators on the project and has played a key role in analysing the sediment cores.
Using novel methods with a high-tech X-ray core scanner, Dr Lamb and post-doctoral researcher Dr Mike Marshall undertook the painstaking work of counting and characterising the annual layers in the Lake Suigetsu cores, many of which were too fine to be distinguished by the naked eye.
The scanner has the capacity to undertake a detailed study of a one metre length of core in a matter of hours, and at intervals as fine as 60 microns - one micron is one thousandth of a millimetre. It would take years to complete the work using conventional analytical methods, and such detail would be impossible.
Over a period of several months the Aberystwyth team studied 40 metres of cores and provided essential data that helped to build the age model that is the basis for the revised radiocarbon calibration announced in this week’s Science article.
The researchers at Aberystwyth worked closely with scientists at the Helmholtz Centre in Potsdam, Germany, who studied the cores using a microscopic layer counting technique. Their findings supported the work undertaken at Aberystwyth.
Dr Lamb said: “Radiocarbon dating is fundamental to archaeology and climate change research. It is key to understanding how the global carbon cycle works. The work we have done on the Suigetsu cores clearly shows that the new high-tech method developed here at Aberystwyth has contributed to significantly enhancing the precision and accuracy of the radiocarbon timescale.
“I always tell my students that “chronology is crucial” in studies of climate change, archaeology, and human evolution. Now I can tell them that Suigetsu provides a “chronology that we can count on”.
The Lake Suigetsu project is led by Dr Takeshi Nakagawa of the University of Newcastle. The principal author on the paper, A complete terrestrial radiocarbon record for11.2- 52.8, is Dr Christopher Bronk Ramsey from the Oxford Radiocarbon Accelerator Unit at the University of Oxford.
Radiocarbon, or C-14, is a naturally occurring, radioactive isotope of carbon that decays at a steady rate. Researchers can calculate the age of an object based on how much radiocarbon it contains relative to its stable cousin, C-12. But, there are several factors that complicate this calculation, since the amounts of radiocarbon in the environment -- and incorporated into growing organisms -- can vary from year to year and from one region to another.
Adjusting for these natural fluctuations in radiocarbon is a process called calibration and requires long, known-age records with associated radiocarbon data. Some of the longest and most important radiocarbon records come from marine sediments or cave formations. However, these need to be corrected using a variety of assumptions about how radiocarbon levels change in ocean water and groundwater.
The Itrax® X-ray Fluorescence Core Scanner
The Itrax® X-ray Fluorescence Core Scanner was installed in the laboratory of Dr Henry Lamb in July 2005. Hand built by physicists at the University of Göteborg, Sweden, it was only the fourth such instrument made, the second to be installed in the UK, and is still one of very few dedicated to analysis of lake sediment cores.
The scanner works by directing a very fine X-ray beam at the core surface, and then identifying the spectral signature of returning (fluorescent) X-rays characteristic of each chemical element present. That allows scientists to determine the composition of a sediment core at very small intervals along the core, very rapidly, and in a non-destructive manner – that is, without physically removing samples from the core.
It can readily obtain data on up to 70 elements at intervals as fine as 60 microns along a 1 metre core section in a matter of hours – very much faster than conventional analytical methods. Nevertheless, for this ultra-high resolution analysis replicated several times over nearly 40 m of core, the scanner had to be engaged for many months. The instrument also provides high-resolution optical and X-radiographic images, which detail the fine structure of the sediment – in this case, the annual laminations or varves in the Lake Suigetsu core.
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Dr Henry Lamb
Institute of Geography and Earth Sciences