October 19, 2012
Radiocarbon Dating Improved With Sediment Measurements
April Flowers for redOrbit.com - Your Universe Online
A research team from Oxford University's Radiocarbon Accelerator Unit has found a more accurate benchmark for dating materials, especially for older objects, from a series of radiocarbon measurements from Japan's Lake Suigetsu.
As far back as 1993, researchers realized sediment cores from Lake Suigetsu would be useful for radiocarbon dating. However, the initial efforts encountered technical problems.
The current team extracted cores of preserved layers of sediment from where they had lain on the bottom of Lake Suigetsu for tens of thousands of years. The cores contained organic material such as tree leaf and twig fossils.
The findings of this study, published in Science, are significant because they provide a more precise way to determine the radiocarbon ages of organic material for the entire 11,000 — 53,000-year time range. Using this process, for example, archaeologists should now be able to pinpoint the timing of the extinction of Neanderthals or the spread of modern humans into Europe with much more accuracy.
"The new results offer an important refinement of the atmospheric radiocarbon record and place the radiocarbon timescale on a firmer foundation," said Jesse Smith, Senior Editor at Science.
Professor Christopher Ramsey of the Radiocarbon Accelerator Unit, along with his colleagues, worked with scientists from two other radiocarbon laboratories — NERC in Scotland and in Groningen, the Netherlands — on the radiocarbon record from Lake Suigetsu as part of a large, international team studying the cores for clues about past climate and environmental change.
Radiocarbon, or C-14, is produced in the upper atmosphere continuously and is incorporated into all living organisms. The radioactive isotope of carbon decays at a known rate when the organisms die. By measuring the radiocarbon levels remaining in samples of ancient organic materials, scientists can work out how old things are. One element that complicates this calculation is the variability of the amount of environmental radiocarbon from year to year and location to location.
The radiocarbon in leaf fossils, such as those found in Lake Suigetsu, comes directly from the atmosphere. This means the processes that can slightly change the levels found in marine sediments or cave formations do not affect it. Previous to this study, the most important radiocarbon dating records came from such marine sediments or cave formations, which needed corrections. The samples from Lake Suigetsu provide a more complete, direct record from the atmosphere without the need for correction.
The cores display layers of light diatoms and darker sediments and in the sediment record for each year, making them unique and giving scientists the means of counting back the years. These counts are then compared to over 800 radiocarbon dates from the preserved fossil leaves. Tree rings provide the only other direct record of atmospheric carbon, but they only go back 12,593 years. The new record from Lake Suigetsu extends backwards 52,800 years, dramatically increasing the direct radiocarbon record by more than 40,000 years.
"In most cases the radiocarbon levels deduced from marine and other records have not been too far wrong. However, having a truly terrestrial record gives us better resolution and confidence in radiocarbon dating,' said Professor Ramsey in a press release. "It also allows us to look at the differences between the atmosphere and oceans, and study the implications for our understanding of the marine environment as part of the global carbon cycle."
The team measured radiocarbon from terrestrial plant fragments spaced throughout the core to construct a radiocarbon record. To place the radiocarbon measurements in time, they also counted the dark and light layers throughout the glacial period. They used microscopes and a method called X-ray fluorescence that identifies chemical changes along the core because many of the layers were too fine to be distinguished by the naked eye.
Some part of the record must be "anchored" in time by assigning some part of it an absolute age. The team managed this by matching the first 12,200 years of their record with the tree ring data, a well-established record that begins in the present. They also used other records from the same period and found that they generally aligned.
"Because of the unique combination of a complete radiocarbon record and terrestrial paleo-climate data, Suigetsu can be a benchmark against which other records can be compared," said Professor Takeshi Nakagawa of Newcastle University.
"From a palaeoclimate perspective, this radiocarbon dataset will also allow very high precision direct correlation between Suigetsu and other terrestrial climate records," said Nakagawa in a press release. "This allows us to see how changes in climate in different parts of the world relate to one another — and particularly where there are leads and lags. Information like this is very useful for studying climate mechanisms."
"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."
To determine the age of objects based on their radiocarbon measurements, scientists generally use a composite record called IntCal. IntCal uses marine records, stalagmites and stalactites, tree rings and multiple other records. The team expects the Suigetsu data will be incorporated into the latest version of IntCal, due to be released in the next few months.