SUERC helps sharpen focus on radiocarbon dating

Published: 19 October 2012

Scientists have used sediment from the bottom of a Japanese lake to calibrate more effectively the tools used to determine the age of ancient materials.

Scientists have used sediment from the bottom of a Japanese lake to calibrate more effectively the tools used to determine the age of ancient materials.

Dr Charlotte Bryant, head of the Natural Environment Research Council Radiocarbon Facility at the Scottish Universities Environmental Research Centre was part of a team of researchers who examined samples from the bed of Lake Suigetsu for a paper published in the journal Science today.

A project led by Professor Takeshi Nakagawa of the University of Newcastle recovered sediment cores which preserve records of the lake and surrounding environment over the past 52,800 years.  The sediment contains layers of tiny, relatively light-colored algae called diatoms, which sink to the floor of the lake each year, followed by a layer of darker sediments. The project set out to use leaves preserved within the sediment to refine the atmospheric radiocarbon record. Radiocarbon, or C-14, is a naturally occurring, radioactive isotope of carbon that decays at a known, constant rate.  Researchers can calculate the age of an object based on how much radiocarbon it contains relative to its stable cousin, C-12.

Converting radiocarbon years to calendar years is a process called calibration and overcomes the complications caused by natural fluctuations in radiocarbon, which varies over time and between regions. Calibration requires long, known-age records with associated radiocarbon data. The radiocarbon in the leaf fossils preserved in Lake Suigetsu’s sediment comes directly from the atmosphere and is not subject to the same processes that affect radiocarbon found in marine sediments or cave formations.

Dr Bryant worked with Professor Christopher Bronk Ramsey and other colleagues of the University of Oxford to reconstruct the lake’s radiocarbon record by measuring terrestrial plant fragments spaced throughout the core. Other members of the team 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.

Dr Bryant said: “The  direct record of atmospheric radiocarbon comes from tree rings and goes back 12,593 years. The new record from Lake Suigetsu extends the direct radiocarbon record by more than 40,000 years. Other, indirect radiocarbon records from marine or cave environments also reach back to roughly 50,000 years ago.

“At our facility based at the Scottish Universities Environmental Research Centre in East Kilbride, we dated around half of the new Lake Suigetsu samples.

 “Our work provides a much more refined toolset for researchers across a wide range of disciplines and will allow them to make more accurate estimates of material dated by radiocarbon. For example, climate scientists could improve their understanding of the timing and rate of environmental changes which will benefit our knowledge of how our own environment responds to change.”

A record of year-to-year changes, such as a sediment core, must be ‘anchored’ in time by assigning some part of it an absolute age. The researchers did this by matching the first 12,200 years of their record with the tree-ring record, a well-established record that begins in the present.

The team also lined up segments of their record with those of other records from the same time periods and generally found good agreement.

Professor Nakagawa said: “Because of the unique combination of a complete radiocarbon record and terrestrial paleoclimate data, Suigetsu can be a benchmark against which other records can be compared.

“From a palaeoclimate perspective, this radiocarbon dataset will also allow very high precision direct correlation between Suigetsu and other terrestrial climate records.

“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.”

Professor Ramsey said: “Although this record will not result in major revisions of dates, for example in archaeology, there will be changes in detail that are of the order of hundreds of years.

“Such changes can be very significant when you are trying to look at human responses to climate, often dated by other methods, for example through the Greenland ice cores. So, a more accurate calibrated time-scale will allow us to answer questions in archaeology, which previously we have not had the resolution to address.”

Researchers generally use a composite record called IntCal to determine the ages of objects based on their radiocarbon measurements. The IntCal record incorporates data from multiple sources, including marine records, speleothem and tree rings, and is updated periodically. The researchers anticipate that their Suigetsu data will be incorporated into the next iteration of the IntCal compilation, to be released in a few months.

The paper, titled ‘A Complete Terrestrial Radiocarbon Record for 11.2 – 52.8 kyr BP’, is published in Science 338, 370-374 2012.

First published: 19 October 2012

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