Japan’s Lake Suigetsu Provides 52,800-Yr Archaeological Clock

A new series of radiocarbon measurements from Japan’s Lake Suigetsu has provided an exquisitely preserved record of the past 52,800 years.

AsianScientist (Oct. 22, 2012) – A new series of radiocarbon measurements from Japan’s Lake Suigetsu has provided an exquisitely preserved record of the past 52,800 years, researchers report.

The research, which appears in the latest issue of the journal Science, could be used to refine estimates of the ages of organic material by hundreds of years.

Archaeologists, for example, may be able to further specify the timing of the extinction of Neanderthals or the spread of modern humans into Europe. And, climate scientists may better understand the chains of events that led to the advance and retreat of the ice sheets during the last glacial period.

In Lake Suigetsu, a layer of tiny, relatively light-colored algae called diatoms blankets the floor each year, followed by a layer of darker sediments. The lake bottom is very still and anoxic, so these layers have remained undisturbed over tens of thousands of years.

Researchers recognized as early as 1993 that sediment cores from Lake Suigetsu might be useful for radiocarbon dating, but the initial efforts encountered technical problems.

“In short, this is a realization of a 20-year-long Japanese dream,” said Takeshi Nakagawa of the University of Newcastle in the United Kingdom. Prof. Nakagawa is one of the primary authors of the study and leader of the Lake Suigetsu (SG06) project.

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 between different parts of the global carbon cycle.

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 terrestrial sediment record from Suigetsu requires no such correction. The radiocarbon in the leaf fossils preserved in the sediment comes directly from the atmosphere and is not subject to the same processes that affect radiocarbon in marine sediments or cave formations.

To construct their radiocarbon record from Lake Suigetsu, Christopher Bronk Ramsey and colleagues measured radiocarbon from terrestrial plant fragments spaced throughout the core. They 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.

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 which extends to 12,593 years ago.

“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,” said Nakagawa.

Bronk Ramsey and colleagues anticipate that their Suigetsu data will be incorporated into the next iteration of a composite record called IntCal, to be released in a few months.

The article can be found at: Ramsey CB et al. (2012) A Complete Terrestrial Radiocarbon Record for 11.2 to 52.8 kyr B.P.


Source: AAAS; Photo: Suigetsu 2006 Research Project.
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