Environment Counts | Continuous atmospheric radiocarbon record for 11,200 to 52,800 years before the present
Author: Geoff Zeiss – Published At: 2012-10-28 13:53 – (837 Reads)
Radiocarbon dating allows us to date material containing carbon back to about 50,000 years. The lack of a continuous record of atmospheric carbon-14 prior to 12.5 thousand years before the present has limited the application of radiocarbon dating to carbon samples from the Last Glacial period (about 110,000 to 10,000 years ago). Lake Suigetsu in Japan contains annual sediments that preserve both paleoclimate proxies (annual sediment layers or varves) and terrestrial plant fossils that are suitable for radiocarbon dating. This article reports over 661 carbon-14 observations correlated with sediment dating from Lake Suigetsu which provide a comprehensive record of terrestrial radiocarbon to the present limit of the carbon-14 method, 52,800 years before the present. Science 338, 370 (2012)
Dating materials using radioactive isotopes
One of the most important techniques for dating materials such as rocks, minerals, and fossils is based on the decay of long-lived radioactive isotopes. The parent isotopes naturally decay to stable daughter isotopes at rates that are not affected by physical or chemical conditions and that can be measured experimentally. The most common isotopes used in dating are
|Half-life (years)||Parent/daughter isotopes|
The parent isotope is incorporated into the material such as rock, mineral of organic material when the material is formed and then decays producing the stable daughter isotope. By experimentally measuring the ratio of parent to daughter isotopes (typically using mass spectrometer) and knowing the half life of the parent isotope, the age of the material can be determined.
Because it has a half life of only 5,730 years, carbon-14, commonly called radiocarbon, is used for dating materials that are between a few thousand and 50,000 years old. But it was difficult to do this reliably for materials older than 12,500 years, because there was no continuous record reflecting atmospheric carbon-14 for the period after 12,500.
This articles reports carbon-14 measurements from Lake Suigetsu, Japan. The carbon-14 measurements are directly compared to dating based on sediments and provide a continuous record of terrestrial radiocarbon to the present limit of the carobon-14 method, 52,800.
Lake Suigetsu contains annually deposited sediments that can be used for dating (using a technique called varve counting) as well as terrestrial plant macrofossils containing carbon that can be used for radiocarbon dating.
In 2006 a set of core samples called SG06 were taken that consist of four parallel cores that together provide a complete record for the past 52,800 years. This article reports 651 carbon-14 measurements for the period between 11.2 and 52.8 thousand years before the present correlated with a time scale primarily derived from varve counting and but also using temporal constraints from other paleoclimate records.
The results are compared with other dating techniques including calcium deposits in caves (speleothems), marine sediments, coral (IntCal09), and radiocarbon dating using tree rings (Late Glacial Pine record). Two sets of speleothem data are compared to the Lake Suigetsu data:
- Bahamas speleothem GB89-25-3
- Hulu Cave speleothem H82
Comparison of raw (black) and five-point moving mean (gray) Lake Suigetsu carbon-14 data against IntCal09 (light blue), as well as GB89-25-3 speleothem data (orange) H82 speleothem data (pink)
Comparison of raw (black) and five-point moving mean (gray) Lake Suigetsu carbon-14 data against IntCal09 (light blue), as well as GB89-25-3 speleothem data (orange) and the Late Glacial Pine record (blue)
As as example of the precision of dates estimated based on this data, the authors have used the radiocarbon sequence from Lake Soppensee, Switzerland to place the onset of the the Younger Dryas (a brief period of cold climatic conditions and drought) at Soppensee at 12,607 (+/- 85) years before the present. This is some 156 (+/- 88) years later than the cooling in Suigetsu and probably after the rapid cooling of Greenland.