These data are important for synchronization of palaeo-climate data and improve calibration of carbon-14 dating for the period before the last ice age.

Dendrochronology

Carbon-14 dating are calibrated to calendar years because the level of carbon-14 in nature is not a constant factor. This level is dependent on the strength of the earth's magnetic field, solar activity, the amount of cosmic radiation and the terrestrial carbon cycle. Traditionally, dendrochronology or tree-ring dating is used in the process of calibration. When tree rings are counted they can provide us with an absolute dating. By also taking in account the carbon-14 dating, we will be able to learn the relationship between the two dating methods and therefore the level of carbon-14 in nature in the past. However, this only accounts for approximately the last 12,500 years as dendrochronology is not available for periods before the last ice age.

The Suigetsu Lake

Fortunately, the Suigetsu Lake in Japan provides us with a solution. Over the last 100,000 years, the sediment has been layered onto the bottom of the lake. Like growth rings of a tree, these layers each represent a year and as such form a calendar. Samples are taken from these layers, in particular of the plant residues. With the help of the carbon-14 method these layers can be dated and this produces a curve.

Gaps in the sediment

Fifteen years ago, the Centre for Isotope Research (CIO) already made such a curve for the last 50,000 years. Sadly, this calibration curve was not consistent with other data. What went wrong? The cause was easily discovered: the CIO researchers "forgot" about the gaps in the sediment. Earthquakes often occur in Japan. These earthquakes had a big impact on the bedding of the lake, resulting in gaps. If one doesn't know how big those gaps are, it will not be possible to make an accurate calibration curve.

Drilling multiple cores

To improve this calibration curve, a Japanese-British team started a new project where they drilled multiple cores adjacent to one another. The levels of carbon-14 of these samples were analyzed in Oxford and Glasgow. Additionally, the results were correlated with the data of the original Groningen measurements. With the aid of independent data, these new sediment cores resulted in a consistent model which produces a proper representation of the carbon-14 variations in nature during the last 50,000 years.

A new carbon-14 calibration curve

Together, the integrated data of the 3 laboratories form one of the most advanced databases, available for archaeologists, paleontologists and climate researchers. A new generic carbon-14 calibration curve, in which the Japanese measurements play a prominent role (among other information), is in progress and will be published in early January in the journal Radiocarbon under the name "intcal13".

More information

Reference
C. Bronk Ramsey et al., a complete terrestrial Radiocarbon record for 11.2 to 52.8 kyr BP, Science 338, pag. 370-374

For more information about this topic, please contact prof. Hans van der Plicht.
Hans van der Plicht is a researcher at the Centre for Isotope Research (CIO) of the University of Groningen and a professor of Isotope Archaeology at Leiden University.