What in reality did we count? Some details of the Suigetsu chronology (SG06_2012 yr BP chronology)

Thursday, 18 December 2014: 5:00 PM
Takeshi Nakagawa, Newcastle University, Newcastle Upon Tyne, United Kingdom; Ritsumeikan University, Kusatsu, Japan
Numerous terrestrial radiocarbon dates from sediment cores from Lake Suigetsu, Japan, associated with high-precision absolute chronology established by varve counting and Bayesian modeling, has been adopted as a major component of the IntCal13 radiocarbon calibration model, and hence has already been actively used by the international users of the radiocarbon age determination method. Despite the number of the users and the range of its influence, however, the reality/details of the varve counting has not necessarily been widely understood. For example, the statements like below are all wrong:

i) Lake Suigetsu sediment is fully varved;

ii) Lake Suigetsu varve chronology is based exclusively on counting;

iii) Lake Suigetsu chronology is reliable because it is based on varve counting;

iv) Error of the Lake Suigetsu chronology is counting uncertainty;

v) The counting error is less than 3 ‰.

In reality,

i) Suigetsu varves are often of suboptimal quality;

ii) We often needed to ‘interpolate’ varves where layers are not clear;

iii) Varve counting supports ‘precision’; of the Suigetsu chronology but its accuracy is mainly controlled by the Bayesian modeling of the 14C dates;

iv) Error of the Lake Suigetsu chronology mainly derives from uncertainties of the U-Th age and of the Bayesian wiggle matching;

v) The counting error alone was more than 3%.

Despite some negative tones of those latter statements, the Suigetsu chronology is still ‘based’ on varve counting and is very precise. In the presentation we explain how varve counting was performed and how the chronology was constructed.