名古屋大学年代測定総合研究センターシンポジウム報告

Radiocarbon calibration is a fundamental stage of the radiocarbon dating process if meaningful calendar ages are to be derived from samples’ radiocarbon determinations. However, the present limit of direct, non-reservoir-corrected, atmospheric radiocarbon calibration is 12,550 calibrated years before present (Reimer et al. 2009), leaving approximately three quarters of the radiocarbon timescale to be necessarily calibrated via less secure marine records. The sediment profile of Lake Suigetsu, Honshu Island, central Japan, offers an ideal opportunity from which to derive an extended, ‘wholly terrestrial’ and continuous record of atmospheric radiocarbon back to the limits of radiocarbon detection (circa 60,000 years before present). The presence of well-defined, annually-deposited laminae (varves) throughout this extended time period provides an independent, high resolution chronometer against which radiocarbon measurements, performed upon plant macrofossil samples retrieved from the sediment column, can be directly related. This site was first exploited for radiocarbon calibration purposes by Kitagawa and van der Plicht (1998a, 1998b), however, issues pertaining to the reliability of the calendar age scale of this work precluded the widespread uptake of this dataset. The work presented in this DPhil thesis represents a significant contribution to the broader, ‘Suigetsu Varves 2006’ project – an international collaboration centring on the re-coring of Lake Suigetsu, which was undertaken in summer 2006 to improve upon the shortcomings of the previous project and, thereby, to fully exploit the site’s potential for both radiocarbon calibration and multi-proxy palaeoenvironmental study (Nakagawa et al. 2011). This DPhil thesis describes the generation of the revised (‘SG06’) terrestrial radiocarbon calibration dataset from Lake Suigetsu, comprising 647 accelerator mass spectrometry (AMS) radiocarbon determinations, and extending across the complete range of the radiocarbon dating method. Furthermore, visual matching of archive SG93 core sections to the continuous SG06 sediment profile was undertaken, allowing the integration of the ≈ 300 radiocarbon determinations from the original Lake Suigetsu project into a higher resolution (≈ 900 radiocarbon measurements), combined Lake Suigetsu radiocarbon calibration dataset, providing a unique reconstruction of atmospheric radiocarbon across the entire radiocarbon dating timescale.

No / his paper addresses the possible connections between the onset of human expansion in Europe following the Last Glacial Maximum, and the timing of abrupt climate warming at the onset of the Lateglacial (Bölling/Allerød) Interstadial. There are opposing views as to whether or not human populations and activities were directly forced by climate change, based on different comparisons between archaeological and environmental data. We review the geochronological assumptions and approaches on which data comparisons have been attempted in the past, and argue that the uncertainties presently associated with age models based on calibrated radiocarbon dates preclude robust testing of the competing models, particularly when comparing the data to non-radiocarbon-based timescales such as the Greenland ice core records. The paper concludes with some suggestions as to the steps that will be necessary if more robust tests of the models are to be developed in the future.

¹⁴C wiggle-matching was applied to two wood samples closely related to geological and archaeological events with associated dendrochronological dates, to demonstrate the accuracy of ¹⁴C dating with accelerator mass spectrometry (AMS). Wiggle-matching on charred wood with bark, excavated from a pyroclastic mud-flow deposited by the huge 10th Century eruption of the Baitoushan Volcano, revealed the eruption age as cal A.D. 935 +8/-5 with 95% confidence. This date is consistent with the eruption age of A.D. 912 to A.D. 972 estimated by dendrochronology on two wooden boards that had clear stratigraphical connections to the B-Tm tephra deposit in Japan, an ash fall layer formed by the eruption of the Baitoushan Volcano. The date is also consistent with an A.D. 937–938 date estimated by the analysis of varved sediments from Lake Ogawarako in Aomori prefecture. The other wooden board collected from the Mawaki archaeological site in Ishikawa prefecture was wiggle-matched as 783 +13/-11 cal B.C. with 95% confidence, which is consistent with the dates of 830 cal B.C. to 759 cal B.C. obtained for seven wooden poles from the same wooden structures as the wooden board. These results are highly encouraging for obtaining accurate dates on wood when dendrochronology cannot be used.