Masataka Hakozaki

Search for Annual 14C Excursions in the Past

Two radiocarbon excursions (AD 774–775 and AD 993–994) occurred due to an increase of incoming cosmic rays on a short timescale. The most plausible cause of these events is considered to be extreme solar proton events (SPE). It is possible that there are other annual 14C excursions in the past that have yet to be confirmed. In order to detect more of these events, we measured the 14C contents in bristlecone pine tree-ring samples during the periods when the rate of 14C increase in the IntCal data is large. We analyzed four periods every other year (2479–2455 BC, 4055–4031 BC, 4465–4441 BC, and 4689–4681 BC), and found no anomalous 14C excursions during these periods. This study confirms that it is important to do continuous measurements to find annual cosmic-ray events at other locations in the tree-ring record.

Stimpson's hard clam Mercenaria stimpsoni; A multi-decadal climate recorder for the northwest Pacific coast

A sclerochronological and radiocarbon-based study of life history traits of Stimpsons hard clam (Mercenaria stimpsoni), collected alive from Funakoshi Bay, northeast Japan, showed the lifespan of the species to be at least 92 years (determined from annual growth line counts). Three M. stimpsoni specimens exhibited the following synchronous growth pattern, suggestive of environmental control; annual increment width increasing after 1955 to a maximum value between 1970 and 1980, subsequently decreasing gradually until 2000, and thereafter remaining constant or increasing slightly. Variations on annual growth patterns, as well as standardized growth indices chronology, were relatively closely linked to the Atlantic Multidecadal Oscillation (AMO), but less so to Pacific Decadal Oscillation (PDO). Carbonate samples collected from ontogenetically younger shell portions, estimated from growth line counts to have been deposited before 1950, contained no nuclear bomb-test radiocarbon, thereby supporting the accuracy of annual growth line counts (versus overcounting from ventral margin). Together with the synchronous annual increment width patterns, this indicated that age and annual growth rate estimations for M. stimpsoni based on growth line counts were reliable and applicable to high-resolution sclerochronological analyses, which should contribute to a deeper understanding of multi-decadal northwest Pacific climate variability.

Fine Structure and Reproducibility of Radiocarbon Ages of Middle to Early Modern Japanese Tree Rings

This study carried out accelerator mass spectrometry radiocarbon (AMS 14 C) measurement of Japanese tree rings dating from the middle to early modern eras to investigate calibration curve fine structure. Tree-ring ages were determined by dendrochronology or δ 18 O chronology for Japanese trees. 14 C ages from the 15th century to the middle of the 17th century followed the IntCal13 calibration curve within measurement error. Different patterns of fluctuations during the latter half of the 17th century to the early the 18th century were observed in different tree samples. In the 19th century, patterns of 14 C ages of different samples appeared similar but did not exactly match each other.