Kunio Omoto

Radiocarbon ages of beach rocks and late Holocene sea-level changes in the southern part of the Nansei Islands, southwest of Japan.

Beach rock is a good indicator of the past sea levels, as it is considered to have been formed within the range of intertidal zone. Radiocarbon dates of beach rocks collected from Iriomote Island, Ishigaki Island, and Miyako Island, in the southern part of the Nansei Islands, indicate that the beach rocks were formed between around 4000 BP and 400 BP. Late Holocene sea-level changes were revealed based on the elevations and (super 14) C dates of the beach rocks. The results indicate that the sea level was similar to the present one for at least the past 4000 BP. Isotopic fractionations (delta (super 13) C) of the beach rocks were between +9.4 per mil and -0.8 per mil, suggesting a different origin for calcium carbonate.

RADIOCARBON AGES AND ISOTOPE FRACTIONATIONS OF BEACHROCK SAMPLES COLLECTED FROM THE NANSEI ISLANDS, SOUTHWESTERN JAPAN

A total of 294 beachrock samples were collected from 116 sites on 15 islands in the Nansei Islands chain,southwestern Japan, and were radiocarbon dated. The beachrocks began to form at about 6900 BP and some are still under development in the islands. Values of isotope fractionations of different materials making up the beachrocks ranged between +9.4%0 and - 5.7%0. Isotope fractionations outside the range of 0 2%0 suggest that these beachrocks were strongly influenced by underground water and running freshwater when they were cemented. The sea level during the late Holocene has remained the same for at least the past 5000 yr, except for several uplifted coasts.

Calibrated 14C Ages of Jomon Sites, NE Japan, and Their Significance

The traditional archaeological chronology in the Japanese Islands during the Jomon period was essentially based on the relative age given to cord-impressed patterns marked on pottery, as well as the shape of the pottery and the thickness of the cultural layers that were excavated. We aimed to correlate the classical archaeological chronology with calibrated radiocarbon dates, to posit a new chronology for the Jomon period in northeastern Japan. We calibrated 80 accelerator mass spectrometry (AMS) 14C dates from NE Japan and reconstructed a chronological timetable for Hokkaido and the Tohoku District. We collected 43 samples from 5 shellmounds and 2 archaeological sites on Hokkaido Island and 4 shellmounds in the Tohoku District in order to determine the calibrated age of their sites. Δ R values used on Hokkaido Island and the Tohoku District were between 282 and -158 yr and between ±0 and -40 yr, respectively. The large Δ R value for the eastern part of Hokkaido Island indicates the influence of the Oyashio Current, while an anomalous Δ R value was obtained from northern Hokkaido Island. These figures show larger apparent Δ R values than those from southwest Japan (Nakamura et al. 2007). The calibrated Jomon period in the investigated area was from 2000 to 200 yr younger than the previous chronology. Calibrated 14C ages of the shellmounds investigated ranged between ~6000 and 3000 yr, correlating to the Early Jomon and Final Jomon periods as indicated by the former archaeological chronology of Honshu Island.

The radiocarbon data base of Japan.

About ten radiocarbon laboratories operate in Japan and have dated more than 25,000 samples since 1960. Geomorphic development and human activities since the last glaciation have been documented with radiocarbon determinations. In order to apply these dates more effectively, the author finds it necessary to create and maintain a radiocarbon database system, which he has been doing since 1985, using a personal computer system linked to a telephone line. A researcher may access, search and retrieve data from the Radiocarbon Database System of Japan.

Development of new beta-counting programs operating under a Windows NT workstation.

beta -counting software programs were developed and installed in new PCs for operation under a Windows (super R) NT workstation. The new software programs allow us to simultaneously control several beta -counting systems, either remotely or from within the laboratory. This setup is much less expensive than our previous arrangement, which required a separate PC for each counter.