Radiocarbon marine reservoir ages in the western Pacific estimated by pre-bomb molluscan shells

doi:10.1016/j.nimb.2007.01.184

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 259, Issue 1, June 2007, Pages 432-437

https://doi.org/10.1016/j.nimb.2007.01.184 Get rights and content

Abstract

Apparent ¹⁴C ages of the pre-bomb molluscan shells show new estimations of ΔR values for the western Pacific including the East Sakhalin Current, Oyashio Current, Kuroshio Current, Kuroshio Countercurrent, and Pacific North Equatorial Current. Clear differences in ΔR between the subarctic and subtropical gyres water masses around the Japanese archipelago are observed.

Introduction

The accurate evaluation of the marine reservoir ages on radiocarbon (¹⁴C) dates is important not only for dating archaeological and anthropological materials but also for the age determination of environmental events recorded in marine sediment cores. In this study, the reservoir ages in the western North Pacific are estimated based on apparent ¹⁴C ages of pre-bomb known-age molluscan shells. We have reported some data regarding on this matter in this area using both recent shells [1] and pairs of marine and terrestrial faunal remains from archaeological shellmidden sites in Hokkaido [2], [3], [4]. However, the reservoir ages around Japan is still not fully understood because of complicated ocean conditions and the lack of reliable known-age marine specimens which had been collected alive.

Although the shells samples of Kyoto University Museum are an exceptionally well-organized collection of pre-bomb molluscan shell collected around Japan, our pilot study showed unexpected large variability among some samples from the same locations and considerable anomalies probably caused by contamination of dead shell or hard-water effect [1]. It was shown that the rigorous selection of samples should be done in the light of their ecology, dietary habit, and living habitat as well as morphological evidence of collection alive, in order to obtain more detailed map of reservoir ages in the NW Pacific. In our previous studies, furthermore, marine reservoir ages as large as 800 ¹⁴C years were observed in the subarctic surface water and the significant difference in ¹⁴C reservoir ages between the Oyashio and Kuroshio Currents along to the Japanese archipelago were expected. This was supported by reservoir ages around 800 ¹⁴C years observed in archaeological remains dated from 5000 to 750 BP around the Volcano Bay, Hokkaido, Japan [2], [3], [4].

Recently some researchers reported the marine reservoir ages in this region. Kuzmin et al. [5] reported 6 radiocarbon dates on the shell collected during the period from 1900 to 1945 on the Pacific coast of Kunashiri Island and two dates for Peter the Great Gulf in the Sea of Japan. They showed that the marine reservoir age on Kunashiri Island (711 ± 46 ¹⁴C years) were significantly larger than that for the Sea of Japan (370 ± 27 ¹⁴C years). The regional means for reservoir corrections for the NW Pacific and the Sea of Japan are based on only these data at the moment (Marine Reservoir Correction Database by Reimer [6]; http://calib.qub.ac.uk/marine/).

In the subtropical Pacific, some estimations on the reservoir effect have been done by analyzing coral heads. Hideshima et al. [7] reported the temporal change in marine reservoir ages during the period from 1899 to 1949 from an annually ringed coral core from Ishigaki Island, Ryukyu Islands. The weighted average of reservoir ages was 355 ± 25 including variable values from 290 to 450. However, this did not agree with data from Naha port, Okinawa Island [8], which showed much smaller reservoir ages around the mean value of 186 ¹⁴C years. It seems this is similar to the reservoir age of 198 ± 60 produced by measuring a gastropod from Ogasawara Islands, although surface water around Guam Island was significantly older (392 ± 50 ¹⁴C years) according to the same source [9]. This discrepancy strongly suggests that more data are required to evaluate reservoir ages in western subtropical Pacific as well.

We measured apparent ¹⁴C ages on a series of known-age molluscan shells from the northwestern and western subtropical Pacific to evaluate marine reservoir ages in this region. In order to estimate the typical values of some major water masses surrounding Japan, samples from the northern region including Sakhalin and Hokkaido, and the southern region including Taiwan, Ryukyus and Ogasawara were investigated. These regions correspond to the water masses of Oyashio Current, East Sakhalin Current in the north, and Kuroshio and its countercurrent in the south (Fig. 1). The data are discussed from the oceanographic point of view.

Section snippets

Materials and methods

The shell materials used for this study have been housed at the University Museum, the University of Tokyo, Japan. The specimens were collected by the Department of Zoology for biological studies. The labels in specimens’ cases generally recorded the sampling locations, dates and collectors’ names, while only rough geographical information, such as an island’s name, were available for several samples. The analyzed samples were carefully selected in the light of morphological evidence that

Results and discussion

Table 1 shows the comparison between apparent ¹⁴C dates treated by the phosphoric acid method ant the pyrolysis in EA-CT. The T values for χ² test suggest that three samples showed good agreement between the results by two different pretreatments with one exception, K-37 [19]. K-37 showed that ¹⁴C date by phosphoric acid method (574 ± 37 BP) is significantly older than that by EA-CT method (454 ± 35 BP). Although the reason of this discrepancy is not clear, it is not likely that this was caused by

Conclusion

The molluscan shells housed at the University Museum, the University of Tokyo, showed the significant variability in reservoir ages in the western North Pacific and surrounding regions. In the northern part, East Sakhalin Current and the Oyashio Current show similar reservoir ages, ΔR values of 399 and 392 ¹⁴C years, respectively. This indicates lower ¹⁴C contents in the subarctic gyre. On the other hand, water masses of Soya Current and Tsugaru Warm Current which showed smaller ΔR values of 94

Acknowledgements

Toshiyuki Kobayashi and Chiaki Kobayashi of Hakuto Co. supported this study by AMS operation, and Yukiko Minoura, Masayo Ihara, Chifumi Kitazaki by sample preparations. This study was financially supported by Grants-in-Aids for Scientific Research (1174048, 13740499, 17107006, 17770214) and the Postdoctoral Fellowship for Research Abroad from the Japan Society for the Promotion of Science to M.Y.

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Radiocarbon marine reservoir ages in the western Pacific estimated by pre-bomb molluscan shells

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusion

Acknowledgements

Nucl. Instr. and Meth. B

Nucl. Instr. and Meth. B

J. Archaeol. Sci.

Nucl. Instr. and Meth. B

Nucl. Instr. and Meth. B

Radiocarbon

Radiocarbon

Radiocarbon

Radiocarbon