Oxygen isotopes of the Japanese stalagmites as global and local paleoclimate proxies

Abstract

Stalagmite oxygen isotopes (δ¹⁸O) have been used to reconstruct terrestrial paleoclimates during the late Pleistocene and Holocene. However, the interpretation of the δ¹⁸O is not straightforward when determining the factor controlling δ¹⁸O; temperature or water δ¹⁸O. In addition, the water δ¹⁸O changes with rainfall intensity (amount effect), rainfall seasonality, and some other factors. Here, we first review the hydrochemical processes and behaviors of the oxygen isotopes and the other proxies in a cave system, which are fundamental for interpretating the paleoclimatic signals. We then introduce the oxygen isotope records of Japanese caves. Some of the Japanese stalagmites demonstrated a δ¹⁸O profile that represented a similar pattern to the Chinese stalagmite records, but had relatively small δ¹⁸O amplitudes, which can be explained mainly by temperature changes rather than the amount effect. This demands a reversal of the relationship between climate and rainwater δ¹⁸O across the Japanese Islands. Using δ¹⁸O data for rainwater samples from four sites in Japan (in Niigata, Fukuoka, Gifu and Mie Prefectures), we presents the results of model calculations to verify how the rainfall intensity and the seasonality relate with the δ¹⁸O of rainwater. A significant correlation coefficient was observed in Niigata, where the rainfall δ¹⁸O decreases with an increase in the annual amount of rainfall, and with a decrease in the winter rainfall. Similar trends were observed in Fukuoka, whereas while the results of Gifu and Mie exhibited no significant trends. Temperature change was would be the main factor controlling the stalagmite δ¹⁸O at the latter two sites. For a better understanding of the stalagmite δ¹⁸O records, the measurement of fluid inclusions and carbonate clumped isotopes can be used to evaluate the effect of temperature on the stalagmite δ¹⁸O, as well as to reconstruct the water δ¹⁸O. We predict that the ¹⁷O excess in stalagmites reconstructs the seasonal shift in the vapor sources.