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Climatic Conditions and Hydrological Change Recorded in a High-Resolution Stable-Isotope Profile of a Recent Laminated Tufa on a Subtropical Island, Southern Japan

¹ Department of Earth and Planetary System Science, Hiroshima University, 739-8426 Higashi-Hiroshima, Japan; kano{at}geol.sci.hiroshima-u.ac.jp
² NHK (Japan Broadcasting Corporation) Miyazaki, 880-8633, Miyazaki, Japan
³ Department of Earth and Planetary System Science, Hiroshima University, 739-8426 Higashi-Hiroshima, Japan
⁴ Department of Earth and Planetary System Science, Hiroshima University, 739-8426 Higashi-Hiroshima, Japan
⁵ Marin Works Japan Ltd., Center of Advanced Marine Core Research, 738-8502 Nankoku, Japan

The stable-isotope composition of a tufa collected on subtropical Miyako Island, southern Japan, records climatic and hydrologic changes over a 15-year period (1989–2003). A total of 450 subsamples taken at increments of 0.2 mm define the high-resolution isotopic profile of a 9-cm-long sample. The oxygen isotope values clearly exhibit cyclical changes that generally correlate with the annual lamination pattern of the tufa. The observed stability of water ¹⁸O values confirms that the seasonal records of water temperature are reflected in the cyclic change of tufa ¹⁸O values. The oxygen isotope profile also exhibits a long-term decreasing trend that is equivalent to a 2.5°C temperature increase over the 15-year period. The range of the long-term change in ¹⁸O values is at least partly compensated for by a warming trend of 0.95°C seen in metrological data for Miyako Island. However, the carbon isotope profile does not vary cyclically, and the values are clearly higher than for previously reported tufas from the temperate Japanese mainland. Vegetation on Miyako Island, which is dominated by C4-type sugar cane, is largely responsible for the high ¹³C values of the tufa. The high-resolution isotopic profile provides data on the annual deposition rate, which clearly increased from 1996 to 1998, when the groundwater table was raised by the construction of two underground dams near the tufa locality. Increased water flow from the spring activated calcite precipitation and increased the depositional rate in the upper part of the core up to > 1 cm/year. This rate is much larger than those recorded for tufas from temperate settings in Japan.