Relationship between Oxygen Stable Isotope Ratios Recorded in a Cascade Tufa from Tokunoshima Island and Pacific Decadal Oscillation

Abstract

Tufas are carbonate sediments that develop in freshwater environments in temperate to subtropical limestone regions. Tufas form annual laminations consisting of dense and porous layers, which can be used for dating. Conventional studies have mainly focused on fluvial tufas, but the problem is that a continuous deposition over 40 years is hardly expected due to changes in flow paths. However, longer records can be found in cascade tufas, which are deposited like stalagmites at waterfalls. In this study, we studied cascade tufas developed on the Obaru beach of Tokunoshima Island, Kagoshima Prefecture, Japan to obtain a longer climate record. A 50 cm long specimen of the cascade tufa has a depositional period of about 200 years, which was recognized by observation of thin-sections and CT scan. By comparing the results of oxygen and carbon isotope ratio measurements with meteorological records, we aim to reconstruct long-term quantitative paleoclimate using tufas. Our preliminary results show cyclicity of about 20 years in the oxygen isotopes, which can be correlated to the change in seawater temperature of the western Pacific. This suggested a close relationship between seawater temperature fluctuations and rainfall.