High-resolution isotopic records for the last 200 years from cascade tufas on Kobaru Beach, Tokunoshima Island, Japan

A stromatolitic tufa with annual growth bands and a large growth rate is a promising archive for high-resolution paleoclimate reconstruction. However, a tufa formed in a fluvial setting typically lacks continuity of deposition. To obtain a continuous record, a potential study material is a cascade-type tufa growing below a waterfall. Here, we study tufas along Kobaru Beach on Tokunoshima Island, Japan, where cascade-type tufas are developed. Oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) of tufa samples collected from three sites with different sedimentary settings provide information about precipitation patterns and water temperature. A tufa sample from a fluvial setting exhibits clear annual growth rings, rapid growth rates, and seasonal changes in isotopic composition over approximately 20 years. Tufas of the stalagmite-type and cascade-type, having less clear annual growth rings and lower growth rates, preserve continuous records over the last 200 years, the longest record from the modern tufas. The δ¹³C of the cascade-type tufa shows a decreasing trend from the 1980s, which is interpreted as a result of fossil fuel consumption and an increase in rainfall. In addition, the δ¹³C of the cascade-type tufa tends to increase in dry intervals and decrease in humid intervals. On the other hand, an approximately 20-year periodicity is observed in the δ¹⁸O. Analysis of rainwater samples implies that the δ¹⁸O of tufa reflects the seasonality of rainfall, which is inherited from the rainwater δ¹⁸O. This interpretation is verified by the consistency between the actual record and the seasonality reconstructed by the tufa δ¹⁸O. While cascade-type tufas do not provide sub-annual resolution, they serve as valuable centennial-scale climate archives, which allow us to investigate decadal-scale climate variability, such as Pacific Decadal Oscillation.