3:52 PM - 4:07 PM
[MIS16-07] Recognition of annual lamination and high-resolurion stable isotopic records of cascade-type tufa.
Keywords:tufa, stable isotopic analyses, paleoclimate, Pacific Decadal Oscillation
Tufas, which develop in freshwater environments in limestone areas, are excellent paleoclimate archives for terrestrial regions in temperate to subtropical zones. Because tufas develop annual laminations and grow at a rate of several millimeters per year, analyses can be done at high resolution. However, fluvial tufas, which develop in riverine water passages, do not continuously accumulate beyond 20-30 years due to changes in the flow path. On the other hand, cascade tufas, which develop under waterfalls, are expected to provide a longer continuous record like stalagmites. This study aimed to reconstruct past climatic conditions using this cascade tufa developed at Obaru beach in Tokunoshima, Kagoshima Prefecture.
Two samples of cascade tufas of 49 cm and 18 cm thick, respectively, were collected from the Obaru beach. Age model based on observation of thin section and X-ray CT scan images indicates that both samples have grown during the last ~200 years. Stable oxygen and carbon isotopes were measured for 1430 and 892 sub-samples respectively collected in 0.2 mm or 0.4 mm intervals. Two samples show a common decadal variability of the oxygen isotope time series, but lack trends reflected from the Global Warming. Time series of the oxygen isotopes reveals saw-tooth pattern of ~25-year cyclicity until late 1970s, and then generally increased. These trends positively correlated with the Pacific Decadal Oscillation (PDO). Oxygen isotopes of tufa are generally controlled by precipitation rather than temperature. Correlation with PDO will be examined with isotopic measurements of local rainwater samples. In addition, carbon isotopes of one sample has been decreasing since the 1960s, which may record the Suess effect of fossil fuel consumption.
Two samples of cascade tufas of 49 cm and 18 cm thick, respectively, were collected from the Obaru beach. Age model based on observation of thin section and X-ray CT scan images indicates that both samples have grown during the last ~200 years. Stable oxygen and carbon isotopes were measured for 1430 and 892 sub-samples respectively collected in 0.2 mm or 0.4 mm intervals. Two samples show a common decadal variability of the oxygen isotope time series, but lack trends reflected from the Global Warming. Time series of the oxygen isotopes reveals saw-tooth pattern of ~25-year cyclicity until late 1970s, and then generally increased. These trends positively correlated with the Pacific Decadal Oscillation (PDO). Oxygen isotopes of tufa are generally controlled by precipitation rather than temperature. Correlation with PDO will be examined with isotopic measurements of local rainwater samples. In addition, carbon isotopes of one sample has been decreasing since the 1960s, which may record the Suess effect of fossil fuel consumption.