Seasonal temperature changes obtained from carbonate clumped isotopes of annually laminated tufas from Japan: Discrepancy between natural and synthetic calcites

Carbonate clumped isotopes provide a novel paleo-thermometer that does not require knowledge of the isotopic compositions of environmental water. Despite considerable effort, there are substantial inter-laboratory disagreements in empirical calibrations between the abundance anomaly of the clumped isotopes (Δ₄₇) and absolute temperature, and discrepancies in paleo-temperature estimates from natural samples. In this study, clumped isotopes using two types of ¹⁷O corrections were analyzed for two sample sets of synthetic calcites and tufas collected monthly from two sites in southwest Japan with known δ¹⁸O values and temperatures of parent water. These measurements form the basis for two new temperature calibrations of carbonate clumped isotopes, the first from our Kyushu University laboratory in Japan. The calcites synthesized at four different temperatures (2.9–61.0 °C) yield a temperature-Δ₄₇ relationship of Δ₄₇=0.0354±0.0013×10⁶/T²+0.290±0.015 (R² = 0.986), which is indistinguishable from the calibrations of other recent studies based on acid digestion experiments at high temperature (70–100 °C). The Δ₄₇ thermometry for the natural tufa samples display the seasonal pattern of water temperature better than the δ¹⁸O thermometry. However, the Δ₄₇ values of the tufa samples deposited at temperatures ranging from 5.6 to 16.0 °C were lower than the values expected from the calibration using synthetic calcites, thus providing another temperature calibration of Δ₄₇=0.0336±0.0036×10⁶/T²+0.301±0.048 (R² = 0.735). Therefore, our results clearly identify a detectable, but fairly uniform, discrepancy in Δ₄₇-temperature calibration between synthetic calcite and natural samples (tufas). The amplitude of Δ₄₇ offset between the tufa and synthetic samples appears to be independent of the seasonal change in water temperature and the chemical properties of water, including pH, pCO₂, and precipitation rate of calcite. The lower Δ₄₇ values have been previously reported from other tufas and travertines, and therefore appear to be associated with processes inherent to tufas and travertines, such as CO₂ degassing. We also find that our results are insignificantly influenced by types of ¹⁷O corrections and almost independent with bulk isotopic composition.