Intra-annual perturbations of stable isotopes in tufas: Effects of hydrological processes

Tufas, which are freshwater carbonates, are potential archives of terrestrial paleoclimate. Time series of stable isotopic compositions commonly show regular seasonal patterns controlled by temperature-dependent processes, and some perturbation intrinsic to the locality. We examined three tufa-depositing sites in southwestern Japan with similar temperate climates, to understand the origin of local characteristics in the isotopic records. Seasonal change in the oxygen isotope is principally reflected by temperature-dependent fractionation between water and calcite but was perturbed after heavy rainfalls overwhelming the stability of the δ¹⁸O value of the groundwater at one site. Isotopic mass balance indicates an undersaturated and relatively small aquifer at this locality. Water δ¹⁸O values at the other two sites were stable, reflecting a regular seasonal change in the δ¹⁸O value of tufa. Perturbation of the δ¹³C values in tufa is largely due to CO₂ degassing from the stream, which significantly increases the δ¹³C values of dissolved inorganic carbon (DIC). At a site with remarkably high pCO₂ in springwater and a sensitive response of flow rate to rainfall, the amount of CO₂ degassing changed distinctly with flow rate. In contrast, the other two sites having low pCO₂ springwater reflect a regular seasonal pattern of δ¹³C in DIC and tufa specimens.