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Intra-annual perturbations of stable isotopes in tufas: Effects of hydrological processes |
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| Authors: | M Hori T Kawai A Kano |
| Institution: | a Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan b OD Scientific Technology, Marine Works Japan LTD., Showamachi 3173-25, Kanazawa-ku, Yokohama 236-0001, Japan c OD Scientific Technology, Marine Works Japan LTD., Kochi Core Center 200 Monobe Otsu, Nankoku City 783-8502, Japan d Division of Evolution of Earth Environments, Graduate School of Social and Cultural Studies at Kyushu University, Ropponmatsu 4-2-1, Chuou ward, Fukuoka 810-8560, Japan |
| Abstract: | 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. |
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