Seasonal patterns of carbon chemistry and isotopes in tufa depositing groundwaters of southwestern Japan

Annually laminated carbonates, known as tufas, commonly develop in limestone areas and typically record seasonal patterns of oxygen- and carbon-isotope compositions. δ¹⁸O values are principally controlled by seasonal changes of water temperature, whereas δ¹³C values are the result of complex reactions among the gaseous, liquid, and solid sources of carbon in the system. We examined the processes that cause the seasonal patterns of δ¹³C in groundwater systems at three tufa-depositing sites in southwestern Japan by applying model calculations to geochemical data. Underground inorganic carbon species are exchanged with gaseous CO₂, which is mainly introduced to the underground hydrological system by natural atmospheric ventilation and by diffusion of soil air. These processes control the seasonal pattern of δ¹³C, which is low in summer and high in winter. Among the three sites we investigated, we identified two extreme cases of the degree of carbon exchange between liquid and gaseous phases. For the case with high radiocarbon composition (Δ¹⁴C) and low pCO₂, there was substantial carbon exchange because of a large contribution of atmospheric CO₂ and a small water mass. For the other extreme case, which was characterized by low Δ¹⁴C and high pCO₂, the contribution of atmospheric CO₂ was small and the water mass was relatively large. Our results suggest that at two of the three sites water residence time within the soil profile was longer than 1 year. Our results also suggested a short residence time (less than 1 year) of water in the soil profile at the site with the smallest water mass, which is consistent with large seasonal amplitude of the springwater temperature variations. The Δ¹⁴C value of tufas is closely related to the hydrological conditions in which they are deposited. If the initial Δ¹⁴C value of a tufa-depositing system is stable, ¹⁴C-chronology can be used to date paleo-tufas.