Seasonal variation in water chemistry and depositional processes in a tufa-bearing stream in SW-Japan, based on 5 years of monthly observations

Annually laminated freshwater carbonate (tufa) is a potential source of high-resolution terrestrial paleoclimate data. In order to understand the processes and characteristics of climate information recorded in tufa, the chemical components of carbonate-rich water, its flow rate and soil PCO2 were recorded monthly during a 5-year period (December 1997 to December 2002) at Shimokuraida (Niimi City, Okayama Prefecture), SW-Japan. Water issues from a limestone cave there flows along a 450-m long stream and deposits tufa that commonly exhibits distinct annual laminations. observed properties, the CO2 partial pressures in the soil and the spring water show the clearest seasonal patterns, which are similarly high in summer–autumn and low in winter–spring. The spring-water PCO2 partly reflects soil PCO2, but is controlled by another temperature-dependent process, natural ventilation between the subsurface and the atmosphere. The concentration of dissolved CaCO3 does not mirror the seasonal pattern of spring-water PCO2, but is largely influenced by rainfall dilution. The development of annual laminations results from seasonal changes in the depositional rate of CaCO3, which was faster from summer to autumn and slower from winter to spring. This seasonal pattern is initially controlled by relatively regular changes in water temperature and flow rate, both of which correlated positively with the calcite precipitation rate. High calcite precipitation in summer–autumn formed a densely calcified texture, whereas a precipitation low rate winter–spring formed a porous texture. Changes in the concentrations of minor dissolved components (Mg2+, Na+, K+, Cl−, NO3−, SO42− and SiO2) exhibited no seasonal pattern. Because the amplitude of the Mg/Ca change is very high at Shimokuraida, the Mg/Ca ratio of tufa (calcite) does not reflect paleotemperatures. While, Mg/Ca ratio of water does not correspond to rainfall intensity, too.