Prior calcite precipitation and source mixing process influence Sr/Ca, Ba/Ca and ⁸⁷Sr/⁸⁶Sr of a stalagmite developed in southwestern Japan during 18.0 4.5ka

We measured Sr/Ca, Ba/Ca, ⁸⁷Sr/⁸⁶Sr ratios, δ¹⁸O and δ¹³C values in a stalagmite, which developed 18.0-4.5thousand years ago (ka) in southwestern Japan. Dripwater and two major bedrocks (limestone and andesite) in the locality were also studied. The ⁸⁷Sr/⁸⁶Sr ratios of the stalagmite are relatively homogeneous (0.706852-0.706921), suggesting a steady source mixing ratio of ~40% from high-⁸⁷Sr/⁸⁶Sr limestone and ~60% from low-⁸⁷Sr/⁸⁶Sr andesite. The stalagmite Sr/Ca and Ba/Ca ratios were higher than the ratio expected from the dissolved fraction of limestone and andesite. The covariance among Sr/Ca, Ba/Ca, and δ¹³C profiles suggests a significant role of prior calcite precipitation (PCP), i.e., carbonate precipitation from infiltrating water before the water drips on a stalagmite. The relationships among stalagmite Sr/Ca, Ba/Ca ratios and δ¹³C values are consistent with the Rayleigh-type fractionation model, supporting that PCP results in successive enrichment of Sr, Ba and ¹³C in the aqueous phase and the resulting stalagmite. The degree of PCP calculated for the stalagmite is highly variable from 0 to 85%, and generally decreased from the last glacial period to the middle Holocene. The large degree of PCP observed during 18-15ka implies a relatively dry climate during this period, which is consistent with weak monsoon intensity inferred by the δ¹⁸O values. The ⁸⁷Sr/⁸⁶Sr ratios of the stalagmite show a slight decrease through the entire period. The increase in the andesite-derived fraction with relatively high ⁸⁷Sr/⁸⁶Sr may result from accelerated silicate weathering in the epikarst with increasing temperature, humidity, and soil pCO₂.