Seasonal cave air ventilation controlling variation in cave air PCO2 and drip water geoch emistry at inazumi cave, Oita, northeastern Kyushu, Japan

Abstract

To compare the influence of cave air PCO2 and drip rate on the drip water geochemistry, approximately one year of cave air monitoring and sampling of drip water were conducted at Inazumi Cave, Oita, northeastern Kyushu, Japan, from February to December 2014. The monitoring revealed that temperature dependent cave air ventilation controlled distinct seasonal variation in the cave air PCO2 and minor variation in temperature and relative humidity with increasing distance from the cave entrance. In addition to traditional sampling of drip water, novel sampling methods were designed to compare the influence of the PCO2 and the drip rate on the karstic and drip water geochemistry. The chemical analysis indicated that the karstic and drip water instantaneously outgassed CO2 once in contact with low cave air PCO2. The drip rate alone, however, had less significant influence on the drip water geochemistry than the PCO2. The drip water PCO2 was repeatedly lower than cave air PCO2, and this is probably due to prior calcite precipitation, to a temporal increase of the cave air PCO2 by anthropogenic CO2, or to storage conditions of the sample; all of which cause alteration of drip water geochemistry from the original state.