Description and environmental monitoring of Hokkai Cave in northern Japan.

Abstract

Hokkai Cave was discovered in 2006 in southwestern Hokkaido, northern Japan, and has been conserved in its natural condition. This paper reports the results of a basic speleological study of the cave's environment conducted during 2006–2009, including a geomorphological survey, monitoring of cave air temperature, airflow and groundwater currents, and an initial interpretation of the cave's speleogenesis. These basic cave data are important to help improve palaeoclimatic speleothem sample selection, because speleothem geochemistry is also controlled by cave geometry and micro-meteorology. Hokkai Cave is a 479.8m-long, zigzag, horizontal system, with two entrances. The cave floor is mainly covered with clay, gravels and breakdown blocks. An underground river flows in the centre of the cave and a dry riverbed, and many springs and dolines are present around the cave. Pure white stalagmites in the cave are possibly suitable for use in Asian Summer Monsoon reconstruction studies. Air temperature in the inner cave (>15m from entrances) was stable throughout the year (7.5–7.7°C); in contrast, air temperature near the entrance was variable because of the influence of the outside air temperature. Inversion of airflow direction is driven by the air temperature gradient between the inside and the outside of the cave. When air temperature inside is warmer than outside, air flows from Ent.U (upper) to Ent.L (lower) and from Ent.U to the inside. When air temperature outside is warmer than inside, air flows from Ent.L to Ent.U and from the inner cave to Ent.U. These results agree with findings from previous studies. Inversion of airflow direction also affects the variation pattern of air temperature near the entrances. Monitoring also indicates that the underground river is usually dry except after heavy rainfall or snow melting.