Silicification of cave corals from some lava tube caves in the Jeju Island, Korea: Implications for speleogenesis and a proxy for paleoenvironmental change during the Late Quaternary

摘要

Cave corals, formed by groundwater seepage, are present in some lava tubes (Socheon, Jingaemot, Susan, Mosimoru, and Bilemot caves) of the Jeju Island, Korea. The cave corals in Socheon Cave consist of calcite, whereas those in Jingaemot and Mosimoru caves are composed of opal-A, calcite, and/or aragonite. Cave corals in Susan and Bilemot caves consist of opal-A. Most show similar morphology to those in limestone caves, but some have morphology resembling frostworks. Internal texture of the cave corals in Jingaemot Cave shows alternating layers of aragonite and calcite, spherulitic-fibrous, and isopachous-fibrous microstructures, respectively. Calcite layers were more susceptible to silicification, and the replacing opal-A commonly retains relic crystals of pre-existing carbonate minerals. The corroded nature of replaced carbonate minerals indicates that the growth of carbonate minerals and silicification were controlled by the pH conditions of intermittent cave water. The chemical conditions of the fluids responsible for the precipitation of aragonite or calcite and for silicification appear to be influenced by the presence of overlying carbonate sediments, the degree of basalt weathering, and the amount of rainfalls, ultimately controlled by local climatic changes in the past.