Opal Cave Corals In The Lava Tubes In Jeju Island, Korea: Implications For Local Paleoenvironmental Change

Cave corals, which have grown by groundwater seepage, can be found within the lava tubes (Socheon, Jingaemot, Susan, Mosimoru and Billemot caves in Jeju Island, Korea. Unlike other speleothems in most lava tubes, these cave corals should have formed after the formation of the lava tubes. The mineralogy and formation process of these cave corals (commonly cited as siliceous sinter) are poorly understood. The cave corals in Socheon Cave only consist of calcite, whereas those in Jingaemot and Mosimoru caves are composed of opal-A, calcite and/or aragonite. Cave corals in Susan and Billemot caves only consist of opal-A. Most cave corals show similar morphology to those found in many limestone caves: some of them resemble the form of the anthodites. Internal texture of the cave corals in Jingaemot Cave show alternating layers of aragonite and calcite. Calcite crystals show isopachous fibrous texture, and aragonite crystals show spherulitic fibrous texture. Most of the terminated portion of both crystals were replaced by opal-A along growth laminae, and the replacing opal-A commonly retain relic crystals of calcite and aragonite. Corroded nature of the former carbonate crystals indicates that the growth of carbonate and siliceous minerals would have been controlled by the pH conditions of cave water from which they precipitated. The mineralogy in cave corals and diagenetic process appear to have a close relationship with the chemical conditions of cave water that percolated down through the overlying carbonate soils and weathered basaltic rocks, and the chemical conditions of cave water were mainly controlled by local climatic variations in the past.