Anthropogenic impact records of nature for past hundred years extracted from stalagmites in caves found in the Nanatsugama Sandstone Formation, Saikai, Southwestern Japan

Abstract

In the Nanatsugama area, Saikai City, Nagasaki Prefecture, Japan, covered by Paleogene calcareous sandstone, the environmental change information for the past hundred years was extracted from growing stalagmites in two limestone caves. Their annual microbanding information was used for dating. From the Shimizu-do Cave stalagmites, the vegetation change from forest to grassland during 1500 to 1700 could be read using the carbon isotope and Mg/Ca ratios of the stalagmites. Before 1500, the stable carbon isotope ratios ranged from −9 to −10‰, which are characteristic of forest vegetation. From 1600 to 1700, the stable carbon isotope ratio increased (δ13C=−2‰), suggesting a drastic change to grassland vegetation probably for the purpose of collecting grasses for agricultural use. The increase in Mg/Ca due to the reduction in the biomass from 1500 to 1700 also showed the vegetation change from forest to grassland. A Ryuo-do Cave stalagmite recorded the change in the SO42− concentration several times during the period from 1600 to 1900. In addition to the sulfate of sea salt origin, the higher concentration and smaller δ34S of sulfate in the Ryuo-do Cave drip water compared to those in the Shimizu-do Cave drip water may be due to the oxidation of biogenic pyrite in the marine Kamashikiyama Tuff Formation, which covers the calcareous sandstone of the Nanatsugama Sandstone Formation. The oxidation of pyrite is promoted by the biological activity of sulfur-oxidizing bacteria under oxic conditions, indicating that the surface cover was repeatedly changed into non-paddy fields. The change in the SO42− concentration could then be related to the changes in the amount of the Nagasaki Prefecture coal production in the early 20th century and the China fossil fuel consumption in the late 20th century transported a long-distance by a monsoon from the China continent. Thus, the combination of annual microbanding information and the Mg2+ and SO42− concentrations and C and S stable isotope ratios of the stalagmites made it possible to extract local and/or global anthropogenic environmental changes in nature. The records extracted in this way were in good agreement with those partially extracted from ancient documents, ancient picture maps and topographical maps.