Reconstruction of environmental changes based on growth rates and carbon and oxygen isotopic compositions of a stalagmite in Imamura Cave, Minami-Daito Island, southwest Japan

Abstract

Speleothems are typical terrestrial carbonates and their formation is dependent on a variety of geological, hydrological, chemical and climatic factors. For the reason, isotopic compositions of speleothems are known as a good indicator of paleoclimate and paleoenvironment. Especially, the  13 C values are considered to be related to vegetation around the limestone cave, decay of organic matter within the soil zone and isotopic compositions of the adjacent carbonate host rocks. Therefore, we attempted reconstruction of environmental changes associated with cultivation by pioneers since 1900, based on carbon and oxygen isotopic composition of a speleothem in Imamura Cave, Minami-Daito Island, southwest Japan. To reconstruct the environmental changes since 1900, when first pioneers settled in the island, a stalagmite collected in 2012 was utilized. Dating of the stalagmite was carried out based on annual bands. Samples for the isotopic study were taken along the growth axis from the top 4-cm of the stalagmite at a sampling interval of 0.5 mm. As a result, an average growth rate is 44.9m/year before 1900, while the rate is 226. 7m/year after 1900. This drastic change of the growth rate is considered due to the change of vegetation around the cave from sub-tropical natural forests to sugarcane fields. The  13 C values before and after 1900 range from -10.94 to -3.98‰ 13 C PDB and from -2.52 to -0.86‰ 13 C PDB , respectively. The abrupt increase of the  13 C values at 1900 means that the contribution of the isotopically heavier carbon derived from sugarcane to ground water increased with cultivation. Further, some fluctuations of the  13 C values are observed since 1900, and those seem to be related to the yield of sugarcane per unit area. On the other hand, the  18 O values range from -8.57 to -6. 70‰ 18 O PDB , and the values before and after 1900 are not so different. However, some fluctuations of the  18 O values are recognized, and it seems that the fluctuations are correlated with the ratio of the precipitation in summer to total annual precipitation. This suggests that the  18 O values of the stalagmite reflect the seasonal