Local hydrology control of radiocarbon in stalagmites from the Kyusendo Cave, Kumamoto, Japan

Stalagmite is an important archive of paleoclimate especially in the region of the East Asian Monsoon. Despite the widespread use of radiocarbon (¹⁴C) dating to explore past environmental changes, the contribution of a¹⁴C-free carbon fraction leached from soil and/or host rocks during stalagmite formation, known as the dead carbon fraction (DCF), impedes its application to stalagmite chronology. Thus, uranium series dating is preferentially used to determine stalagmite ages. However, both U/Th and ¹⁴C dating can be applied to stalagmite samples, U/Th can be used to calibrate the radiocarbon ages by assuming a relatively constant DCF contribution over time. Studies exploring DCF changes from glacial to interglacial periods remain scarce, suggesting that temporal and speleothem-specific DCF studies are needed. Here, we present findings on DCF changes over the last 38 ka in speleothems obtained from southwestern Japan. Our analysis includes measurements of both ¹⁴C in drip water and speleothem calcite alongside U/Th dating of three stalagmites collected from Kyusendo Cave, located in southern Japan. The DCFs reconstructed from these stalagmites exhibited variations of 37.8%–73.9% between 4.2 and 38.3 ka, which exceeded the typical DCF values reported previously. Intra-test variations of the DCF values in Kyusendo Cave also revealed differences of up to 30%–40% among the three stalagmites. A higher drip water DCF and dripping rate showed a strong negative correlation, indicating that variations in DCF may reflect changes in local hydrology. The study findings suggest that speleothem-specific and temporally varied DCF should be considered in paleoclimate reconstructions using speleothems.