Late Pleistocene buried humic soils within a tephra-soil sequence near Unzen volcano, Kyushu, Japan

Abstract

Relatively few late Pleistocene buried humic soils in volcanic areas have been studied to date, because they are not common worldwide. The present study examined the origin of soil organic matter (SOM) in mainly late Pleistocene buried humic soils within a tephra-soil sequence near Unzen volcano, northeastern Shimabara Peninsula, Kyushu, SW Japan. The buried humic soil horizons were divided into seven horizons in a tephra-soil sequence. The total carbon contents were very high for late Pleistocene soils, with a value of >60 g kg −1 obtained for the oldest buried humic soil (8Ab horizon). The main phytolith types in the buried humic soils are Pleioblastus sect. Nezasa in A and 2Ab, and Sasa sect. Crassinodi in 3Ab, 4Ab, 5Ab, 6Ab, 7Ab/C, and 8Ab. SOM was derived mainly from Gramineae. In the oldest buried humic soil (8Ab horizon), the δ 13C values indicate that C 3 plants contributed more than 50% of the plant-derived carbon in the SOM, and the phytoliths indicate the dominance of C 3 grasses (e.g., Sasa sect. Crassinodi) in the NE and SE of the peninsula. The late Pleistocene humic soils formed due to an abundant supply of organic matter from mainly grass vegetation such as Sasa sect. Crassinodi, regardless of climate change at ∼30 ka or earlier in the Unzen volcano area. The present study contributes to the paleoenvironmental reconstruction for the late Pleistocene.