AMS 14C dating of lacustrine and pyroclastic deposits in summit crater of Nantai volcano, NE Japan: Evidence of Holocene eruption

摘要

Nantai volcano, northeast Japan, is a stratovolcano whose recent eruptive activity is poorly known. This paper presents the characteristics and ages of a newly discovered Holocene succession of volcanic and sedimentary units exposed at the northern sector of the present summit crater of Nantai volcano. The stratigraphic relationships and three accelerator mass spectrometry (AMS) ~(14)C ages for eruptive products and lacustrine deposits of the northern sector of the summit crater show that Nantai volcano was active after the 15-14 cal ka BP eruption, which was thought to be the latest eruption of the volcano. After the 15-14 cal ka BP eruption, the summit crater was filled by a lake in which a water-chilled volcanic and tuff breccias (Goshinbutsunagi Volcanic Breccia) and the overlying lacustrine deposit (Nantai Lacustrine Deposit 1: NLD1) were emplaced. The ~(14)C age of ～ 8 cal ka BP obtained from a wood fragment in the bottom of NLD1 defines the initiation of the deposition of NLD1 in the crater lake. The crater lake dried up before 7 cal ka BP, following which a phreatomagmatic eruption occurred in the summit cater, forming Nantai-Yudonoyama Tephra (NYT). NYT is the latest recognized eruption product of Nantai volcano, and it underlies the second lacustrine deposit (Nantai Lacustrine Deposit 2) formed in the renewed crater lake. The young age of 7 cal ka BP obtained from the two charred wood stumps in NYT highlights that Nantai is an active volcano.