[H-QR04] Deep time perspective on the geological response to climate change

[HQR04-P08] Identification of tephra in sediment cores in the Oki Islands, northwestern Japan, and evaluation of radiocarbon marine reservoir effects

Toshimichi Nakanishi², *Mitsuru Okuno¹ (1.Osaka Metropolitan University, 2.Museum of Natural and Environmental History, Shizuoka)

Keywords:Sedimentary facies, radiocarbon age, tephra, marine reservoir effect

Sedimentary facies and radiocarbon ages of a sediment core were determined to detect tephras in the Oki Islands, northwestern Japan. Based on these analyses, artificial soil, floodplain deposit, deltaic deposit, estuary deposit, and weathered basalt were identified from top to bottom. A tephra layer was recognized at the upper part of the deltaic sediment. This tephra consists of alkali feldspar, few amphibole, few volcanic glass, and few biotite. This mineral composition was similar to the tephras obtained from the Ulleung volcano, approximately 100 km east of the coast of the Korean Peninsula, such as U-2, U-3, and U-4 tephras. This tephra would be correlated with the U-1 reported on Ulleung Island and Lake Suigetsu based on the mineral composition and the sedimentary age. The conflicting results of depositional age and brown glass shard between ours and the previous report from the Lake Suigetsu imply several late Holocene eruptions of the Ulleung volcano. Radiocarbon marine reservoir effects, 360±270 years, were also measured from six age pairs of terrestrial plants and marine shells at the same horizons of shallow marine deposits during 1,700 to 7,800 cal BP. These results were similar to previously determined values from Hakata Bay, approximately 370 km southwest of the study area, and the Korean Peninsula.