Microscopic observations of varve sediments from Lake Ni-no-Megata and Lake San-no-Megata, Oga Peninsula, NE Japan, with reference to the fallout age of the B-Tm Tephra

Abstract

This paper presents the results of microscopic observations of thin sections of sediment cores from Lakes Ni-no-Megata and San-no-Megata, Oga Peninsula, NE Japan. Micro-sedimentary structures indicate that cycles of light-dark lamina couplets represent biogenic varves, marking annual laminations. We identified 14 complete annual cycles and 1 partial annual cycle between the thin tephra layers of Towada-a (To-a) and Baitoushan-Tomakomai (B-Tm) in sediment cores from the two lakes, suggesting that B-Tm was deposited in AD 929. We also determined that To-a was deposited during early spring, while B-Tm was deposited during one or more eruption events during spring-summer.