Varved Lacustrine Sediments in Japan : Recent Progress.

Abstract

Annually laminated (varved) lacustrine sediments provide a potentially continuous , high-resolution records of the last glacial and Holocene paleoenvironments. In 1991-1993, long sequences of laminated lacustrine sediments were successfully taken from two lakes in southwestern Japan, Lake Suigetsu and LakeTougouike. Based on lamina counting and AMS 14C dating, we clarified that these laminated sediments were varves. Clay mineral composition in these varved sediments would be reflected by eolian dust concentrations, transported from Chinese Loess Plateau, and by precipitations around both lakes. Also, formation of iron sulfides and carbonates in varves would be caused by cyclic changes of seawater invasion, originated from sea-level changes. Annual to decadal oscillations of dust and iron mineral concentrations since the last glacial were detected in varved sequences of both lake sediments. These detailed sedimentological analysis of these sediments revealed varve chronology , process of varve formation and annual to decadal changes of sea-level and climate. Some of the climatic changes may correlate with abrupt changes (Younger Dryas and Heinrich events) observed in Greenland ice cores and marine sediments.