Annual-Resolution Precipitation Record of Lake Suigetsu Based on Lamina Thickness and Its Chemical Composition during the Last 350 Year

Abstract:

Lake Suigetsu sediment has distinct lamination since ca. 1664 A.D. when Urami trench was cut to lower the lake level that increased because of the closure of the outlet due to the 1660 A.D. earthquake. Approximately 3 m deep Urami Trench allowed intrusion of brackish water that caused density stratification within the lake and development euxinic bottom water. This distinct parallel lamination is considered as varves, but previous “varve”counting fails to prove its annual origin. In this study, we sampled top several tens of centimeter of the Lake Suigetsu sediment using Limnos Sediment Sampler. A high-resolution age-depth model based on radioisotopes ¹³⁷Cs and ²¹⁰Pb profiles and ¹⁴C dating are compared with the age-depth model based on varve counting. The two curves agreed within the error that is less than 10 years at the bottom. Thus, the lamination is proved to be varves. This age model allows us to examine annual-resolution record of river discharge, eolian dust flux, and seismic events.

Lamination is generally from 1 to 2 mm thick, dark gray in color and rich in diatom. In addition, there are a few thicker (>2mm) lamina characterized with sharp and slightly erosional at the bottom and gradational at the top. Based on these characteristics, we call them “Event layers”.

Light gray Event layers are common in the Suigetsu sediments, and interpreted as representing flood events although supporting evidence is insufficient. We correlated them to contemporary observational precipitation record. These light gray Event layers are well correlated to the historical record of the flood disasters in Lake Suigetsu within +/_ 3 years during the past 70 years. Assuming these light gray event layers represents flood events, we fine-tuned the age-depth model and examined the correlation between precipitation record and flux of detrital materials estimated from the sedimentary record. The result will be presented and implication will be discussed.