天然の「時計」・「環境変動検出計」としての湖沼の年縞堆積物 [in Japanese] Non-Glacial Varved Lake Sediment as a Natural Timekeeper and Detector on Environmental Changes [in Japanese]
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年縞(non-glacial varve)を用いた高精度年代測定法は,第四紀の年代決定法の中で最も有力な方法の一つである.本論文では,第1にスウェーデン年縞編年学(Swedish Varve Chronology)を例として取りあげ,年縞研究の現状と問題点をレビューした.その結果,1.最新年縞年代と現在との関係,2.年縞の計数方法および,3.年縞でない葉理の付加および年縞の消失が,編年に大きな影響を与えることが指摘できた.第2に,福井県三方五湖の水月湖における数本の堆積物コアから得られた,過去16,000年間にわたる年縞堆積物を用いた編年学について詳しく述べた.この年縞堆積物では歴史記録から年代をさかのぼる計数,および広域テフラや泥流堆積層による対比や計数の検証が行われつつあり,Younger Dryas期や最終氷期極相期の編年が数年単位で明らかにできる可能性を指摘した.第3に,集水域が狭く,周辺からの粗粒砕屑物が流入しない水月湖の細粒堆積物中の鉄鉱物や粘土鉱物組成の変動が,大陸起源の風成塵変動,日本海の海水準変動および若狭湾周辺の降水量変動を反映しており,年縞計数の結果から過去8,830年間の変動を1年～数年単位で明らかにした.
Varve chronologies are regarded as one of the most accurate dating techniques for the Quaternary. In this article, I aim to review the latest investigations on the process of varve formation and to examine the accuracy of varve chronology. As a result, there are three errors on varve counting in time scale: 1) the link to the present, 2) problems of correlation among varve sequences, 3) extra and missing varve.<br>Non-glacial varved sediments during the last 16, 000 years were discovered from four cores in Lake Suigetsu, central Japan. There are marker beds of historical events such as civil engeering for drawing, eruptions, earthquakes and floods in the varved sediments. These marker beds provide isochronous horizons across all depositional environments and are very useful to link the latest age of varved sediments to the present day. In other words, the presence of these marker beds such as a known tephra within different varved sequences would provide evidence for both age and correct correlation. Also, age and duration of the Younger Dryas and the Last Glacial Maximum may be able to be determined in an annual scale by these varve correlations. Varved sediments may be of use as a timekeeper year by year.<br>In Lake Suigetsu, clay mineral composition in bottom varved sediments would be reflected by eolian dust concentrations, transported from Chinese loess plateau, and by precipitation around Lake Suigetsu. Also, formation of iron sulfides and carbonates in varves would be caused by cyclic changes of sea-water invasion, originating from sea-level changes. Annual oscillations of dust and iron mineral concentrations during the last 8, 830 years were detected in varved sequence of SGP 2. Varved sediments may be useful as detectors for annual environmental changes.
- The Quat. Res.
The Quat. Res. 34(3), 135-149, 1995-08-31
Japan Association for Quaternary Research