5:15 PM - 6:30 PM
[MIS16-P06] Reconstruction of environmental and flooding histories since AD1250 at Lake Kortta, an annually laminated lake at the Central Finland presumed by Diatom and Chrysophyceae assemblages
Keywords:Annually laminated sediment, Finland, Climatic Change, Flooding sediments, Diatom, Chrysophyceae cists
Lake Kortta, located at the central part of Finland, is a small freshwater lake with 1000m length and 400 width. It has continued more than 3000 years for the sedimentation of laminated lake deposits. The average sedimentation rate was about 4mm/year (after AD1950) and 0.8-1.0mm/year (before AD1950). In this paper, we used the 80cm sedimentary core which was taken in 2010. The age of the base of the core was presumed as AD1250.
The annually laminations of Lake Kortta was related to melting flooding at early spring. The flooding layers were composed of whitish gray sands and were contrast to dark gray clay at other seasons. Among the fine laminated layers, we recognized the several thick whitish flooding layers, dated AD1260, AD1460, AD1780, AD1914-1918, AD1967 and AD1986 respectively. AD1260, AD1460 and AD1780 were presumed to be cold winter in North Europa by historic records, and AD1967 and AD1986 were recorded as lowering of winter temperatures by Meteorological observation in Finland. AD1914-1918 was the period of WWI, and the huge deforestation occurred.
Diatom assemblages were dominated by freshwater planktonic diatoms as Aulacoseira ambigua, A. valida, Stauroneis spp., those presumed the freshwater lake environment has continued since AD1250 to the present. In addition to those lake living diatoms, Eunotia spp., Pinnuralia spp., peat living diatoms, were observed abundantly. They presumed to be enter into the lake from the surrounding peat lands during the flooding Early Spring season.
Cists of Chrysophyceae increased at cold climatic years as AD1260, AD1460, AD1780, and AD1986. In addition to them, the ratio of Cists of Chrysophyceae increased during the Little Ice Age, about AD1780-1900. Chrysophyceae made cists when lake water conditions became unsuitable for growing, and the ratio of cists was presumed to be related to the climatic and flooding histories at the lake for these 750 years.
The annually laminations of Lake Kortta was related to melting flooding at early spring. The flooding layers were composed of whitish gray sands and were contrast to dark gray clay at other seasons. Among the fine laminated layers, we recognized the several thick whitish flooding layers, dated AD1260, AD1460, AD1780, AD1914-1918, AD1967 and AD1986 respectively. AD1260, AD1460 and AD1780 were presumed to be cold winter in North Europa by historic records, and AD1967 and AD1986 were recorded as lowering of winter temperatures by Meteorological observation in Finland. AD1914-1918 was the period of WWI, and the huge deforestation occurred.
Diatom assemblages were dominated by freshwater planktonic diatoms as Aulacoseira ambigua, A. valida, Stauroneis spp., those presumed the freshwater lake environment has continued since AD1250 to the present. In addition to those lake living diatoms, Eunotia spp., Pinnuralia spp., peat living diatoms, were observed abundantly. They presumed to be enter into the lake from the surrounding peat lands during the flooding Early Spring season.
Cists of Chrysophyceae increased at cold climatic years as AD1260, AD1460, AD1780, and AD1986. In addition to them, the ratio of Cists of Chrysophyceae increased during the Little Ice Age, about AD1780-1900. Chrysophyceae made cists when lake water conditions became unsuitable for growing, and the ratio of cists was presumed to be related to the climatic and flooding histories at the lake for these 750 years.