山中湖湖底堆積物に記録された過去2500年間の環境変遷史 : 花粉の運搬・堆積過程の基礎的研究とその応用  [in Japanese] Environmental history during the last 2500 years recorded in sediments of Lake Yamanaka, based on the fundamental study of transportation and sedimentation of pollen grains  [in Japanese]

花粉化石群集の古生態解析の精度を上げるために, 山中湖とその周辺環境における表層花粉の分析と, 山中湖湖底ボーリングコアの花粉および珪藻化石の分析を行った。表層花粉の分析の結果, 湖深・湖岸型(Fagus, Quercus), 湖岸型(Picea), 湖岸・山麓型(Gramineae, Cyperaceae, Artemisia), 広域型(Pinus, Cryptomeria, Abies, Ulmus-Zelkova, Tsuga, Betura, Larix)などの花粉の堆積の類型が区分された。また, 母植物の被度に対して過大評価(Pinus, Cryptomeria), および過小評価(Fagus, Quercus, Larix)されるものが識別された。ボーリングコアの珪藻分析の結果, 2500〜1850年前には湿原が, 1850年前の湖の成立から300年前まではやや低水位の湖が存在し, それ以降に水温の低下と水深の増加があり現在に至ったと推定された。ボーリングコアの花粉分析の結果, 2500〜1000年前は花粉の出現率が過大評価されるスギの優占ではなくカラマツやコナラ亜属優占の森林が, 1000〜300年前は温帯落葉広葉樹優占の森林(気候の温暖・乾燥化, 湖水位低下:「中世の温暖期」)が, 300年前以降はスギ林の拡大(気候の冷涼・湿潤化, 湖水位上昇:「小氷期」)が, それぞれ推定された。

Environmental history during the last 2500 years around Lake Yamanaka in the neighbor region of Fuji volcano, was reconstructed based on paleoecological information as fossil pollen and diatom assemblages, and stratigraphical one as tephra and radio-carbon analyses. The study was progressed by three steps as follows; 1) Transportation and sedimentation patterns on pollen grains were researched in the present environments in and around the lake, by means of comparison in the distribution of pollen grains in surface sediments with that of surrounding vegetation. 2) Sequense on sedimentary environments in the lake was reconstructed based on fossil diatom assemblages in the bore-hole core sample obtained from the central part of the lake. 3) Vegetational history around the lake was reconstructed based on fossil pollen assemblages from the bore-hole core and datings using tephrochronology and radio-carbon method. As a result, the history of Lake Yamanaka and the surrounding vegetation during the last 2500 years was evidently clarified as follows; (1) 2500-1850 years BP. when Lake Yamanaka was not formed yet, the swampy land had been expanded in the valley. (2) The lava flow, erupted from the Fuji volcano, abruptly dammed up to make Lake Yamanakaat about 1850 years BP. (3) Contrary to the dramatic formation of the lake, conifer forest composed of Cryptomeria and Larix, and temperate broad-leaved deciduous forest composed of Quercus subgenus Lepidobalanus had kept to be distributed around the lake. The vegetation suggests the climate at the time was not so different from that of the present. (4) At about 1000 years BP., the vegetation around the lake changed to be dominant in the temperate broad-leaved deciduous forest mainly composed of Quercus subgenus Lepidobalanus and decreased in Cryptomeria. And the lake level commenced to be lowered. Those suggest that the climate changed relatively drier and warmer at the time. (5) At about 300 years BP., Cryptomeria conifer forest has been growing up to be dominant again around the lake. The temperature of the lake water was relatively lowered and the lake area was expanded. Those suggest that the climate at the time became wetter and cooler, and that the peculiar climate corresponds to the "Little Ice Age".