日本琵琶湖火山灰之研究

Abstract

琵琶湖地處日本本州的中央，長期地質構造穩定，是研究日本火山灰沉積最適合的陸上場所之一。1982~1983年間，京都大學研究團隊於湖泊中心偏南處 (35°13'06'N, 136°00'49'E)鑽取一口1422公尺的岩芯。本實驗選取此岩芯上部640公尺以上的21層火山灰進行研究，希望藉由分析火山灰的地球化學特性，來追蹤岩芯中火山灰來源。同時從火山灰的分佈及顆粒大小，來探討古環境的改變。 利用火山玻璃中地球化學組成的特性，配合火山灰層中礦物組成及玻璃折射率等分析，與陸源資料對比發現本研究的21層火山灰層中，有3層來自本州西南地區、5層來自九州中部地區、8層來自九州南部地區，其中有1層來自日本海的Ulreungdo，值得一提的是另外3層來自琵琶湖東面的本州中部地區。唯編號為Layer-7的火山灰並未找出較符合的來源區可作對比。且所有火山灰層中，共有13層火山灰，可詳細對比出噴發事件。由於噴發事件的時間可提供本岩芯的年代控制點，因此可估計岩芯上部237公尺以上的沉積速率，其介於0.16至1.24 mm/yr之間。 而從火山灰粒徑分佈分析資料顯示：(一)當火山噴發量越大，岩芯中火山灰顆粒粒徑越大；(二)噴發距岩芯位置越近，岩芯中火山灰粒徑也越大；(三)岩芯中同來源火山灰粒徑大小分佈與冰期及間冰期的風速改變無明顯關係。 由結果再次確認，火山灰在西日本廣域分佈，本研究也對於日本海陸地區，建立更可靠的火山灰年代學，進而提供日本西南地區於第四紀研究的指標層。

Located in the central of Honshu, Lake Biwa is one of the most important mines for the volcanic ash study in Japan area. In 1982-1983, a 1422 m length core was raised from southern basin of the lake (35°13'06'N, 136°00'49'E) by the research team of Kyoto University. The 21 tephra layers with more than 1 cm thickness in the upper 640 m of this core were found. By analyzing the geochemistry characteristics and refractive index of glass shards and mineral assemblages of tephra deposits, we expected to trace and identify the sources of each layer of tephra in the core and furthermore discuss the paleo-environment changes in southwestern Japan through the results of the grain size distributions in the ash deposits. Comparison of the results of this study with previous reports inland, the sources of tephra layers in Lake Biwa were obviously traced back in the down wind direction, three layers from the Daisen and Sanbe caldera of southwestern of Honshu, five layers from central of Kyushu, eight layers from southern of Kyushu and one layer from Ulreungdo island in Japan sea, respectively. Another notable result is that there happened three layers from eastern side of Lake Biwa, central of Honshu in the upper wind direction. All sources of the tephra were identified except the tephra layer labeled “Layer-7”. During the confident correlation between tephra layers in the core and source eruption events, there were only 13 layers of tephra could be certainly identified the eruption events, and consequently, the dating of these layers of tephra were obtained and the sedimentation rate of the upper 237m of the core was then estimated to be between 0.16 and 1.24 mm/yr. Other several important phenomenons observed in this research, they are summarized as following. First, in the larger eruption volume, the bigger the particle sizes are. Second, in the shorter transporting distance, the bigger the particle sizes are. Third, the particle size distributions show no obvious correlation with the changes of wind speed estimated from the glacial and interglacial periods in Japan area. In generally, this research offered the conceivable geochemical and isotopic data of each tephra layer in Lake Biwa. Therefore, the more reliable chronology and correlation of the volcanic ashes can be successful as the index beds of Quaternary researches in southwestern Japan.