西太平洋鄂霍次克海1.8百萬年來之環境變遷

Abstract

鄂霍次克海位於太平洋的西北方及亞洲的東北方，是世界上最大的邊緣海，具有季節性海冰變化的特徵。MD012414岩芯採樣位置位於鄂霍次克海的中央，是目前本海域的紀錄中最長的一根岩芯。據古地磁定年的結果，本岩芯可記錄鄂霍次克海中央過去約一百七十七萬年來的環境變遷。本研究利用礦物組成分析、主要化學元素分析、海冰漂礫物質與沉積物粒徑組成等方法對岩芯沉積物進行分析，以探討過去一百七十七萬年來，鄂霍次克海的環境變遷。 本岩芯沉積物的主要礦物組成大致可區分成三種：陸源碎屑性礦物以石英及長石礦物為主；生物源礦物即方解石與蛋白石；和沉積環境氧化還原指示礦物，如白雲石等。其中，生物源礦物含量變化，可以將整根岩芯的記錄分成四大時期：第一時期，約一百七十七萬年前到一百二十五萬年前，蛋白石含量峰值出現頻率為整根岩芯中最高的時期，可表示鄂霍次克海表層有較高的生產力；第二時期，約一百二十五萬到八十萬年前，生物源沉積物的含量最少，沉積物以陸源礦物為主且IRD含量略高，是生物記錄稀少的時期；第三時期，四十八萬年到八十萬年前左右，生物源礦物的組成幾乎只有方解石，可反應營養鹽組成變化，而造成鈣質殼體生物成為優勢；第四時期，約四十八萬年以來的沉積物記錄大致可以與晚第四紀全球氣候十萬年週期的變遷相呼應。 元素分析的結果中，SiO2/Al2O3及CaO/Al2O3與蛋白石及方解石變化相呼應，可看出相似的趨勢；TiO2/Al2O3、Al2O3/Al2O3、Fe2O3/Al2O3和K2O/Al2O3等比值則與陸源碎屑物質的供應有關；而晚第四紀的沉積物記錄中，間冰期初期，P2O5/Al2O3與MnO/Al2O3比值都會有突然增加的情形，反映因沉積速率突增而造成沉積物表層或次表層中的氧被耗盡而形成還原環境的情形。海冰漂礫物質的分析結果顯示，八十萬至八十五萬年前左右，鄂霍次克海的沉積環境發生明顯的轉變，可能因為海域東部的堪察加半島上山嶽冰川擴張的結果所致，此記錄可能與全球水氣循環的平衡有關。另外，岩芯沉積物中細顆粒陸源物質的粒徑變化與礦物及主要元素等結果的變化沒有明顯的相關性，可能因為沉積作用中另有海冰的參與，而增加其複雜性，令傳統海洋岩芯沉積物粒徑分析的參數在此並不適用。

The Okhotsk Sea located in the northwestern Pacific and also the northeastern Asia is the largest marginal sea in the world and is characterized by seasonal sea-ice. Core MD012414 is collected in the central of the Okhotsk Sea and is the longest sedimentary core in the area. According to the paleomagnetic stratigraphy, the core recorded the environmental changes in the past 1.77 Myrs. The proxies including mineral assemblages, major elements, sea-ice rafted materials and grain size analyses of the core sediments were utilized to discuss the environmental changes. Three mineral assemblages of core sediments can be distinguished and described as followed; the first, terrigenous minerals including quartz and plagioclase; the second, minerals of biogenous shells containing calcite and opal; and the last, the dolomite peaked out in early of interglacial periods of late Quaternary and related to the oxic-reduced condition. Based on the variations of biogenous minerals, the whole core records can be distinguished into four periods. The first period, from 1.77 Ma to 1.25 Ma, the Okhotsk Sea was in a circumstance of high siliceous productivity. The second period, from 1.25 Ma to 0.8 Ma, showed low biogenous mineral contents and represented a severe environment. The third period, from 0.8 Ma to 0.48 Ma, calcite was the major biogenous minerals instead of opal. It implied a different nutrient condition. The fourth period characterized by five 100 kyrs cycling were corresponded to the late Quarternary glacial-interglacial cycles. As the results of major elements, the variations of SiO2/Al2O3 and CaO/Al2O3 were related to opal and calcite, respectively; the ratios of TiO2/Al2O3、Al2O3/Al2O3、Fe2O3/Al2O3 and K2O/Al2O3 were associated with detridus materials, and the ratios of P2O5/Al2O3 and MnO/Al2O3, increased suddenly in the early of deglacial periods of late Quaternary were sensitive to the oxic-reduced conditions. The record of sea-ice rafted materials suggests a change of the sedimentation environment occurred in 0.8 Ma. It may be correlated to the extensive mountain glaciers of Kamchatka when the global atmospheric circulation was in a different condition. The results of grain size analyses of the core sediments showed no close relationships with the variations of mineral and major elements. It infers that the sea-ice rafted materials disturbed the grain size distributions.