鄂霍次克海岩心MD012414之磁學研究—180萬年來東北亞古氣候及古環境變遷

Abstract

本研究對2001年IMAGE VII航次採自鄂霍次克海中部Deyugin Basin中心編號MD012414海洋岩心進行古地磁及磁學參數分析，岩心點址經緯度為(149°34.80’E, 53°11.77’N)，水深1123公尺。 古地磁學研究分析岩心中沈積物之自然殘磁方向及強度，主要目的為建立該岩心之古地磁地層及古地磁場長期變化型態，提供岩心對比及年代控制機制。而磁學性質之研究則包含磁感率、逆磁滯殘磁及等溫殘磁等，這些磁學參數主要探討沈積物中磁性礦物種類、粒度及含量變化，進而探討研究地點古環境、古氣候變遷模式。 自然殘磁之結果分析指出本岩心總長度53.88公尺涵蓋了松山(Mayuyama)反向世代上半部與布倫(Brunhes)正向世代，可提供本研究區域近180萬年來更新世 (Pleistocene) 相當完整的資料。記錄中出現多次地磁場反轉紀錄，分別為Upper Olduvai Event、Cobb Mountain Event Jaramillo Event、及Mayuyama – Brunhes Boundary，其年代各為177萬年、124~121萬年、107~99萬年、及78萬年前。由年代模式，計算沈積物平均沈積速率約3 cm/kyr。記錄中亦出現多次古地磁場極性異常事件(Excursion)。 磁感率隨深度變化結果與沈積物顆粒大小及氧同位素分析初步結果相比較(李孟陽等,未發表資料)，我們發現磁感率高值區出現在粗顆粒百分比高之沈積物區，同為冰期時之產物，由於磁感率高低反應磁性礦物含量相對之多寡，因此推論冰期時磁性礦物含量較高，而間冰期時則下降。樣品所得逆磁滯殘磁除以磁感率之結果，可去掉磁性礦物量變化影響，明顯反映出磁性礦物粒度大小變化，本岩心分析之結果顯示磁性礦物之粒度與一般沈積物顆粒粒度之變化相當一致，顯現冰期時沈積物中磁性礦物顆粒粒度較粗，間冰期時磁性礦物顆粒較細的特性。從這些結果與前人之研究，我們認為冰期時鄂霍次克海沈積物可能主要來自東邊的勘察加半島(Kamchatka Peninsula)火山地區，由冰川刮蝕帶來之沈積物中具有含量較高顆粒較粗之磁性礦物，而在間冰期時，則可能由周遭帶來較多海洋及陸源沈積物，其中磁性礦物較少顆粒較細。 等溫殘磁測量結果中，出現多處加磁變化異常區域，當弱磁場(50~75 mT)加磁時等溫殘磁值下降，加磁至強磁場時才增加，此現象有可能是磁學特性特殊之礦物，如菱鐵礦(Siderite, FeCO3)所造成。由等溫殘磁所得結果與S-ratio及剛性等溫殘磁結果指出，沈積物中主要磁性礦物為磁鐵礦；排除等溫殘磁加磁變化異常區域，我們在岩心深度9.20~14.98公尺、39.06~39.76公尺及40.67~41.88公尺處發現具有較高含量的赤鐵礦或褐鐵礦，初步推論這些磁性礦物的來源可能與風積物有關。其可能機制為：風成黃土在乾冷的冰期形成，當蒙古及西伯利亞地區高壓產生時，吹起近地表強烈乾冷的風，將泥沙從黃土高原吹送上平流層，經由西風吹送，最後落入鄂霍次克海及附近區域沈積。 本岩心中也發現數層火山灰，其中3.35公尺處之火山灰層應相當於Gorbarenko(2002)報導之K2火山灰層，可能為在26 ka時位於Onecotan島之火山Nemo – III 所噴發。This study presents the magnetic results of core MD012414 taken from Deyugin Basin in the middle part of the Okhotsk Sea during IMAGES VII cruise in 2001. The locality of the coring site is 149°34.80’E in longitude and 53°11.77’N in latitude, and the water depth is about 1123 m. The total recovered length of this core is 53.88 m. Paleomagnetic results indicate that the boundary of Brunches and Matuyama epochs appeared at the depth of 28.77 m having the age of 0.78 Ma. Also the Jaramillo event, the Cobb Mountain event and the upper limit of the Olduvai event could be recognized at the depths between 35.8 m and 38.1 m, between 41.76 m and 42.15 m, and 53.8 m. The age intervals of these events are 0.99~1.07 Ma, 1.21~1.24 Ma, and 1.77 Ma, respectively. The average sedimentation rate is estimated to be about 3 cm/kyr. In addition, several events and excursions both in the Brunhes and Matuyama epochs could be identified. Results of magnetic susceptibility (c) point out that high values are usually observed at the glacial periods and low values at the interglacial periods. In consideration of the environment of the site studied, it is proposed that much more magnetic minerals were brought from the Kamchatka peninsula located at the east into the coring site during the glacial times by ice river, and more terrigenous sediments with less magnetic mineral content came from the Asia continent during the interglacial times. The acquisition pattern of the IRM shows that most samples acquired saturated IRM before 300 mT treatment, which reveals that magnetite is the major magnetic mineral contained in most of the samples. However, significant amount of hematite and goethite could be found in there parts, such as the depths of about 9.20~14.98 m、39.06~39.76 m and 40.67~41.88 m. These data might support the evidence of existing the deposition of Aeolian dust in Okhotsk Sea at those periods. In addition, several short portions of the core studied show an abnormal acquisition pattern of IRM: dramatically intensity drops down occurred at relative low applied fields (of about 50-75 mT), then slightly increases at different high level, but never reached the saturation. This phenomenon might suggest the existence of siderite or other magnetic minerals at these parts. Besides, the parameters ARM/c and SIRM/c all show that low values appear at the glacial time and high values at the interglacial time. Generally, low value represents the dominance of coarse magnetic grain in the sample. Thus, it could be concluded that coarse grain magnetic mineral deposited during glacial time and relative fine grain magnetic mineral deposited during interglacial time. This is consistent with the observation of the distribution of the coarse-grained fraction in the samples made by Lee (unpublished data). Some ash layers were found in this core. One of them appeared at the depth of 3.35 m could be correlated to the K2 ash layer reported by Gorbarenko (2002) due to the eruption of the Nemo – III volcano of the Onecotan island in 26 ka.