Magnetic Study of Core MD012414 from Okhotsk Sea ─ Paleoclimate and Paleoenvironment Changes of Northeastern Asia Since 1.8 Ma

• Main Authors: CHOU, YU-MIN, 周祐民
• Other Authors: Lee, Teh-Quei
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/28970307074543397895

碩士 === 國立臺灣師範大學 === 地球科學研究所 === 91 === ABSTRACT 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.