Magnetic Study of Core MD012396 from the South China Sea - Environmental Changes of the South China Sea Since 460 ka
Date Issued
2006
Date
2006
Author(s)
Chen, Kuo-Feng
DOI
zh-TW
Abstract
Paleomagnetic and environmental magnetic methods were used to study an IMAGES core, MD012396, raised from the northern shelf of the South China Sea, in order to find out the changes of the paleomagnetic field, the paleoenvironment and the paleoclimate of this area.
Natural remanent magnetization vs anhysteresis remanent magnetization (NRM/ARM) after 20 mT alternating field demagnetization was used to simulate the paleo-intensity variation of the earth magnetic field. In addition to the 14C dating, the oxygen isotope stratigraphy, the last-appearance-datum (LAD) of Globigerinoides ruber (pink) and the LAD of Pseudoemiliania lacunosa, the obtained paleo-intensity pattern was compared to the Sint-800 curve compiled by Guyodo and Valet (1999) in order to establish the age model of this core. 10 magnetic polarity reversal events and magnetic excursions have been delimited. The results pointed out that core MD012396 could provide the information for the last 460 ka.
The parameter S-ratio has the value larger than 0.93 in general. This suggests that magnetic minerals contained in the sediments of this core dominate the low coercivity magnetite. Generally, relative higher values of some magnetic proxies, such as magnetic susceptibility (χ), saturated isothermal remanent magnetization (SIRM) and ARM, have been found during the interglacial periods than those during the glacial times since 460 ka. This implies that more abundant magnetic minerals were deposited during interglacial times than during glacial times, and shows that the environment was different during glacial periods and during interglacial periods. Besides, magnetic proxies show clear changes happened at about 425 ka and 365 ka. In between, gradual shifting has been observed. It may indicate a climate transition at this period. In the early Marine oxygen Isotope Stage 8 (MIS 8), magnetic proxies and δ18O suggest that the climate was warmer and wetter than the other glacial times. In addition, magnetic proxies show a phenomenon that low-abundance, high-oxide magnetic minerals appeared at MIS 7.4 (215~230 ka). Such behavior looks just as at a glacial period. Thus, it is considered to be a severe cooling event during this stage. Furthermore, there were 7 peak values observed from the patterns of the magnetic proxies, which might reflect the volcanic activities in the surrounding area.
Subjects
環境磁學
古海洋學
南海
environmental magnetism
paleoceanography
the South China Sea
Type
thesis
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