南海岩芯MD012396之磁學研究：46萬年來南海的環境變遷

Abstract

本研究分析南海北部岩芯MD012396之古地磁學與環境磁學訊息，目的在找出南海的古地磁場變化，並重建該地區的古環境與古氣候變化。 利用NRM/ARM模擬的古地磁場強度和Sint-800比較，輔以14C定年、氧同位素變化、Globigerinoides ruber ( pink )與Pseudoemiliania lacunosa生物末現面得到年代控制點，並建立本岩芯的年代模式，估計整根岩芯的年代約涵蓋過去46萬年，由此獲得古地磁與磁學參數隨時間變化的資訊。 古地磁學方面，本岩芯記錄過去46萬年來南海的古地磁場的長期變化，包括10次的極性異常事件，其年代和前人的報導相當一致。 環境磁學的部分，S-ratio都在0.93以上，表示磁性礦物絕大部分為低矯頑力的磁鐵礦。整體而言，反映磁性礦物含量的三個參數(磁感率、ARM、SIRM)均呈現間冰期較高(含量較多)、冰期較低(含量較少)的情形，顯示磁性礦物含量的變化，明顯反映南海的古環境(如海水面升降、雨量多寡、冬夏季風強弱)在間冰期與冰期有顯著的不同。除此之外，磁學參數在42.5萬年前和36.5萬年前有明顯的轉變，在這之間呈現漸變的過程，是氣候的轉變期。MIS 8早期的磁性礦物量偏多，同時氧同位素值偏負，表示環境較暖濕，是一個較溫暖的冰期，或與前人報導此時夏季季風較一般冰期強有關( Wei et al., 2003 )。磁學參數在MIS 7.4(約21.5~23萬年前)出現劇烈的變化：磁性礦物含量減少、氧化程度增高(S-ratio降低、HIRM增加)、ARM/驟降，呈現類似冰期的特徵，應是間冰期中的一個強烈變冷事件。本岩芯有7次磁性參數異常峰值的訊號，在時間上和南海地區岩芯MD972142(蔡榮浩，2000)的火山灰層有良好的對應關係，推斷是火山活動的紀錄。

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.