| 摘要: | 隨著岩芯鑽取技術的進步和越來越多深海岩芯的研究，低緯度赤道地區，已普遍被認為是控制全球氣候變遷的重要一環。其中較重要的影響因子包括：西太平洋暖池（Western Pacific Warm Pool）和間熱帶幅合帶（Inter-Tropical Convergence Zone）等。本研究係利用位於赤道西太平洋地區的邊緣海，所鑽取的三根深海岩芯來進行磁學參數的研究和小波頻譜（wavelet spectrum）的分析。這三根岩芯包括位於班達海（Banda Sea）的岩芯MD012380，年代大約包含過去82萬年；位於南中國海（South China Sea）北部的岩芯MD012396，年代大約包含過去46萬年；以及位於珊瑚海（Coral Sea）北部的岩芯MD052928，年代大約包含過去38萬年。主要分析的磁學參數包括磁感率（magnetic susceptibility: ?）、飽和等溫殘磁（IRM: isothermal remanent magnetization）、逆磁滯殘磁（ARM: anhysteretic remanent magnetization）和磁感率的比值（ARM/??），以及S比值（S-ratio）。藉由比較磁學參數和氧同位素（?18O）之間變化的異同，來探討冰期和間冰期的氣候變化，對岩芯中磁性物質所造成的影響。 冰河時期在班達海和南中國海北部這兩個區域，由於受到全球海水面下降、陸地相對上升的影響，導致眾多洋流流動的通道被封閉，流動的強度因而降低。因此洋流所能攜帶至岩芯站址附近沉積的磁性物質，其總量會減少且顆粒較粗。而來自陸源（陸地或大陸棚）氧化程度較高的磁性物質也會相對較多。在間冰期則相反，磁性物質之總量會較多，顆粒度較細且受到氧化程度亦較低。而在珊瑚海北部，雖然全球海水面的升降同樣主控此區域磁性物質的改變，但是區域性降雨量的變化似乎也造成影響。在間冰期時，岩芯站址附近的沉積物，主要來自新幾內亞（New Guinea）中部地區。這些沉積物是由數條主要河川帶進珊瑚海北部，再由沿岸的洋流（Hiri Current）帶到岩芯站址附近沉積。由於此較遠的搬運過程，磁性物質的總量變得較少且粒度較細。而在海水面下降的冰期，沿岸洋流（Hiri Current）水道可能改變（往海洋方向後退），因此由新幾內亞中部地區帶來的磁性物質的總量變少。而此時來自新幾內亞東南端，受到冰川侵蝕作用所帶來的粗顆粒磁性礦物總量則相對增加。 此外，藉由小波頻譜對這些參數的分析，得知對磁性物質的總量及顆粒度變化較敏感的參數（?、SIRM、ARM/??），記錄著很強的米蘭科維奇（Milankovitch）之週期，特別是10萬年的偏心率（eccentricity）週期，同時此週期也和冰期、間冰期的循環週期有關。而反應磁性物質種類改變的參數（S-ratio），除了10萬年的週期之外，也很明顯受到地球自轉軸傾角變化（obliquity，週期4萬年）和歲差（precession，週期2萬年）的影響。由此結果同樣可推測，在晚第四紀的西太平洋地區，這些岩芯中沉積的磁性物質之變化，主要受控於全球海水面的升降，因此有很強烈的10萬年之週期。此外區域性的雨量改變也可能同樣影響著磁性物質的變化，且可能受到地軸傾角變化和歲差的控制，因此在岩芯中記錄著4萬年和2萬年的週期。With increased evidence from marine sediment cores, the tropical ocean has been considered as a key role of modulating global climate change on orbital/suborbital time scales. In this study, oxygen isotopic and magnetic data from three IMAGES cores are presented. These cores includes MD012380 (covering an age period of 820 ka) from the Banda Sea, MD012396 (covering an age period of 460 ka) from the southern South China Sea (SCS) and MD052928 (covering an age period of 380 ka) from the northern Coral Sea. Four high-resolution magnetic records are presented, including magnetic susceptibility, SIRM, ARM/??and S-ratio. These parameters are compared with the ?18O to verify the relationship between the magnetic records and marine isotopic stratigraphy (MIS) variation. Both in the Banda Sea and northern SCS, the sea-water currents (such as the Indonesian Throughflow) intensity might decrease due to global sea-level reduction in glacial periods. Magnetic minerals carried by the currents therefore became fewer, coarser and more oxidative. On the contrary, there should be more, finer and less oxidative magnetic minerals due to opposite environmental conditions in interglacial periods. In the northern Coral Sea, although magnetic records are also dominated by global sea-level change, the amount of regional precipitation may significantly influence their variation. In interglacial periods, the sediments, sourcing from central New Guinea, are brought by the Hiri Current and then deposited around the northern Coral Sea. In glacial periods, because of reduction in sea-level, route of the Hiri Current might be back seaward. Sediments source therefore shifted to southeastern PNG due to this change. Furthermore, wavelet spectrum is computed to analyze the dominant periods embedded in these time-series data. Both characteristics of the magnetic abundance and magnetic grain size vary simultaneously with MIS change and present the Milankovitch periods, especially the 100-ka period (glacial-interglacial cycle). However, variation of the magnetic mineralogy shows good relationship to the solar insolation variation with periods of 40-ka and 20-ka in addition to a 100-ka period. These characteristics indicate that the magnetic records are dominated by sea-water currents related to global sea-level change and may also influenced by the amount of regional precipitation related to the solar insolation in the western Pacific for the late Quaternary. |
