西菲律宾海780ka以来气候变化的岩石磁学记录:基于地磁场相对强度指示的年龄框架

ROCK MAGNETIC RESPONSE TO CLIMATIC CHANGES IN WEST PHILIPPINE SEA FOR THE LAST 780KA:BASED ON RELATIVE PALEOINTENSITY ASSISTED CHRONOLOGY

文章对取自西菲律宾海本哈姆高原东部的两个钻孔岩芯进行了岩石磁学和古地磁研究,两个岩芯长度近4m,水深均超过5000m。在岩芯146底部出现了磁倾角倒转和相对磁偏角偏移,被认为是布容-松山极性转换(780ka);而岩芯89中没有发现磁倾角倒转和磁偏角偏移,表明其底部年龄比780ka年轻。岩石磁学结果表明沉积物的磁学特征符合磁性"均一性"条件:即主要载磁矿物为低矫顽力的磁铁矿,磁性矿物粒度在假单畴-单畴范围内,并且磁性矿物含量变化不超过10倍;据此构建了地磁场相对强度(RPI)的3个指标NRM/κ,NRM/ARM和NRM/SIRM。综合古地磁和地磁场相对强度与Sint800的对比结果,我们得到了两个岩芯的深度-年龄对比,在此基础上获得的岩石磁学记录表明,岩芯记录了Brunhes期以来的气候演化:冰期时,磁性矿物的粒度变粗,细粒磁性矿物含量降低;反之,间冰期时磁性矿物的粒度变细,细粒磁性矿物含量增加,与南海ODP1143孔记录的氧同位素具有一致的变化,这反映了间冰期时加强的化学风化对源区物质的影响而不是海面升降造成的沉积分选。而且,在间冰期时磁学参数反映的粒度大小亚峰谷值与氧同位素的亚峰谷值一一对应,表明间冰期沉积过程能反映气候的细微变化。这种磁学参数对气候的响应也出现在南海钻孔中,表明该特征至少是东亚海域沉积物的共同规律。对细粒磁性矿物比较敏感的ARM显示了与磁学粒度参数相似的变化,在间冰期为高值,冰期为低值,而对粗颗粒更敏感的κ和SIRM则缺乏这种与冰期-间冰期旋回的对应关系。与此同时,S-ratio变化不大并且缺乏与亚洲风尘记录的对应关系,表明自从780ka以来沉积区的低矫顽力磁性矿物一直是主要磁性载体,并且暗示物质来源应以来自陆地的悬浮体为主,而非风尘。研究区内自西向东降低的沉积速率表明了沉积物的主要来源是西部的吕宋岛和东亚大陆。400kaB.P.前后,磁学参数出现转折,粒度由逐渐变粗到稳定,磁性矿物含量降低以及变化幅度增加;同时,碳酸盐含量的变化也相应降低,反映了全球海区出现的碳酸盐溶解加剧即中布容事件。磁性矿物含量和粒度与冰期-间冰期的同步变化反证了地磁场相对强度指示年龄的正确性,同时岩石磁学参数对稳定氧同位素和碳酸盐含量的响应也揭示了岩石磁学方法是进行古气候和古海洋研究的有效工具。

We investigated rock magnetic and paleomagnetic properties from two deep sea sediment cores from western Philippine Sea, located at the east of the Benham Rise with lengths of 4m and water depths over 5000m. Inclination and relative declination at the end of core 146 indicate a reversal which is recognized as the Brunhes-Matuyama Polarity Boundary （BMPB）. No such a reversal occurred in core 89, impling a younger age of the bottom of core 89 than that of core 146. Rock magnetic results reveal the magnetic uniformities in mineralogy, concentration, and grain size along two cores and relative paleointensity variations are acquired. ARM, K and SIRM are normalizers to get relative paleointensities. The three normalization results are averaged to indicate paleointensity of the two cores and further stacked together to get a synthetic curve for the west Philippine Sea （ referred to as WPSS00 in this paper）. Several paleointensity lows occurred in WPSS00, among which the lowest （under 20% of maximum intensity） at 780ka just corresponds to BMPB. Based on the magnetic correlation between cores and paleointensity to SintS00, we transfer the changes of rock magnetic parameters from depth to time. Furthermore, the astronomically tuned oxygen isotope from ODP site 1143 in the northern South China Sea is used for the glacial and interglacial indicator. Three concentration proxies of ARM, K and SIRM and grain size indicators （κARM/SIRM, κSRM/κ） are examined according to the paleointensity assisted chronology. The grain size changes in the two cores display a consistent pattern with climatic changes embodied by oxygen isotope. The magnetic sizes are usually coarser in glacial periods and finer in interglacial times, which may be resulted from chemical erosion to source sediment rather than fining from sea level rising. Furthermore, the sub-peaks in interglacials almost mimic that of oxygen isotope records, which means that sedimentation can indicate the subtle changes during interglacials. This kind of magnetic response to climatic fluctuation is also found in South China Sea, which shows a common pattern of magnetic signals to climate within East Asian marine scope. The concentration of ARM （representing more about fine grain）also shows a similar reflection to glacial and interglacial cycles, that is, high in interglacial and low in glacial; but K and SIRM （reflect more about coarse grain）lack the relationship with warm-cold cycles. At the same time, S-ratio lacks the correlation with aeolian dust record and rhythmic changes, indicating that the dominant source of main magnetic carrier（low coercivity magnetite）is the suspended matter instead of dust. The decreasing trend of sedimentation rate from west to east also show that the material are mainly from west-Luzon and adjacent land. Around 400kaB. P., grain size first became coarse and then stable. At the same time all magnetic contents were lowered and amplitude of magnetic mineral changes increased. The magnetic transition around 400kaB. P. is simultaneous with the decreases of carbonate content, reflecting a global carbonate dissolution event-mid Brunhes event. The synchronization of magnetic content, size with climatic cycles of glacials and interglacials imply the validity of paleointesnity-assisted chronology. Furthermore, the response of rock magnetic signals to stable oxygen isotope changes and to carbonate variation reveals that rock magnetism method can be an efficient tool for paleoclimatic and paleoceanographic research.