中国科学院海洋研究所机构知识库

西北太平洋位于西风和东亚冬季风的下风向，是亚洲风尘的重要“汇”，其深海沉积物中的风尘组分在构造时间尺度上记录了东亚古气候的变化。奄美三角盆地是西北太平洋西北部的一个深水盆地，“国际大洋发现计划”（IODP）351航次在此处获得了长时间尺度连续的沉积岩芯（U1438孔），为中新世以来东亚古气候变化的研究提供了极好的机遇。

石英是沉积物的重要组成部分，其物理和化学性质相对稳定，能够作为指示沉积物源区的可靠指标，并可以用来指示气候变化。石英在U1438孔中普遍存在，为识别石英的来源，作者首先在区域上对菲律宾海的石英来源进行了研究。在西菲律宾海吕宋岛附近、本哈姆隆起和深水盆地，以及东菲律宾海的九州-帕劳海脊附近和帕里西-维拉海盆选取代表性的表层和次表层沉积物样品，通过一系列物理和化学方法分离出了其中的石英单矿物，并对石英的粒度、形貌特征、δ¹⁸O值和结晶度指数进行分析，结果表明，菲律宾海沉积物中的石英主要由三个端元组成，EM1端元（众数粒径平均2.9 μm）、EM2（众数粒径平均8.2 μm）和EM3端元（众数粒径平均为21.7 μm）。EM1端元石英颗粒具有典型的风尘石英的形貌特征，结合石英的δ¹⁸O值和结晶度指数，EM1端元石英主要来自于塔克拉玛干沙漠。EM2端元的石英呈棱角状，表面有溶蚀痕迹，EM3端元呈棱角状，表面痕迹较少，结合石英的δ¹⁸O值，EM2和EM3端元石英来自于火山岛弧。因此菲律宾海中EM1端元的石英可用于东亚古气候的研究。

对U1438孔中石英的粒度、形貌特征、δ¹⁸O值和结晶度指数进行分析结果表明，石英的粒度主要呈双峰态、三峰态或四峰态的分布特征，用Weibull分布可分离出不同的粒度端元：EM1端元众数粒径约为3.11 μm，EM2端元众数粒径约为9.15 μm，EM1和EM2端元的石英呈次棱角状/次圆状，表面有碰撞凹坑和突起，是典型的风尘石英的特征；EM1和EM2端元石英δ¹⁸O值高，结晶度指数低，EM1端元石英的δ¹⁸O值和结晶度指数位于火山石英和塔克拉玛干沙漠的混合线上，EM2端元石英的δ¹⁸O值和结晶度指数位于火山石英和蒙古戈壁的混合线上，所以我们认为EM1端元的石英主要是由西风从塔克拉玛干沙漠搬运而来，EM2端元的石英主要由冬季风从蒙古戈壁搬运而来。EM3端元众数粒径约为27.76 μm，EM4端元众数粒径大于50 μm，EM3和EM4端元粒径分布范围较大；石英颗粒呈棱角状，有的表面痕迹较少，有的表面有溶蚀痕迹；石英的δ¹⁸O值低，结晶度指数高，与太平洋火山岛弧的类似，推测EM3和EM4端元的石英来自于火山岛弧。

U1438孔沉积物中石英的含量和通量，以及EM1和EM2端元石英的通量变化类似，可以分为四个阶段：（1）23-15 Myr，石英的含量、通量，以及EM1和EM2端元石英的通量均很低，变化不大；（2）15-7.6 Myr，从约15 Myr开始，石英的含量和通量，以及EM1和EM2端元石英通量逐渐增加；（3）7.6 Myr-3.6 Myr，各粒级石英通量明显增加，尤其是上新世以来增加更为明显；（4）3.6 Myr至今，石英的含量和各粒级石英的通量明显增加，其中2.5 Myr以及1.2 Myr时，石英以及EM1端元的石英通量增加更为明显。石英通量的变化指示了亚洲内陆自15 Myr干旱化程度的增加，尤其是上新世和第四纪以来干旱程度明显增加，北太平洋风尘通量以及黄土的堆积速率变化类似，并且与深海氧同位素记录的全球变冷趋势一致，但与青藏高原隆升不存在耦合关系，因此作者认为中新世以来亚洲内陆的干旱主要受控于全球变冷。

U1438孔EM1和EM2端元石英的众数粒度变化指示了西风和冬季风强度的变化。中新世以来西风和冬季风的强度整体呈增强趋势，其变化可分为四个阶段：23-15 Myr、15-7.6 Myr、7.6-3.6 Myr、3.6 Myr至今。7.6 Myr之前，全球变冷和青藏高原的隆升对西风和冬季风的强度影响较小，西风和冬季风没有呈现明显的变化趋势。在7.6 Myr，EM1和EM2端元石英众数粒径的增大指示了西风和冬季风的强度出现明显的增强，3.6 Myr后，EM1端元众数粒径趋于稳定，指示了西风强度没有明显的变化，EM2端元众数粒径迅速增加，指示了冬季风的迅速增强。作者认为中新世以来全球变冷和青藏高原的隆升可能是西风和冬季风增强的两个可能机制。

The northwestern Pacific is located in the downwind direction of the westerly and the East Asian winter monsoon, and it is an important ‘sink’ area of Asian dust. The aeolian component in the deep sea sediment record tectonic-scale paleoclimate change of East Asia. The Amami Sankaku Basin is an abyssal oceanic basin and located in the northwestern Pacific. International Ocean Discovery Program (IODP) Expedition 351 retrieved a long and continuous advanced piston core (Hole U1438) in the basin, which provide a good opportunity to address research on the paleoclimate change of East Asia since Miocene.

Quartz is an important component of sediment, and has relatively stable physical and chemical characteristic, so it is a reliable proxy of sediment source and paleoclimate change. Quartz is ubiquitous in hole U1438. In order to identify the source of quartz in the hole, the regional characteristics and source of quartz in the Philippine Sea are studied. The author select some typical surface and core-top sediments in sites near the Luzon Island, on the Benham Rise and deep water basin in the western Philippine Sea, and near the Kyushu Palau Ridge and in the Parece Vela Basin in the eastern Philippines Sea. The quartz in the sediments was seperated with a sequential physical and chemichal leaching procedure, and then the grain size, morphology, δ¹⁸O values and crystallinity index of quartz were analysized. The results show that the quartz in the Philippine Sea is composed by three endmembers: the endmember 1 (EM1, average mode at 2.9 μm), endmember 2 (EM2, average modeat 8.2 μm) and endmember 3 (EM3, average modeat 21.7 μm). EM1 quartz has typical aeolian quartz morphology, with higher δ¹⁸O values and lower crystallinity index, the author argue that EM1 quartz is mainly derived from the Taklamakan desert. The EM2 quartz is angular with corrosion marks on the surface, and EM3 is also angular but with little corrosion. The EM2 and EM3 quartz has lower δ¹⁸O values and higher crystallinity index.The author argue that the EM2 and EM3 quartz are from volcanic islands. So the EM1 quartz in the Philippine Sea can be used as proxy of paleoclimate in the East Asia.

The grain size, morphology, δ¹⁸O values and crystallinity index of the quartz in hole U1438 were analysed. The grain size distribution of quartz are bimodal, trimodal and four model characteristics. The Weibull distribution function is used to separate different endmember: the grain size of EM1 quartz, vary from 0.04 μm to 21.18 μm, with a mode at ~3.11 μm, the grain size of EM2 quartz range from 0.04 μm to 65.34 μm, with a mode at ~9.15 μm. The EM1 and EM2 quartz are subangular/subrounded, with collision pits and upturned plate on the surface, which is the typical characteristics of aeolian quartz. EM1 and EM2 quartz have higher δ¹⁸O values and lower crystallinity index. In the plot of δ¹⁸O values against crystallinity index of quartz, EM1 quartz is on the mixing line of volcanic island and Taklimakan Desert, while EM2 quartz is on the mixing line of volcanic island and Mongolia Gobi. Therefore, the author argue that EM1 quartz is transported from Taklimakan Desert by prevailing westerly, and EM2 quartz is transported from Mongolia Gobi by winter monsoon. The grain size distribution of EM3 and EM4 quartz are wider with mode at ~27.76 μm and >50 μm, respectively. The EM3 and EM4 quartz are angular, and with smooth surface or dissolution pits in some samples. The low δ¹⁸O values and high crystallinity index for EM3 and EM4 quartz are similar with those in northwestern Pacific islands, which indicate they are derived from the volcanic islands.

The vertical variation of aeolian quartz content, flux, and EM1 and EM2 quartz flux are similar and can be divided into four stages: (1) 23-15 Myr, they are very low and keep stable, (2) 15-7.6 Myr, they gradually increase at approximately 15 Myr, and have a peak at 10-11 Myr, (3) 7.6-3.6 Myr, significant increase occurred during this period, and peak at 5.3 Myr, (4) 3.6 Myr to present, significantly increase occurred at 2.5 Myr and 1.2 Myr. The variation of quartz flux indicate an enhanced aridificaiton in Taklimakan Desert and Mongolia Gobi since 15 Myr, especially at Pliocene and Quaternary, which is similar with the dust flux variation of North Pacific, and coupling with the global cooling trend recorded in the deep sea oxygen isotopes, but inconsistent with uplift of Tibetan Plateau. Therefore, the author argue that the aridification of East Asia continent is mainly controlled by global cooling.

The variation of mode grain size of EM1 and EM2 quartz of hole U1438 indicate the intensity variation of the westerly and winter monsoon, respectively. The westerly and winter monsoon strengthened since Miocene and can be divided into four stages: 23-15 Myr, 15-7.6 Myr, 7.6-3.6 Myr and 3.6 Myr to present. There isn’t significant variation in the intensity of westerly and winter monsoon before 7.6 Myr, indicating a weak influence of global cooling and Tibetan Plateau on the atmospheric circulation. At 7.6 Myr, the increased EM1 and EM2 quartz modal grain size indicate the strengthening of westerly wind and winter monsoon. After 3.6 Myr, the stable of EM1 quartz modal grain size indicates that the intensity of westerly wind is stable, while the rapid increase of EM2 quartz modal grain size indicates that the intensity of winter monsoon increase. The author argue that global cooling and Tibetan Plateau may be two potential mechanisms that drive the strengthening of westerly wind and winter monsoon.