中国科学院海洋研究所机构知识库

    通过对东阿拉伯海IODP U1456站位上部141.5m沉积物碎屑态的堆积速率、粒度、粘土矿物组合及常微量元素组成的综合研究，并结合有孔虫 δ¹⁸O 及AMS¹⁴C等方法建立的年代框架，本文追踪了1.2 Ma以来U1456站位碎屑沉积物的来源并建立了东阿拉伯海的印度夏季风演化代用指标。通过与前人重建的季风记录对比，本文探讨了1.2 Ma以来印度夏季风的长期演化历史及其驱动机制。得到以下主要结论：

         U1456站位的粘土矿物组合主要由蒙脱石（平均59%）和伊利石（平均33%）组成，并含有少量的绿泥石（平均5%）和高岭石（平均3%）。物源分析表明蒙脱石主要来自于德干高原玄武岩的化学风化；而伊利石、绿泥石和高岭石主要来自于印度河，以西喜马拉雅山脉地区的物理剥蚀产物为主；来自于印度南部片麻岩区的贡献可以忽略。

    在千年时间尺度上，研究站位蒙脱石/（伊利石+绿泥石）和K/Al比值分别能够指示德干高原物源端元相对于印度河物源端元的贡献大小以及印度河流域的化学风化作用强度。30 ka以来两者与前人重建的印度夏季风演化大体同步，表明晚第四纪以来印度夏季风的演化很可能是影响所研究站位沉积物来源变动以及陆源区风化剥蚀作用强度的重要因素。除此以外，晚第四纪以来所研究站位的沉积记录还受到了海平面变化的控制。

    在轨道时间尺度上，研究站位蒙脱石/（伊利石+绿泥石）和α^Al_K比值与前人重建的印度夏季风的演化一致，且具有大致的冰期-间冰旋回特征。在强夏季风时期（如间冰期），陆源区化学风化作用增强，更多来自于德干高原的风化剥蚀产物进入研究区；在弱夏季风时期（如冰期），陆源区化学风化作用减弱，尤其是西喜马拉雅山脉地区的机械剥蚀作用加剧，更多的特征物理剥蚀产物（伊利石和绿泥石）进入研究区。由此，所研究站位沉积物的粘土矿物及元素地球化学组成蕴含了印度夏季风演化的信号，从而可以作为季风演化的有效指标。

    基于U1456站位所遴选出的有效季风指标，可将1.2 Ma以来印度夏季风的长期演化可以大致分为两个主要阶段：1.2-0.9 Ma阶段以高频变化为特征的印度夏季风和0.9 Ma以来以低频变化为特征的印度夏季风。在中更新世气候转型期（1.2-0.9 Ma），MIS 13期及中布容事件（~MIS 11）印度夏季风都出现了增强的趋势。频谱分析结果进一步表明，本研究所重建的印度夏季风演化历史除具较明显的岁差周期（23 kyr或19 kyr）外，在0.9 Ma前后还出现了由29 kyr或41 kyr（斜率）周期向100 kyr（偏心率）周期的转型变化，表明在轨道时间尺度上印度夏季风的演化受到了高纬过程和低纬过程的双重驱动。

   Combined with δ¹⁸O and AMS¹⁴C chronology, a multiple-record of siliciclastic mass accumulation rate, grain-size, clay mineralogy and elemental geochemistry from the upper 141.5 mof the International Ocean Discovery Program Site U1456 have been investigated to trace the provenance of detrital sediments, and to establish proxies of Indian summer monsoon intensity, thus to constrain the evolution of the Indian summer monsoon and its forcing mechanisms by comparison with other monsoon records of previous studies. The major conclusions we have drawn are summarized as followed:

    The clay mineral assemblages at Site U1456 mainly consist of smctite (with an average value of 59%) and illite (with an average value of 33%), with minor chlorite (with an average value of 5%) and kaolinite (with an average value of 3%). The provenance analysis indicates that smectite predominatly originated from the chemical weathering of basalts in the Deccan Traps, while illite, chlorite, and kaolinite were primarily derived from the Indus River, which are the products of physical erosion from the Western Himalya; the contribution from the gneissic rocks in the southern India is insignificant.

    At millennial timescales, smectite/(illite+chlorite) and K/Al ratios reflect the relative contribution of the Deccan Traps to the Indus River and the intensity of chemical weathering in drainage basin of the Indus River, respectively. Both ratios show coherent variation patterns to the pervious Indian summer monsoon records, which suggests that evolution of the Indian summer monsoonshould probably be responsible for the provenance changes at Site U1456 as well as weathering/erosion intensity in the source areas during the late Quanternary. In addition, the sea-level variations also palyed an important role in controlling the sedimentary records at Site U1456 in the late Quaternary.

   At orbital timescales, variations in paleoclimate indicators of smectite/(illite+chlorite) and α^Al_K ratios at the study site are coincident with the evolution of Indian summer monsoon intensity revealed by previous studies, characterized by general glacial-intergalcial cycles. During the period of strong summer monsoon (e.g., interglacial periods), the enhanced chemical weathering in the source areas resulted in higher input of smectite from the Deccan Traps. During the period of weakened summer monsoon (e.g., glacial periods), a weakening in chemical weathering is observed in the source areas. In particular, more siliciclastic sediments (illite and chlorite) were derived from the Western Himalya due tointensified physical erosion. Therefore, the clay mineralogy and elemental geochemistry composition at Site U1456 recorded the singals of the Indian summer monsoon variations, thus can be used as reliable proxies to indicate the evolution of summer monsoon.

    According to the selected monsoon proxies, the Indian summer monsoon evolution can be divided into two intervals since 1.2 Ma: (i) a period of high-frequency variability of Indian summer monsoon during 1.2-0.9 Ma; and (ii) a period characterized by low-frequency in the Indian summer monsoon since 0.9 Ma. A strengthened Indian summer monsoon was observed during the Mid-Pleistocene Transition (1.2-0.9 Ma), Marine Isotope Stage (MIS) 13 event and the Mid-Brunhes Event (~MIS 11). Spectral analysis results further revealed that, except for the obvious precession cycle (23 kyr or 19 kyr), a transiton from 29 kyr or 41 kyr (obliquity) periodicities to strong 100 kyr (eccentricity) periodicity was recorded in the monsoon proxies. Such results suggest that the evolution of Indian summer monsoon at orbital timescales is controlled by both high-and low-latitude processes.