海洋地质与环境重点实验室知识库

本文通过对印度洋北部、孟加拉湾南部的东经九十度海岭上的ODP 758站岩芯沉积物的陆源物质堆积速率、粘土矿物、常微量元素和Sr‒Nd‒Pb同位素组成及浮游有孔虫混合种属Nd同位素开展了系统分析，重建了孟加拉湾始新世以来的物源演化史，探讨了源区喜马拉雅地区化学风化强度的变化及南亚季风演化，提出了指示喜马拉雅风化通量的新指标‒ΔɛNd，进而重建了喜马拉雅地区风化通量历史，揭示了河流演化、风化剥蚀与构造隆升和全球变冷之间的联系。

粘土矿物和Sr‒Nd同位素指示，研究区物源在晚始新世‒渐新世(37‒22 Ma)主要来自印度半岛片麻岩和滇缅马苏地块岩石风化产物，从早中新世(~22 Ma)开始，喜马拉雅陆源碎屑物质逐渐占据主导，在晚中新世(~8 Ma)以来，伊洛瓦底江和布拉马普特拉河成为了研究区最主要和稳定的物源。结合古地理重建和喜马拉雅‒青藏高原隆升历史，提出伊洛瓦底江和布拉马普特拉河可能起源于早中新世(~22 Ma)，并在晚中新世(~8 Ma)最终发育成为与现代相似的流域地貌。喜马拉雅造山带的两期构造隆升是南亚河流发育和孟加拉湾南部物源变化的主要控制因素。

常微量元素、粘土矿物以及Sr‒Pb同位素被用以探讨源区化学风化历史，发现晚中新世以来南亚地区的化学风化强度在7.8‒3.8 Ma期间较弱，在8.6‒7.8 Ma，3.8‒3.2 Ma，2.4‒1.8 Ma以及0.7 Ma以来较强。结合前人有关南亚季风强度的研究，我们认为南亚夏季风在7.8‒3.8 Ma期间明显减弱，在距今约7.8 Ma南亚气候变得相对干旱，这种气候特征维持了约4个百万年，随后在约3.8、2.4和0.7 Ma南亚夏季风增强，南亚气候变得相对湿润。

通过研究浮游有孔虫混合种属的Nd同位素比值，结合印度洋古海洋演化历史，认为印度洋北部海水来源在始新世‒渐新世由南极底层水水团主导，随后喜马拉雅风化剥蚀产物输入逐渐增强，12‒9 Ma期间来自太平洋的印尼贯穿流水团混合对印度洋北部海水组成产生短暂显著影响。基于浮游有孔虫Nd同位素组成，结合前人有关印度洋沉积岩芯碳酸盐组分Nd同位素的工作，重建了新生代喜马拉雅风化侵蚀通量的演化历史。发现新生代喜马拉雅侵蚀通量自35 Ma以来呈现逐渐增长的趋势，暗示了喜马拉雅造山带的隆升起始及对新生代全球变冷的重要贡献。

Based on the terrigenous mass accumulation rate, clay mineralogy, major and trace element geochemistry, Sr‒Nd‒Pb isotopic geochemistry and Nd isotopic composition of planctonic foraminifera mixed species, clay‒sized sediments from ODP Site 758 in the northern Indian Ocean, southern Bay of Bengal on the Ninetyeast Ridge since the late Eocene. According to these analysis methods, this study has reconstructed the evolution history of river systems in South Asia, and revealed the linkage of development of rivers, tectonic uplift activity and global climate change; It has been proposed the variation of the chemical weathering intensity in the Himalayas since the late Miocene, which has been the source region of clay‒sized sediments, combined with the evolution history of the South Asian Monsoon reconstructed by the chemical weathering proxies, revealed that the linkage between the evolution history of chemical weathering history in the Himalayas and the South Asian Monsoon; The source of seawater in the northern Indian Ocean has also been used to establish its bottom water evolution history since the late Eocene. The ΔɛNd proxy was proposed to measure the Himalayan weathering flux, and reconstruct the evolution history of silicate weathering in the Himalayas. The conclusion reveals the persistently increase of the Himalayan weathering flux during the Cenozoic.

Provenance analysis indicates that sediment source have changed from the sediments mainly originated from the weathering of gneissic rock of the stable southern Indian continent and Sibumasu rocks of the west Burma block (37‒22 Ma) to terrigenous material eroded from the eastern Himalaya with minor contributions from southern India and the Sibumasu rocks (22‒8 Ma), and finally the Irrawaddy and Brahmaputra Rivers became the main sources of the Site 758 after ~8 Ma. Considering the timing of tectonic events and climate change in South Asia, this study indicated that the two stages of tectonic uplift in the Himalayas during the early and late Miocene (~23 and 8 Ma), respectively, are the main control mechanism of drainage reorganization in South Asia and provenance change in the southern Bay of Bengal. Therefore, This study suggest that the Brahmaputra and Irrawaddy Rivers might have been initiated and fully established at that two‒stage time, respectively.

Based on the chemical weathering proxies like major and trace element geochemistry, clay mineralogy, and the Sr‒Pb isotopic compositions, the chemical weathering intensity of silicate minerals at Site 758 since the late Miocene have been suggested that it was weak during 7.8‒3.8 Ma, and strong during 8.6‒7.8 Ma, 3.8‒3.2 Ma, 2.4‒1.8 Ma and 0.7‒0 Ma. By comparing previous studies on the South Asian Monsoon intensity and the evolution history of the South Asian monsoon since the late Miocene reconstructed by the chemical weathering proxies in this study, it is suggested that the South Asian summer monsoon intensity became weak during 7.8‒3.8 Ma, and appeared the warm humid and cold dry alternate monsoon climate after ~3.8 Ma.

Nd isotopic composition of planktonic foraminiferal mixed species combined with the paleoceanic history of the Indian Ocean indicates that, the source of bottom seawater in the northern Indian Ocean was dominated by AABW water mass from Eocene to Oligocene, and then gradually mixed with the weathering input of Himalayas, resulting in the gradual increase of ΔɛNd until 12 Ma. The ITF water mass from the western Pacific Ocean mixed into Indian Ocean and had a great impact on the seawater composition for the northern Indian Ocean during 12‒9 Ma. Based on the Nd isotope composition of planktonic foraminifera, combined with other previous works on the Nd isotopic composition of the carbonate fraction of the Indian Ocean sedimentary cores, this study reconstruct the evolution history of the Himalayan weathering flux during the Cenozoic. The Himalayan weathering flux has persistently increased since 35 Ma during the Cenozoic, implying the beginning of the uplift of the Himalayan region and its significant contribution to global cooling during the Cenozoic.

摘 要 4

Abstract 6

目 录 9

第1章 引言 11

1.1 喜马拉雅-青藏高原隆升和河流演化历史 11

1.2 新生代全球大陆硅酸盐风化剥蚀历史 14

1.3 海水Nd同位素示踪印度洋古海洋演化历史 18

1.4 孟加拉湾地质地理背景 21

1.5 孟加拉湾长时间尺度沉积岩芯研究进展 22

1.6 选题依据和研究目的 25

第2章 研究材料与方法 26

2.1 研究材料 26

2.2 研究方法 29

2.2.1 粘土矿物组成分析 29

2.2.2 陆源物质堆积速率分析 31

2.2.3 常微量元素地球化学分析 31

2.2.4 Sr‒Nd‒Pb同位素地球化学分析 32

2.2.5 浮游有孔虫Nd同位素分析 33

第3章 晚始新世以来孟加拉湾物源演化及其构造气候联系 35

3.1 矿物和沉积学记录 35

3.1.1 陆源物质堆积速率 35

3.1.2 粘土矿物 35

3.1.3 Sr‒Nd‒Pb同位素 36

3.2 始新世以来孟加拉湾南部粘土粒级沉积物来源 38

3.2.1 潜在源区分析 38

3.2.2 粘土矿物约束物源 40

3.2.3 Sr‒Nd同位素约束物源..............................................................................42

3.3 始新世以来南亚古环境演化与驱动机制 47

3.4 小结 53

第4章 晚中新世以来南亚风化历史及季风演变 54

4.1 沉积物地球化学记录 54

4.1.1 常量元素 54

4.1.2 稀土元素 55

4.2 晚始新世以来南亚区硅酸盐化学风化历史及其控制因素 58

4.2.1 化学风化指标及演变 58

4.2.2 晚中新世以来南亚季风演化历史 61

4.3 小结 63

第5章 新生代喜马拉雅风化通量增长的有孔虫Nd同位素证据 65

5.1 浮游有孔虫Nd同位素及其来源 65

5.1.1 Nd同位素 65

5.1.2 海水Nd同位素变化影响因素 66

5.2 晚始新世以来印度洋北部古海洋演化及其驱动因素 68

5.3 新生代喜马拉雅风化通量演变历史重建 70

5.4 小结 76

第6章 结语与创新点 78

参考文献 80

致 谢 107

作者简历及攻读学位期间发表的学术论文与研究成果 109