海洋地质与环境重点实验室知识库

本论文对国际大洋发现计划（IODP）369航次于澳大利亚西南岸外曼达岬海盆所钻取的U1514站位长岩芯进行了沉积物碎屑态组分的堆积速率、粒度、粘土矿物、常/微量元素、Sr-Nd同位素组成及全样沉积物的Hg含量和有机地球化学组成进行分析，研究了始新世（52～34 Ma）期间东南印度洋曼达岬海盆的沉积演化历史，重建了澳大利亚西南部大陆的风化剥蚀历史，并探讨了构造运动和气候变化对上述沉积物源—汇过程的影响。

研究结果表明，始新世期间曼达岬海盆的沉积物来源发生过两次重大改变：早—中始新世阶段（52～43 Ma），曼达岬海盆的碎屑沉积物主要来自澳大利亚西南部大陆上的太古代和中元古代地块，主要由形成于白垩纪的西向大型流域系统搬运而来；中始新世后期（43～38 Ma），伴随着澳大利亚与南极大陆的快速分离，研究区及其周边发生了大规模的火山活动，由此形成的火山物质及其蚀变产物混合着来自澳大利亚西南部大陆的陆源碎屑进入到研究区；晚始新世阶段（38～34 Ma），受澳大利亚西南部大陆区域性构造抬升和内陆干旱化的影响，流经伊尔岗克拉通的大型古河流系统改道和重组，从而限制了距研究区较远的陆源地块（如伊尔岗克拉通和阿尔巴尼—弗雷泽造山带）的物质输入，而相应地近源地块（珀斯盆地和露纹杂岩）的贡献则显著增加。

在此基础上，本文重建了始新世（52～34 Ma）期间澳大利亚西南部大陆构造时间尺度的风化剥蚀历史。早始新世气候适宜期（EECO）是近65 Ma以来全球气候最为温暖的时期，此时陆源区的化学风化强度为中等程度，其风化产物主要是蒙脱石，而源区较强的水文循环则将较多的陆源碎屑物质带入到研究区；EECO之后（50～43 Ma），尽管全球气候进入了逐渐变冷的阶段，但澳大利亚西南部大陆仍处于相对温暖湿润的气候条件下，陆源区的化学风化作用相对稳定，仍为中等强度。但此时研究区的碎屑物质输入通量显著增加，这源自于澳大利亚和南极大陆自43 Ma开始的快速分离，该过程伴随着的强烈火山活动向研究区输入了大量粗粒的火山碎屑物质，并造成了曼达岬海盆的构造下沉。中始新世晚期（43～38 Ma），所研究站位的高岭石含量和物质堆积速率明显增加，这指示陆源区化学风化强度和水文循环的显著增加，这可能与此时澳大利亚西南部出现的气候异常变暖密切相关。这一异常温暖时段是始新世中—晚期全球长期变冷过程中的一次非常重要的气候逆转，其持续时间（～5 Myr）远长于其他短暂的极端变暖事件（如始新世中期气候适宜期，其持续时间约500 kyr），这一区域性气候事件造成了陆源区降雨量的增加，进而导致了大量河流物质向研究区的输入。38～37 Ma期间，受晚始新世塔斯曼地区海底扩张开始加速的影响，澳大利亚西南部沿岸的海平面急剧下降，同时该地区还发生了区域性的构造抬升，导致了曼达岬海盆碳酸盐补偿深度的升高。在海平面的快速降低和区域构造抬升的双重作用下，澳大利亚西南部的部分陆架快速出露，并遭受到强烈的物理剥蚀作用，大量未经化学风化的碎屑沉积物被冲刷搬运到研究区，导致海洋表层硅质生物生产力在短时间内的快速升高。37～34 Ma期间，伴随着全球气候的变冷，澳大利亚西南部大陆逐渐干旱，陆源区化学风化强度也相应降低。

本次研究首次重建了始新世阶段东南印度洋曼达岬海盆的沉积记录，并探讨了南半球中—高纬度地区沉积物源—汇过程对构造运动和气候变化的响应，这为深入理解暖室期地球气候系统对关键气候环境事件的反馈提供了新的证据。

In this study, we present a multi-proxy record of the siliciclastic mass accumulation rate, grain size, clay mineralogy assemblages, major and trace element compositions, Sr-Nd isotopic compositions, bulk Hg contents, and organic geochemical compositions for the Eocene sediments from IODP Site U1514 drilled in the Mentelle Basin. We reconstructed the sedimentary evolution history of the Mentelle Basin in the southeastern Indian Ocean, as well as the chemical weathering and physical denudation history of the southwest Australian continent during the Eocene (52~34 Ma). Furthermore, the impacts of tectonic movements and climatic changes on the abovementioned sedimentary source-to-sink processes were also discussed.

Our results suggest that the sediment sources of the Mentelle Basin changed significantly twice during the Eocene. During the early-middle Eocene (52~43 Ma), the siliciclastic sediments in the Mentelle Basin were mainly derived from the Paleo-Mesoproterozoic terranes of the southwestern Australian continent, which were mainly transported by large westward paleodrainage systems that developed from the Cretaceous. During the late middle Eocene (43~38 Ma), sediments entering Site U1514 were the mixture of terrigenous materials derived from the southwest Australian continent and volcanic materials from the adjacent Naturaliste Plateau. The volcanic materials were likely resulted from extensive volcanic activities occurred in and around the study area, which is associated with the onset of fast separation between Australia and Antarctica at 43 Ma. During the late Eocene (38~34 Ma), major sediment provenance shifted from distal source areas (e.g., the Yilgarn Craton and the Albany‒Fraser Orogen) to relatively proximal sources (e.g., the Leeuwin Block and Perth Basin). We interpret that the regional uplift in southwestern Australia and coeval aridity of the interior continent resulted in the diversion and inactivation of large drainage systems, thus blocking the transportation of sediment from distant regions, and the contributions from proximal source areas were correspondingly increased.

Furthermore, we also reconstructed the chemical weathering and physical erosion history of the southwestern Australian continent on a tectonic time scale during the Eocene (52~34 Ma). The Early Eocene Climatic Optimum (EECO) was the warmest period of global climate since nearly 65 Ma. The chemical weathering intensity in the southwestern Australian continent was moderate at that time and the weathering products were mainly smectite. However, our result suggests that the enhanced hydrological cycle in the source regions could bring more terrigenous materials into the study area. Following the end of EECO (50~43 Ma), although the global climate encountered a long-term cooling period, the southwest Australian continent was still under relatively warm and humid conditions, therefore, the chemical weathering in the sediment source areas was relatively stable and still moderate for the formation of smectite. A tectonic subsidence of the Mentelle Basin occurred at ~43 Ma in response to the onset of separation between Australia and Antarctica at that time. In addition, increased terrigenous input from the southwest Australian continent as indicated by both increase in kaolinite content and mass accumulation rates during 43~38 Ma, suggest intensified chemical weathering and hydrological cycle in the terrestrial source areas, which may be closely related to the anomalous warming of southwestern Australia during the late middle Eocene. This anomalously warm period was a very significant warming reversal that interrupted the long-term global cooling throughout the middle to the late Eocene. The much longer duration (~5 myr) of this warming event compared to other transient hyperthermals (e.g., the MECO, which lasted for ~500 kyr) occurred during the early-middle Eocene, suggest that this event was more complex than the other hyperthermals. This regional warming event resulted in increased rainfall in southwest Australia, which further led to increased terrigenous materials transported to the study region by large paleodrainage systems. During 38~37 Ma, the onset of accelerated seafloor spreading in the Tasman region resulted in sudden sea level fell along the southwestern Australian coast. The carbonate compensation depth of the Mentelle Basin increased between 38 and 37 Ma, in response to the sea level change and the regional tectonic uplift in southwestern Australian at that time. Some parts of the continental shelf in southwest Australia were exposed to experience intense physical erosion due to the abovementioned tectonic changes. Therefore, large amounts of unweathered detritus were delivered to the study site, resulting in a rapid increase in the productivity of siliceous organisms in the ocean surface within a short period of time. As for the time interval of 37~34 Ma, the chemical weathering in the source area became relatively weaken, as the southwest Australian continent became progressively drier due to the global cooling during the late Eocene.

In this study, we presented the first detailed sedimentary record of the Mentelle Basin in the southeast Indian Ocean and discussed the sedimentary source-to-sink processes in response to tectonic movements and climatic change in the mid- to high-latitude southern hemisphere during the Eocene. This helps to provide new evidence for improving our understanding of the feedback of the Earth's climate system to key climate and environmental events during the “warmhouse” conditions.

第一章 引言 ...................................................................................... 1

1.1 选题背景和研究意义................................................................. 1

1.2 始新世中期古气候研究............................................................. 4

1.3 南半球中—高纬海区的沉积记录........................................... 10

1.4 研究内容与目的........................................................................ 13

第二章 区域地质概况 ..................................................................... 15

2.1 研究区位置及地质构造............................................................ 15

2.2 澳大利亚地质概况..................................................................... 16

2.3 研究区构造背景......................................................................... 17

2.4 区域气候特征..............................................................................19

2.5 区域洋流和水团特征..................................................................20

第三章 研究材料与方法 .................................................................. 23

3.1 研究材料...................................................................................... 23

3.2 研究方法...................................................................................... 24

3.2.1 碎屑态堆积速率...................................................................... 24

3.2.2 粒度分析................................................................................... 25

3.2.3 粘土矿物分析........................................................................... 26

3.2.4 常/微量元素分析..................................................................... 28

3.2.5 有机地化分析........................................................................... 29

3.2.6 Sr-Nd 同位素分析 ................................................................. 29

第四章 始新世曼达岬海盆沉积演化历史 ...................................... 31

4.1 年代框架........................................................................................ 31

4.2 矿物学和沉积学记录................................................................... 35

4.2.1 碎屑态堆积速率 ....................................................................... 35
4.2.2 粒度分布..................................................................................... 36

4.2.3 粘土矿物..................................................................................... 36

4.3 沉积物地球化学记录.................................................................... 37

4.3.1 沉积物有机地球化学................................................................. 37

4.3.2 常量元素...................................................................................... 38

4.3.3 微量元素与稀土元素.................................................................. 43

4.3.4 Sr-Nd 同位素组成 ..................................................................... 46

4.4 沉积物源识别.................................................................................. 47

4.4.1 潜在物源分析............................................................................... 47

4.4.2 火山活动示踪............................................................................... 50

4.4.3 Sr-Nd 同位素组成约束源区 ..................................................... 52

4.4.4 微量及稀土元素约束源区........................................................... 56

4.4.5 粘土矿物来源分析....................................................................... 60

4.5 沉积物输运及其控制机制.............................................................. 66

4.5.1 沉积通量估算................................................................................ 66

4.5.2 中始新世晚期火山物质输入：构造运动影响.......................... 69

4.5.3 晚始新世陆源碎屑物质输入：构造与气候变化影.................. 72

4.6 曼达岬海盆构造及海洋环境演化历史.......................................... 74

4.7 小结.................................................................................................... 77

第五章 始新世澳大利亚西南部陆表风化剥蚀历史 .......................... 79

5.1 风化剥蚀指标.................................................................................... 79

5.2 陆源区风化剥蚀历史........................................................................ 81

5.3 小结..................................................................................................... 87

第六章 结论 ............................................................................................. 89

参考文献 ................................................................................................... 91

附 录 ........................................................................................................ 113

致 谢 ........................................................................................................ 119

作者简历及攻读学位期间发表的学术论文与研究成果 .................. 121