中国科学院海洋研究所机构知识库

   本文通过对国际大洋发现计划（IODP）369航次于东南印度洋曼达岬海盆所钻取的U1513、U1514和U1516三个岩芯进行了沉积物碎屑态堆积速率、粒度、粘土矿物、常微量元素、Nd-Hf同位素和有机地球化学分析，分别研究了晚白垩世（99–84 Ma）及早新生代（62–52 Ma）暖室期曼达岬海盆沉积演化史及其对极端气候事件的响应，在此基础上详细重建了大洋缺氧事件2（Oceanic Anoxic Event 2，OAE 2）期间碎屑沉积物源-汇过程及其对南半球高纬度地区水文循环的响应，并基于新地球化学指标（△εHf_clay）探讨了OAE 2期间大陆源区硅酸盐风化历史及其古气候指示意义。

   整体研究结果表明：晚白垩世曼达岬海盆碎屑沉积物主要来自于邻近的澳大利亚西南部大陆，而研究区周边与凯尔盖朗地幔柱活动相关的火山成因产物的贡献可忽略不计。在构造时间尺度上，在晚森诺曼期-早土伦期（~94 Ma）后，曼达岬海盆的陆源碎屑物质贡献显著减少，转变为以深海碳酸盐占主导的沉积模式，这与该阶段曼达岬海盆快速热沉降及全球海侵所导致的研究区相对海平面升高有关。在此背景下，曼达岬海盆的沉积记录揭示了在森诺曼期-土伦期界限（~94 Ma）和中森诺曼期（~96 Ma）发生的两次快速的气候波动，分别对应于OAE 2和中森诺曼期事件（MCE I），尤其以前者最具代表性。

   本文聚焦OAE 2事件，基于稀土元素和粘土粒级（< 2 μm）Nd同位素进行的物源分析结果表明：U1516站位碎屑沉积物主要来自于澳大利亚西南大陆不同地块的混合，由发育于晚白垩世的西向流域系统搬运而来。在OAE 2阶段，尤其是δ¹³C显著正偏的中期阶段，河流搬运碎屑物质输入迅速增加，且伴随着物源区的显著变化——远源地块（伊尔岗克拉通和阿尔巴尼亚弗雷泽造山带）相对于近源地块（珀斯盆地和鲁汶地块）的贡献显著增多。上述物源变化指示着当时南半球高纬度地区水文循环的加剧，可能与晚白垩世极端温室气候条件下水汽向两极传输量增加及西风带降水的纬向移动有着密切联系，由此引起了陆源区降水的增加，进而导致内陆大型流域系统的再次活跃。与此同时，大量河流物质（营养盐和淡水）的输入进一步促进了研究区硅质生物生产力的勃发及曼达岬海盆底层水缺氧条件的发展，两者的共同作用很可能是导致OAE 2期间U1516站位富有机质黑色页岩的形成的主要因素。

   基于由粘土粒级碎屑沉积物Nd-Hf同位素的解耦所建立的新地球化学指标△εHf_clay，本文重建了OAE 2期间澳大利亚西南大陆的硅酸盐风化历史。对比其他大陆边缘站位粘土粒级Nd-Hf同位素组成，研究结果表明Nd-Hf同位素的解耦不受反风化作用的显著控制。此外，U1516站位△εHf_clay的变化也不受锆石效应、物源变化和冰川风化的影响，由此认为△εHf_clay是可靠的硅酸盐风化指标。在δ¹³C显著正偏的初期，△εHf_clay出现了短暂的快速降低，这可能与水文循环加剧初期降水快速增加导致的物理剥蚀和冲刷作用有关，进而在短期内将大量未经风化或风化较弱的碎屑沉积物搬运至曼达岬海盆。在此之后，受极端温室气候条件及水文循环加剧的共同控制，陆源区硅酸盐风化作用显著增强。对比现代河流记录，推测南半球高纬度地区在OAE 2极端温室气候的峰值期可能达到了现代热带气候条件的相当水平。

    此外，本研究还基于U1514站位的沉积记录初步探讨了早新生代（62–52 Ma）曼达岬海盆的沉积演化史。物源分析表明，碎屑沉积物主要来自于鲁汶地块和珀斯盆地。粘土矿物组合及地球化学组成分析结果表明：大陆源区在中晚古新世（62–56 Ma）以季节性干湿交替的气候条件为主，且保持长期稳定；在早始新世（56–52 Ma）逐渐转变为暖湿气候条件。其中在古新世-始新世极热事件期（PETM，~55.5 Ma）记录到了一次显著的气候波动，表现为高岭石含量显著增加、大陆源区风化作用增强及陆源碎屑物质输入增多，指示南半球高纬度地区降水显著增加。此外，硅酸盐风化指标△εHf_clay与高岭石含量及碎屑态磷含量呈现出显著相关性，可能指示着△εHf_clay与土壤中富镥的磷酸盐矿物循环间的密切联系。

   本研究首次重建了晚白垩世及早新生代古气候与古环境演化在曼达岬海盆的沉积记录，并进一步探讨了南半球高纬度地区在极端气候事件OAE 2和PETM期间的沉积响应，两者均揭示了碳循环重大扰动时期大气圈、水圈及岩石圈等多个圈层之间的相互作用与密切联系，为深入理解典型暖室期及极端气候背景下地球系统的反馈及全球性和区域性响应的共性和差异提供了最新证据。

In this study, sediments from Site U1513, U1514 and U1516 during the International Ocean Discovery Program Expedition (IODP) 369 in the Mentelle basin of the southeastern Indian Ocean were analyzed for siliciclastic accumulation rate, grain size, clay mineralogy, major and trace elements, Nd-Hf isotope and organic geochemistry, in order to investigate the sedimentary evolution of the Mentelle Basin during Late Cretaceous (99–84 Ma) and early Cenozoic (62–52 Ma) and their responses to extreme climatic events. Additionally, the sediment source-to-sink processes and its implications to hydrological cycle at southern high latitudes, and the history of silicate weathering in the source regions and its paleoclimatic implications based on a novel geochemical index (△εHf_clay) during Oceanic Anoxic Event 2 (OAE 2) were investigated in detail.

   The results indicate that detrital sediments in the Mentelle Basin were mainly derived from adjacent southwest Australia during Late Cretaceous, while the contributions from volcanogenic sediments associated with the activity of the Kerguelen mantle plume were negligible. Over tectonic timescale, the contributions of terrigenous sediments transported to the Mentelle Basin decreased significantly after the late Cenomanian-early Turonian (~94 Ma), indicating a transition to a sedimentary pattern dominated by deep-sea carbonate deposits. This might result from the rapid thermal subsidence of the Mentelle Basin and relative sea-level rise associated with global transgression. In general, sedimentary records of the Mentelle Basin were punctuated by two rapid climate fluctuation events occurred during the Cenomanian-Turonian boundary (~94 Ma) and the Middle Cenomanian (~96 Ma), which correspond to OAE 2 and Middle Cenomanian Event (MCE I) respectively, the former of which is more representative.

In terms of OAE 2, provenance analysis based on rare earth element compositions and clay (< 2 μm) Nd isotope suggest that detrital sediments at Site U1516 were mainly derived from the mixing of different terranes in southwest Australia, delivered mainly by the westward river systems developed in the Late Cretaceous. During OAE 2, especially the middle phase with the most significant positive δ¹³C excursion, fluvial terrigenous input increased rapidly, accompanied by a major provenance shift, i.e., significantly increased contributions from more distant terranes (Yilgarn Craton and Albany-Fraser Orogen) relative to proximate terranes (Perth Basin and Leeuwin Block). This observation indicates the enhancement of hydrological cycle during OAE 2, which might be closely related to increased water vapor transport to the poles under the extreme greenhouse climate and the migration of rain belt of the westerlies during the Late Cretaceous, thus leading to the rejuvenation of inland large drainage systems due to increased precipitation in the source regions. Meanwhile, increased fluvial input (nutrients and fresh water) further stimulated siliceous biological productivity and the development of anoxic conditions of the bottom water, both of which are likely the main factor leading to the formation of organic-rich black shale at Site U1516 during OAE 2.

A novel geochemical proxy (△εHf_clay) based on decoupling of Hf-Nd isotopes of clay-sized siliciclastic sediments was applied in this study to reconstruct the history of silicate weathering in southwest Australia during OAE 2. By comparing to Hf-Nd isotopes of clay-sized sediments from other continental margin sites, the results indicate that the decoupling of Nd-Hf isotopes is not significantly influenced by reverse weathering. Additionally, the variation of △εHf_clay at Site U1516 is not affected by potential factors such as zircon effect, provenance change and glacial weathering. Therefore, we propose that △εHf_clay can be used as a more reliable indicator for silicate weathering. During the early stage of the most significant positive δ¹³C excursion, the abrupt decrease in △εHf_clay most likely resulted from physical erosion and massive flushing of un-weathered or weakly weathered sediments to the Mentelle Basin driven by intensive precipitation at the onset of the enhanced hydrological cycle. Following this interval, a major increase in silicate weathering intensity can be inferred from a positive shift in △εHf_clay, probably resulting from a combination of extreme greenhouse condition and enhanced hydrological cycle. A tropical-like climate condition at southern high latitudes during the warmth peak of OAE 2 can be inferred when compared with modern river clays.

In addition, the sedimentary evolution of the Mentelle Basin during the Early Cenozoic (62–52 Ma) was preliminarily investigated based on Site U1514. Provenance analysis indicates that detrital sediments of Site U1514 were mainly derived from the Leeuwin Block and Perth Basin. Clay mineral assemblage and geochemical results reveal that the source regions were dominated by climate with marked seasonality during the middle and late Paleocene (62–56 Ma) without significant fluctuations. During the early Eocene (56–52 Ma), the climate gradually became warm and humid, punctuated by a rapid climate fluctuation during Paleocene‐Eocene thermal maximum (PETM, ~55.5 Ma). PETM is characterized by a rapid increase of kaolinite, enhanced weathering in the source regions, and increased terrigenous input, indicating a significant increase in precipitation at southern high latitudes. Notably, the weathering proxy △εHf_clay shows a significant correlation with kaolinite content and detrital phosphorus concentrations, which may indicate that △εHf_clay is closely related to the cycling of Luteium-rich phosphate minerals in soil.

This study presents for the first time the sedimentary records of the Late Cretaceous and Early Cenozoic paleoclimate and paleoenvironmental evolution in the Mentelle Basin, and investigate the sedimentary responses of the southern high latitudes to the extreme climate events (OAE 2 and PETM). Our study reveals the coupling and close interaction of multiple layers (e.g., atmosphere, hydrosphere and lithosphere) during significant carbon cycle perturbations, and provides new evidence for understanding the similarities and differences between global and regional responses and the feedbacks of the Earth system under greenhouse climate conditions.