藏南Cenomanian-Turonian界线大洋缺氧事件(OAE2)的高分辨率年代标尺

Oceanic Anoxic Event 2 (OAE2), which occurred at the Cenomanian-Turonian boundary (~93.5 Ma), represents an abrupt climate change in the Cretaceous greenhouse world and can provide insight into understanding rapid climate changes and associated environmental consequences under greenhouse warming conditions. Key to deciphering its initiation, development, and termination is to precisely determine its duration and rate of changes. Previous estimates of the OAE2 duration were derived mainly from highly condensed pelagic sequences and vary widely by up to a factor of 2-3. Also, durations of sub-stages of OAE2 have rarely been determined. Here we propose to carry out a high-resolution rock magnetic and carbon isotope study of the type OAE2 section in southern Tibet that contains a highly expanded OAE2 interval, indeed the thickest OAE2 interval among the records in the Atlantic-Tethy's realm. The proposed study aims to establish an orbital timescale, delineate carbon isotope stages of OAE2 by comparison with those from other OAE2 reference sections, and determine the OAE2 duration and the durations of its each sub-stage with a significantly improved precision. This study will provide a chronologic basis for delving into not only abrupt changes in the sedimentologic, biotic, and chemical processes during the OAE2, but also complicated feedbacks among these processes.In the meantime, the proposed study will investigate the possible mechanisms for mineral magnetic properties in recording environmental changes in anoxic or dysoxic conditions. This will be useful for interpreting magnetic records in deep-time research.

发生于白垩纪Cenomanian-Turonian界线（约93.5 Ma）的快速气候变化-大洋缺氧事件（OAE2）深受科学界关注，一个重要原因是其能为认识温室效应引起的全球快速气候变化及其环境效应提供重要参考。精确确定OAE2的时限及变化速率是探究其发生、发展及终结过程的关键。OAE2的时限估算多来自于沉积速率很低的深海相地层，不仅误差大（可相差2-3倍），而且很难细化OAE2不同阶段的时限。本项目选取大西洋-特提斯域OAE2沉积厚度最大的藏南定日剖面，对OAE2层位开展高分辨的岩石磁学和碳同位素地层学研究，建立一个轨道尺度的年代标尺；通过与全球典型OAE2剖面对比，厘定OAE2的不同碳同位素阶段并确定各阶段的时限，为进一步解析与OAE2相关的沉积、生物、地球化学等过程的快速变化与反馈提供时限约束。探究矿物磁性在海洋缺氧或贫氧状态下对沉积环境变化的响应机理，为解译深时岩石磁性记录提供参考。

大洋缺氧事件2 （OAE2）是发生于白垩纪中期（~93.9Ma）温室气候状态下的快速气候变化事件。因其能为认识温室效应引起的全球快速气候变化及其环境效应提供重要参考而深受科学界关注。精确确定OAE2 的时限及变化速率是探究其发生、发展及终结过程的关键。然而，现有的对OAE2时限的估算多来自沉积速率很低的深海相地层，不仅误差大，而且很难细化OAE2不同阶段的时限。. 本项目选取赛诺曼/土伦（Ｃ／Ｔ）界线附近沉积厚度很大的藏南定日贡扎剖面开展了高分辨的岩石磁学和碳同位素地层学研究。该剖面Ｃ／Ｔ段地层为泥岩、泥灰岩和灰岩，下段夹页岩。取样间隔为１０－２０厘米。取样层位涵盖了Ｃ／Ｔ界线，共７６.3米。Ｃ／Ｔ段地层的磁化率显示显著的韵律特征。频谱分析显示轨道尺度沉积旋回。据此建立了基于短偏心率周期的高分辨率年代标尺。高分辨率碳同位素结果揭示出六个碳同位素演化阶段（Ｃ１至Ｃ６）。其中Ｃ２－Ｃ５阶段详尽刻画了ＯＡＥ２事件从起始、发展到终结的演化细节。结合新建立高分辨率年代标尺，更准确地估算了Ｃ２－Ｃ５各个阶段的时限，确定ＯＡＥ２事件的持续时间为约８７０kyr。. 另外，对比磁化率与碳同位素结果发现，磁化率和碳同位素的变化均受到短偏心率周期的轨道调谐控制。二者在ＯＡＥ２事件之前以及中间的大部分时间内同步变化。但在C5阶段中期之后则呈反相位变化。这表明全球大洋在C5中期可能发生了深刻变革，使其由缺氧状态逐渐恢复到正常状态，即终结了OAE2。由于C5中期正好对应于偏心率由小变大的过程，我们推测低偏心率可能预示着海洋缺氧状态的终结。环境磁学结果表明，ＯＡＥ２事件层位内不仅存在铁硫化物， 而且存在针铁矿和赤铁矿等磁性矿物。这说明研究区在ＯＡＥ２期间为准厌氧环境，而不是缺氧环境。本项目研究提升了对ＯＡＥ２事件演化细节的认识水平，研究结果为进一步探究ＯＡＥ２事件的时空演化奠定了坚实的基础。