晚塞诺曼期东特提斯洋古海洋环境探析

发生于白垩纪中期塞诺曼期—土伦期之交的大洋缺氧事件2（OAE2）被认为是研究大洋缺氧事件形成与其他地质过程异常的重要窗口。重建晚塞诺曼期OAE2事件发生之前的古海洋环境对于理解OAE2的成因机制至关重要。因而选择沉积速率快且有高分辨率年代标尺的西藏定日地区OAE2剖面事件层位之下冷青热组地层开展了详细的岩石磁学研究，旨在为重建研究区晚塞诺曼期古海洋环境演化提供新约束。对剖面-5.20 m 至30 m的地层以10 cm间隔采样352个，并测得这些样品的磁化率，重点对-5.20 m至0 m样品测量其非磁滞剩磁，饱和等温剩磁等岩石磁学参数。结合已有的0 m 至37.2 m的岩石磁学数据，获得了-5.20 m至37.2 m的完整的岩石磁学记录，并将其年龄限定为（95.58±0.15） Ma至（94.55±0.15） Ma。岩石磁学结果显示1）95.58~95.10 Ma期间磁性矿物含量增多，反映沉积区物源供给逐渐增多。这很可能是由于晚塞诺曼期全球海平面下降（KCe4）所导致，且（95.10±0.15） Ma时海平面下降至最低。2）磁性矿物种类变化反映定日地区古海洋环境在~94.7 Ma发生了较显著变化，从95.10~94.70 Ma期间亚氧化为主的环境逐渐演变为~94.7 Ma后趋于缺氧的海洋环境。这很可能是由于95.10 Ma海平面上升及相伴的低氧带（OMZ）扩张至研究区所致。研究限定的（95.10±0.15） Ma作为全球海平面上升的初始时间可为研究其他OAE2剖面海平面上升对OAE2形成的影响提供重要的年代约束。结合晚塞诺曼期活跃的火山活动，我们认为晚塞诺曼期全球性海平面上升和火山活动共同作用导致了OAE2事件的发生。

An Investigation of the Late Cenomanian Paleoenvironmental Changes in Eastern Tethys: Insight from the sedimentary archive from Tingri,Tibet, China

Oceanic Anoxic Event 2 （OAE2）， which occurred near the Cenomanian?Turonian boundary in the mid-Cretaceous， provides an important window for investigating the initiation of oceanic anoxic events and their links to other anomalous geological processes during the mid-Cretaceous greenhouse. Reconstruction of the late Cenomanian paleoenvironment of the ocean prior to OAE2 is crucial to gaining an understanding of the genesis of OAE2. We carried out a detailed rock magnetism study of the Gongzha section of the Lengqingre Formation in southern Tibet， China， where one of the most extended OAE2 intervals occurs with a high-resolution time scale. This study is focused on the strata below the OAE2 interval， with the aim of providing new constraints on the reconstruction of late Cenomanian paleoenvironmental changes in eastern Tethys. The magnetic susceptibility was measured in 352 samples collected from the -5.2 m to 30 m interval at 10 cm spacing. In addition， other magnetic parameters （magnetic susceptibility， anhysteretic remanent magnetization， saturation isothermal remanent magnetization） were measured for samples from the -5.2 m to 0 m interval. Together with existing rock magnetic data from 0?37.2 m， a complete rock magnetic record of the -5.2 m to 37.2 m interval was obtained， constraining the ages to the interval 95.58 ± 0.15 to 94.55 ± 0.15 Ma. The rock magnetic data show: （1） The magnetic mineral content increased between 95.58 and 95.10 Ma， indicating an increased supply of detritus to the depositional area. This was most likely caused by the global sea-level drop （KCe4） in the late Cenomanian； the lowest sea-level was reached at 95.10 ± 0.15 Ma. （2） Changes in the types of magnetic mineral indicate that around 94.7 Ma the depositional environment in the Tingri area changed significantly from dominantly suboxic at 95.10?94.70 Ma to a reducing environment after 94.7 Ma. This environmental change was most likely due to the sea-level rise after 95.10 Ma and the associated lateral and vertical expansion of the oxygen minimum zone （OMZ） that affected the study area. The timing of the onset of the global sea-level rise defined in this study （95.10 ± 0.15 Ma） in the late Cenomanian provides an important chronological constraint for studying the effect of sea-level rise on the initiation of OAE2 in other sections. As volcanism has been shown to have intensified in the late Cenomanian， we propose that the combination of global sea-level rise and intensified volcanism in the late Cenomanian caused the widespread OAE2 event.