An Investigation Of The Late Cenomanian Paleoenvironmental Changes In Tingri,Southern Tibet,China

The discovery of geomagnetic anomalies and ocean anoxic event 2(OAE2)in the middle Cretaceous greenhouse world is of great significance for understanding the coupling of the abnormal events in the different spheres of the Earth in the Cretaceous.The detailed record of geomagnetic field intensity(paleo-intensity)of Late Cenomanian is important for the comprehensive understanding of the abnormal geomagnetic field(Cretaceous normal superchron,CNS)of the Middle Cretaceous.Reconstruction of the late Cenomanian paleoenvironment of the ocean prior to OAE2 is crucial to understanding the genesis of OAE2.We carried out a detailed paleomagnetic and rock magnetism study of the Gangbacunkou Formation and Lengqingre Formation of the Gongzha section in southern Tibet,China,where one of the most extended OAE2 intervals is archived and has been constrained with a highresolution orbital time scale.This study is focused on the strata below the OAE2 interval with the aim to find out the paleo-intensity variation of Late Cenomanian and to provide new perspective on the late Cenomanian paleoenvironmental changes in eastern Tethys.A total of12 oriented samples were collected from three sites from the-5 m to 65 m interval.The anisotropy of magnetic susceptibility was measured,and stepwise thermal demagnetization and alternating field demagnetization were performed.Then,the magnetic susceptibility was measured in 352 unoriented samples collected from the-5.20 m to 30 m interval at a 10 cm spacing.In addition,other magnetic parameters(magnetic susceptibility,anhysteretic remanent magnetization,saturation isothermal remanent magnetization)were measured for samples from the-5.20 m to 0 m interval.Together with existing rock magnetic data from 0 m to 37.2 m,a complete rock magnetic record of the-5.2 m to 37.2 m interval was obtained,spanning 95.58± 0.05 to 94.55 ± 0.05 Ma.The demagnetization results from this section failed to yield reliable primary remanent magnetization and thus it is not possible to obtain a reliable paleo-intensity record.The rock magnetic data show that(1)The magnetic mineral content increased between95.58 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 ± 0.05 Ma.(2)Changes in the types of magnetic mineral indicate that the depositional environment in the Tingri area changed significantly from dominantly suboxic state during 95.10–94.70 Ma to a reducing environment after 94.7 Ma.This environmental change was most likely caused by the sea-level rise after 95.10 Ma and the associated lateral and vertical expansion of the oxygen minimum zone(OMZ)that drowned the study area.The timing of the onset of the global sea-level rise defined in this study(95.10 ±0.05 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,it is proposed that the combination of global sea-level rise and intensified volcanism in the late Cenomanian caused the widespread OAE2 event.