Paleoclimatic Evolution Of The Early Jurassic In The Northern Qaidam Basin And Its Response To The Toarcian Ocean Anoxic Events

Early Jurassic Toarcian Oceanic Anoxic Event?T-OAE?can be revealed by a large amount of marine sedimentary organic matter and the significant negative carbon isotope excursion?CIE?.The study of T-OAE in marine strata has been basically mature,but the investigation for response characteristics of T-OAE in terrestrial ecosystems is still insufficient,and researches on the similarities and differences of response characteristics in marine ecosystems and terrestrial ecosystems and the consistency of their genetic mechanisms are limited.Based on the theories and methods of sedimentary petrology and geochemistry,taking the Lower Jurassic in the northern Qaidam Basin as an example,field exploration,samples collection and contrast of paleontological fossils,the geochemical analysis of trace elements,zircon U-Pb dating,total organic carbon?TOC?analysis and organic stable carbon isotope(?¹³C)testing were conducted to determine the chronological attribution of the Lower Jurassic in the northern Qaidam Basin,to restore the palaeoclimate and palaeoenvironments of Toarcian and recognize T-OAE location in continental sediments of Qaidam Basin,whose characteristics and genetic mechanisms were clarified by comparing with the corresponding characteristics of T-OAE in the marine sediment records of the West Tethys.The main conclusions are as follows:?1?Definition of paleobiostratigraphy:Based on the systematic fossil data of paleofauna and flora,the chronological assignment of the Early Jurassic sedimentary strata in the northern Qaidam Basin was sorted out.The Huxishan Formation of the Lower Jurassic corresponds to the Hettangian,the Xiaomeigou Formation corresponds to the Sinemurian,the Huoshaoshan Formation and Tianshuigou Formation correspond to the Pliensbachian,and the Yinmagou Formation correspond to the Toarcian.The early Jurassic volcanic rocks in the northern Qaidam Basin are not recorded.The clastic zircons from the Jurassic in the northern Qaidam Basin are collected.The age peaks of latest two U-Pb dating are 220±0.24 Ma,which can be used to determine the absolute age of the Lower Jurassic in the northern Qaidam Basin.?2?Paleoenvironment analysis:Based on the element geochemical data and the analysis of proxy records change characteristics of paleoenvironment geochemical,the paleoclimate vertical evolution characteristics of the Early Jurassic in northern Qaidam Basin are summarized,that is,the climate of Xiaomeigou Formation changed from humid to arid and then to humid.The climate of Huoshaoshan Formation changed from humid to arid.The climate of Tianshuigou Formation changed from humid to arid.The paleoclimate of the black mudstone member of the Yinmagou Formation changes from humid climate to drought in three cycles,and the paleoclimate of the variegated mudstone section of the Yinmagou Formation is humid,in which,the temperature drops rapidly to a minimum value at the beginning of the black mudstone member of the Yinmagou Formation,and the value of delta?¹³C is-30.22‰?111.90 m?.Then the temperature rises.At this time,the value of?¹³C is-23.27‰?103.01 m?,and the value of?¹³C increased by about 6.95‰.The early temperature raised in the black mudstone member of the Yinmagou Formation is consistent with the temperature raised in the global Early Toarcian.?3?Responses of T-OAE in strata of the northern Qaidam basin:in this paper,T-OAE occurred in the black mudstone member of the Yinmagou Formation of the Early Jurassic in northern Qaidam basin,which was recognized from the macroscopic rock characteristics,the vertical variation of paleoplants and TOC.Its cross-layer thickness is 47.03m?84.03-131.06 m?,the organic carbon isotopes in the Dameigou Section of Lower Jurassic in the Northern Qaidam Basin have obvious negative migration,which possesses the similar trend with the stable isotope profile of organic carbon in the contemporaneous strata of marine sediments of the West Tethys and Sichuan Basin,e.g.,two positive migration modes and one negative migration mode.And the three changes stages of organic carbon stable isotope during T-OAE occurrence were clearly identified in the black mudstone member of the Yinmagou Formation in the northern Qaidam Basin.?4?The revelation of T-OAE origins from the continental sedimentary records of northern Qaidam Basin:it may because the rise of the global atmospheric CO₂concentration and temperature in Early Toar Stage,which aggravates the terrestrial water cycle and paleoweathering effect,triggers a series of sedimentary responses in continental lake basins and then changes and regulates the global carbon cycle.It is concluded that the light carbon entered the ocean and atmosphere in the form of impulses in the Toarcian of Early Jurassic and took place a series of reactions,resulting in the occurrence of anoxic events in the ocean.