The Reconstruction Of Paleo-redox Environment Of Early Cretaceous Lake Sihetun In Western Liaoning

The Jehol Biota,characterized by Eosestheria-Ephemeropsis-Lycoptera fossil assemblage traditionally,is praised as a "Mesozoic Pompeii",which is widely distributed in eastern and central Asia.Abundant exceptionally preserved fossils of the Jehol Biota,including early birds,feathered theropod dinosaurs,primitive mammals,early angiosperms,insects and a variety of aquatic organisms,have been discovered from lacustrine deposits(Lake Sihetun)of the Early Cretaceous Yixian Formation in western Liaoning Province(northeastern China).The Jianshangou unit,located in the lower part of the volcanic-sedimentary system of the Early Cretaceous Yixian Formation in western Liaoning Province,is a fossil-bearing lacustrine sedimentary stratum.Geochemical data were obtained from the mudstones,shales and volcanic ash of the 7.4-m-thick fossil-bearing lacustrine sedimentary section,which aims to determine the redox environment of lacustrine sediments and the variations of paleoclimate,and to further constrain the causes of mass mortality events and preservation mechanism during this period.The Enrichment Factors(EF)of trace mental elements,together with the relationships between trace metal element contents and total organic carbon(TOC),relevant element ratios,and Ce anomaly were used to reconstruct the depositional environment of Lake Sihetun.In combination with enrichment of several trace mental elements(U,V,Mo,Ni,Zn),low Th/U ratios(<2),high V/Ni ratios(>3),high V/Cr ratios(>2),and high Ce anomaly(>0.8)in the samples of mudstone and shale,it is suggested that an overall anoxic or even euxinic environment was dominated over the deposition of Lake Sihetun,of which the segment ? was more euxinic than the segment I.In addition,the relatively good correlation is revealed between EFv and TOC in mudstones and shales in segment I,in contrasting with weak correlation between EFv,EFMo,EFv and TOC in the segment ?.However,EFNi and TOC shows good correlation throughout these two segments.These geochemical data indicate that the water column of Lake Sihetun was euxinic prevailingly at the beginning,and gradually changed to anoxic during the deposition of 7.43-m-thick layers.Except for volcanic ash sample S08-1,most volcanic ash was deposited in oxic-suboxic conditions,which may be related to the dissolved oxygen brought by the high-density underflow formed by rivers in rainy seasons.The preservation of abundant fossils like fish,Eosestheria,and Ephemeropsis from the segment ? may link with the expansion of sulfidic water at the bottom of lake which may kill organisms and help to preserve their body fossils.Ephemeropsis and Eosestheria in the segment I may migrate to the deep water for survival due to the breakdown of stratification system in the lake.Subsequently,these organisms died because of hypoxia with the establishment of stable stratification system.There are different types of fossils assemblages in the sediments deposited in euxinic and anoxic environment,indicating that the euxinic environment is more conducive to preservation of fossils.Most well-preserved fossils of the Jehol Biota occurr in mudstone and shales with sedimentary laminae,while the volcanic ash layers contain almost no fossils.Besides,some less well-preserved fossils are covered with volcanic ash.These observations indicate the volcanic ash may not facilitate the fossil preservation.Therefore,euxinic(anoxic),high sedimentation rate,and undisturbed sedimentary environment are key factors to the preservation of Jehol Biota.