The Response Of Calcareous Nannoplankton To The Paleocene-Eocene Thermal Maximum (PETM) In Western Tarim Basin, Southwestern China

The Paleocene-Eocene Thermal Maximum event was a dramatic global warming⁵5.93 Ma ago that resulted in biological extinction events,lithological changes,and major deviations in?¹³C and?¹⁸O.The southwestern Tarim Basin of China exposes successive Paleogene strata as a result of Tethys evolution and is considered an ideal region for PETM research.Calcareous nannofossils are abundant in the Qimugen Formation exposed in the Bashibulake Section.The preservation and diversity of the nannofossils varies from poor to moderate.A total of 57 different taxa that belong to 28 genera have been identified.Based on the observed distribution of the nannofossils,the identified assemblage has been divided into Subzone NP9a,NP9b and Zone NP10.In the overlying Gaijitage Formation,calcareous nannofossils disappear completely.Based on calcareous nannoplankton biostratigraphy,we also used stable isotopes and XRD to analyze the Paleocene-Eocene transition in the Tarim Basin.At the Bashibulake Section,the PETM interval is characterized by?1?an abrupt negative shifts in?¹³Ccarb,?¹³Corg and?¹⁸O?-3‰,-4.5‰and-3‰respectively?;?2?an obvious negative correlation between the K-mode?Discoaster,Fasciculithus,Ericsonia,Sphenolithus and Rhomboaster?and r-mode?Biscutum,Chiasmolithus,Toweius?nannofossil taxa coincident with a short exist Rhomboaster-Discoaster assemblage;and?3?a significant increase in the percentage of detrital input along with an increase in gypsum content.Together with the data from planktonic foraminfera and previous study,the Paleocene-Eocene boundary was refined inner the black mudstone bed of the Qimugen Formation?between Sample 12BB4-M25to12BB6-27?.The age of Qimugen Formation belongs to late Paleocene to early Eocene.These evidences indicate that the southwestern Tarim basin was a warm shallow continental shelf during the deposition of the Qimugen Formation.From the early Eocene,the environment changed conspicuously.Evaporation increased and sea level fell,which led to an acid climate.This climate mode continued the Gaijitage Formation,characterized by the deposition of thick evaporates.Consequently,most of the marine plankton became disappear,because of the significant climate shift.