Calcareous nannofossil changes linked to climate deterioration during the Paleocene–Eocene thermal maximum in Tarim Basin, NW China

Abstract

The Paleocene–Eocene Thermal Maximum (PETM) event was a dramatic global warming ∼55.93 Ma ago that resulted in biological extinction events, lithological changes, and major deviations in δ13C and δ18O. 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.Based on calcareous nannoplankton biostratigraphy, we also used stable isotopes and XRD to analyse the Paleocene–Eocene transition in the Tarim Basin. At the Bashibulake Section, the PETM interval is characterized by (1) an abrupt negative shifts in δ13Corg, δ13Ccarb and δ18O (−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 robust Rhomboaster-Discoaster assemblage; and (3) a significant increase in the percentage of detrital input along with an increase in gypsum content. In the upper part of the Qimugen Formation Micrantholithus and Braarudosphaera are commonly found right up to the top where most of the nannofloras suffer a sharp decrease. In the overlying Gaijitage Formation, calcareous nannofossils disappear completely. These events 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 within the youngest unit studied, the Gaijitage Formation, characterized by the deposition of thick evaporates. Consequently, most of the marine plankton, i.e. calcareous nannoplankton, became disappear, because of the significant climate shift.