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

The Paleocene-Eocene Thermal Maximum(PETM) event was a dramatic global warming w55.93 Ma ago that resulted in biological extinction events, lithological changes, and major deviations in σ13 C and σ18 O.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 σ13 C_(org), σ13 C_(carb) and σ18 O(-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.

Stratigraphy of the Lower Cretaceous Sediments from the Carpathian Bend Area,Romania

The Lower Cretaceous sediments of the Ceahlǎu Nappe （from the bend region of the Romanian Carpathians） were investigated from lithological and micropaleontological （calcareous nannoplankton） points of view. Our investigations revealed that the studied deposits were sedimented within the latest Tithonian-Albian interval. The calcareous nannofossil assemblages of the turbidite calcareous successions （the Sinaia Formation） were assigned to the NJK-？NC5 calcareous nannofossil zones, which cover the Late Tithonian-Early Barremian interval. The sandy-shaly turbidites, which followed the calcareous turbidites of the Sinaia Formation, are Early Barremian-Early Albian in age （interval covered by the ？NC5-NC8 calcareous nannofossil zones）. Because the studied deposited are mainly turbidites, many reworked nannofossils from older deposits are present in the calcareous nannofloras. Thus, some biozones （i.e., NC5）, defined based on the last occurrences of nannofossils, could not be identified. The calcareous nannofossil assemblages are composed of Tethyan taxa （which dominate the nannofloras） and cosmopolitan taxa. During two intervals （the Late Valanginian and across the Barremian/Aptian boundary）, Tethyan and cosmopolitan nannofossils, together with Boreal ones, were observed. This type of mixed calcareous nannoplankton assemblage is indicative for sea-level high-stand, which allows the nannofloral exchange between the Tethyan and Boreal realms, within the two-above mentioned intervals.