Response of calcareous nannoplankton to the Paleocene–Eocene Thermal Maximum in the Paratethys Seaway (Tarim Basin, West China)

The Paleocene-Eocene Thermal Maximum (PETM) was a rapid global warming occurred 56 million years ago and has been widely viewed as an ancient analogue to the ongoing warming driven by anthropogenic CO2 emissions. The complete and continuous Paleogene shallow marine strata well preserved and outcropped in the Tarim Basin, northwestern China are ideal to study the paleoenvironmental change of the Paratethys Seaway during the PETM. To date, no high-resolution calcareous nannofossil biostratigraphy has been performed for the PETM interval in the Tarim Basin. Outcrop samples taken from the Qimugen Formation in the Kuzigongsu section contain abundant, moderately well preserved calcareous nannofossils, allowing for the establishment of a high-resolution biostratigraphic framework. Overall, 73 species of calcareous nannofossils from 33 genera were observed, with the dominant species including Coccolithus pelagicus, various Toweius species, Pontosphaera exilis, and Micrantholithus flos. The five calcareous nannofossil datums allow for the recognization of nannofossil Zone NP6 through Zone NP10. The common occurrence of shallow-water taxa (Micrantholithus) throughout the section suggests a middle to outer neritic setting as the depositional environment of the Kuzigongsu section. The stratigraphic distribution of “excursion taxa” (Coccolithus bownii, Discoaster araneus, D. acutus, Rhomboaster spp.) is consistent with the range of negative excursions in δ13Ccarb and δ18Ocarb, indicating that these “excursion taxa” are micropaleontological markers for identifying the presence of the PETM in the Paratethys Seaway. During the PETM, the deteriorated preservation and extremely low abundance of nannofossils and near-zero wt% CaCO3 values suggest that ocean acidification occurred in the shallow water of the Paratethys Seaway. In addition, a significant increase in the species Neochiastozygus junctus, which is a high productivity indicator suggests increased surface ocean productivity. Higher primary productivity may be triggered by enhanced continental weathering delivering increased nutrient through river runoff.