Ecosystem regime shift in shallow marine environment during the Paleocene-Eocene Thermal Maximum (PETM): A case study of calcareous nannoplankton from the Kuzigongsu section in the Tarim Basin

The Paleocene-Eocene Thermal Maximum (PETM) event occurred 56 million years ago has drawn significant attention due to its similarity to anthropogenic CO₂ driven global warming. The Tarim Basin has well-preserved, fully exposed, and continuous Paleogene shallow marine strata, offering ideal materials for studying the paleoenvironmental changes of the eastern Tethys Ocean across the PETM. However, there is a lack of research on the regime shifts of plankton assemblages in the Tarim Basin across the PETM. This study selected the Kuzigongsu section (39° 44' 19″ N, 75° 18' 20″ E) located in the northeastern part of Wuqia County, Kashgar region, Xinjiang, as the research site. Samples were collected at ca. 10 cm intervals, resulting in a total of 480 samples with a total thickness of the section ca. 48 m from the Qimugen Formation. By selecting one out of every four samples, 123 samples were obtained for calcareous nannofossil study. PCA analysis and STARS mutation detection of the abundant calcareous nannofossil assemblages from the Kuzigongsu section revealed two significant regime shifts across the PETM. The first regime shift occurs between sediment thicknesses of approximately 19.61 m to 24.56 m and coincides with the PETM onset. The second regime shift occurs between sediment thicknesses of approximately 29.60 m to 31.06 m, roughly corresponding to the PETM recovery. Prior to the PETM, the calcareous nannofossil assemblages were dominated by Coccolithus pelagicus, Toweius pertusus, and T.eminens, with an average relative abundance of 61.6%. As the ecosystem shifted to another stable state, the relative abundance of T.pertusus and T.eminens decreased, with average values decreasing from 20.5% and 8.0% before the PETM to 13.0% and 3.3% during the PETM, respectively. At the same time, "excursion taxa" such as Discoaster araneus and Discoaster acutus emerged. Increases in nutrient index such as phosphorous(P) concentration and Zn/Al ratio suggest elevated primary productivity in the eastern Tethys Ocean during the PETM, which is consistent with the significant increase of the high-productivity Neochiastozygus junctus. Our analysis also showed that the shallow marine ecosystem in the North Atlantic, as reconstructed from the sedimentary record off Maryland shelf, also experienced two regime shifts across the PETM, which largely correspond to the timing of those in the Tarim Basin. This further suggests that the regime shifts in shallow marine ecosystems caused by the PETM may be a global rather than a regional change.