Larger benthic foraminiferal assemblages and their response to Middle Eocene Climate Optimum in the Kohat Basin (Pakistan, eastern Tethys)

The Middle Eocene Climatic Optimum (MECO) at -40 Ma is a significant global warming event associated with pronounced changes in the hydrosphere, atmosphere, and biosphere. The Kohat Formation in the Kohat Basin (eastern Tethys, Pakistan) is studied for identifying the response of larger benthic foraminifera (LBF) to MECO. The LBF assemblages in the Kohat Formation, covering from the Shallow Benthic Zones (SBZ) 15 to 17, suggest middle Lutetian to early Bartonian in age. Microfacies analyses indicate a lagoonal (inner carbonate ramp facies belt) to open marine (middle carbonate ramp facies belt) paleodepositional environment of the Kohat Formation. A distinct positive d13C shift marks the stratigraphic position of the MECO in this formation. At the Peak-MECO event that is marked by the onset of the positive carbon isotope excursion (CIE), no evident compositional variation in the LBF assemblages is observed. However, significant changes in the LBF assemblages with the local first and last occurrences of some LBF genera can be observed in the Post-MECO and CIE recovery phase. These changes are verified by the sudden disappearance of Alveolina and orthophragminids and initial dominance of larger shell-size Nummulites fabianii, Heterostegina, and Linderina species accompanied by an increase in the species diversity. Here, we argue that the change in the observed LBF assemblages in the uppermost part of the Kohat Formation might be related to a larger foraminiferal turnover occurring during the Post-MECO event and corresponds to the CIE recovery phase. (c) 2020 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.

Automated summary

The study shows that there were no evident compositional variations in the larger benthic foraminifera (LBF) assemblages during the Middle Eocene Climatic Optimum (MECO) event, but significant changes occurred in the Post-MECO and Carbon Isotope Excursion (CIE) recovery phase. This suggests a larger foraminiferal turnover happening during the Post-MECO event, leading to compositional changes in the observed LBF assemblages in the uppermost part of the Kohat Formation.