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 δ¹³C 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.

大约 40 Ma 的中始新世气候最适 (MECO) 是一个重要的全球变暖事件，与水圈、大气和生物圈的显着变化有关。对 Kohat 盆地（巴基斯坦东部特提斯）的 Kohat 组进行研究，以确定较大的底栖有孔虫 (LBF) 对 MECO 的响应。Kohat 组中的 LBF 组合，覆盖浅底底带 (SBZ) 15 至 17，表明年龄为鲁特阶中期至巴托阶早期。微相分析表明，Kohat 组具有泻湖（内碳酸盐岩斜坡相带）至开阔海相（中碳酸盐岩斜坡相带）古沉积环境。一个明显的正 δ ¹³C 位移标志着 MECO 在该地层中的地层位置。在以碳同位素正偏移 (CIE) 开始为标志的 Peak-MECO 事件中，未观察到 LBF 组合的明显组成变化。然而，在后 MECO 和 CIE 恢复阶段可以观察到 LBF 组合的显着变化，其中一些 LBF 属的局部首次和最后出现。这些变化得到了Alveolina和 orthophragminids的突然消失以及较大贝壳大小的Nummulites fabianii、Heterostegina和Linderina 的初始优势的证实伴随着物种多样性的增加。在这里，我们认为在 Kohat 组最上部观察到的 LBF 组合的变化可能与后 MECO 事件期间发生的更大的有孔虫周转有关，并且对应于 CIE 恢复阶段。