Paleoenvironmental reconstruction and transitional role of the Fergana Sea (Central Asia) during the middle Eocene through the lens of Micropaleontology

The Fergana Basin, one of the largest depressions in Central Asia, is situated within the intermountain range of the southwestern Tien Shan. The final stage of marine sedimentation in the basin, represented by middle Eocene layers, marks the onset of the Asian aridification. This shift in paleoenvironmental conditions coincided with significant global changes, including the restriction of the Peri-Tethys, the isolation of Asian realms, and alterations in oceanographic and climatic systems. Microfossil assemblages provide important information about changes in the palaeoenvironment. Despite the abundance and importance of foraminifera and ostracods in the Fergana Basin, the paleoenvironmental conditions, species diversity, and paleobiogeography remain poorly understood, largely due to the outdated literature, most of which is in Russian. The present study focuses on foraminiferal and ostracod assemblages to gain new insights into environmental changes within the Fergana Basin and its connections to other marine basins during the middle Eocene. The material analyzed in this study comprises foraminifera and ostracods collected from the Sauk-Tanga section (40° 2'50.82"N, 70°15'42.70"E) and the Chamangul section (40° 1'50.34"N, 70°26'7.82"E), both located in the southern part of the Fergana Basin. The study site is located within the Madygen Geopark in the Batken Region of Kyrgyzstan, and the sections comprise an expanded middle Eocene clay sequence.

A total of 17 foraminiferal species and 11 ostracod species were identified. The foraminiferal community is characterized by three distinct assemblages: one dominated by Cribroelphidium, one comprising a combination of Ammonia, Cribroelphidium, and Nonion, and another dominated solely by Nonion. A comprehensive analysis of the micropaleontological assemblages indicates that conditions were shallow and fully marine during the Middle Eocene epoch. The environmental conditions are marked by elevated oxygen levels within the water column, extending from the intertidal zone to the high subtidal regions, in contrast to the low oxygen conditions prevalent within the sediments. The microfauna recovered is comparable to faunal communities in Europe and western Siberia, as well as neighboring regions such as the Tajik and Tarim basins. However, it differs significantly from the microfauna of the Turan Basin. The findings indicate that the Fergana Basin functioned as a transitional zone between the western Peri-Tethys and the easternmost realms, contradicting previous hypothesis that it merely served as a gulf. This novel perspective contributes to a more comprehensive understanding of the region's paleogeography and its significance in broader paleoenvironmental changes.