Calcareous nannofossil micropalaeontology of the Eocene–Oligocene offshore sedimentary sequence of the Mannar Basin, Sri Lanka: deciphering palaeoenvironmental dynamics of the equatorial margin of the Northern Indian Ocean

Abstract

The Eocene–Oligocene transition (EOT) marks a pivotal shift from a global warm climate to the glaciated conditions observed in the Oligocene Era. Despite its significance, research on the palaeoenvironmental and palaeoclimatic conditions during this transition in the equatorial margin of the Northern Indian Ocean around Sri Lanka remains limited. This study addresses this gap by reconstructing these conditions using calcareous nannoplankton assemblages from sediment cores collected in the Mannar Basin in the Indian Ocean. A total of 54 species spanning 17 genera and 7 families were identified, indicative of a diverse marine ecosystem during this period. The assemblages were distributed across NP23 to NP16 nannofossil zones, corresponding to the Bartonian to early Rupelian stages. Warm-water species such as Coccolithus pelagicus and Umbilicosphaera bramlettei, alongside cold-water species like Reticulofenestra dictyoda, suggest variations in sea surface temperatures and cooler water layers or upwelling zones. The rapid extinction of warm-water taxa and the absence of certain species during the Eocene–Oligocene Transition indicate a significant decrease in temperature. Notably, the decline in abundance of k-mode taxa such as Coccolithus, Discoaster and Ericsonia suggests a shift from warm and oligotrophic conditions to cooler and eutrophic environments. Key index nannofossils, including Coccolithus formosus and Discoaster deflandrei, indicate relatively high sea surface temperatures and oligotrophic environments. These findings shed light on the transition from the late Eocene to the early Oligocene period and provide valuable insights into past climatic and environmental dynamics in the Indian Ocean.