Impact of changing Neogene climate on the plankton community: evidence from the northeast Indian Ocean (Andaman and Nicobar Basin)

Some major climatic changes occurred during the Neogene Period, which encompasses the Miocene and Pliocene epochs. The marine flora and fauna either adapted to or became extinct due to changes in climatic conditions and tectonic activity. The Andaman and Nicobar Basin in the northeast Indian Ocean hosts deep-water marine facies with a few shallow-water sequences. The Neogene sediments of the basin archive well-preserved fossil phytoplankton and zooplankton represented by diatoms, radiolarians, silicoflagellates, calcareous nannofossils, including planktonic and benthic foraminifers, etc. The Neogene marine sediments exposed on various islands in the Andaman and Nicobar Group offer a unique and valuable geological archive. Combined with micropalaeontological data obtained from deep-sea sediment cores, these records provide an excellent foundation for comprehensive biostratigraphy and for reconstructing the overall palaeoenvironment and palaeoceanography. The significant events identified by the analysis of microfossils are: the Miocene Climate Optimum (MCO) during the middle Miocene; intensification of the Indian Summer Monsoon (ISM) coupled with upwelling and nutrient-rich conditions during the late Miocene; and low biogenic silica and a cooling event during the early Pliocene. Morphometric analysis of long-ranging calcareous nannofossil species that existed during ~16.4 to ~4.54 Ma reveals that they survived in different environments due to variations in their dimensions. Estimation of radiolarian shell volume reveals a gradual decrease in shell volume with younger age during the late Neogene. Silicoflagellates, another group of phytoplankton, are also useful for estimating relative sea surface palaeotemperature. Datasets generated from fossilised phytoplankton and zooplankton, both from onshore and offshore sequences of the northeast Indian Ocean (Andaman and Nicobar Basin), provide valuable information about the overall Neogene palaeoceanographic scenario.