Neogene calcareous nannofossil biostratigraphy of the northern Indian Ocean: Implications for palaeoceanography and palaeoecology

Abstract

Neogene sediments of the Andaman and Nicobar Islands in northern Indian Ocean, specifically the lower Miocene (Burdigalian) to lower Pliocene (Zanclean) sequences, were analysed for calcareous nannofossils that yielded well-preserved and diverse assemblages. The majority of samples contained stratigraphic marker taxa that allowed us to identify successions within the Burdigalian-Langhian (uppermost lower Miocene to lowermost middle Miocene) on the Havelock Island, Tortonian (upper Miocene) on the Neil Island and Zanclean (lower Pliocene) on the Car Nicobar Island. The nannofossil assemblages of uppermost lower Miocene to lowermost middle Miocene sediments reflect strong preference for warm tropical climate and nutricline condition i.e., depth dependent high variation of nutrient content. The presence of relatively unusual assemblage compositions, especially the presence of abundant sphenoliths and small reticulofenestrids, in conjunction with rare discoasters and alongside common diatoms, are indicative of high-productivity surface waters and probably upwelling conditions in the late Miocene. In the Tortonian (upper Miocene), low abundance of discoasters and high abundance of small reticulofenestrids (<3 μm) indicate eutrophic condition. The high-productivity surface-water environments can be linked with the intensification of the Indian Summer Monsoon. Well-preserved calcareous nannofossils have been also been recovered from the Zanclean (lower Pliocene) sediments on Car Nicobar Island. During the Zanclean warm water condition prevailed and from marginal to deep water setting i.e., transgressive event was recognised in the northern Indian Ocean.