Tortonian silicoflagellates from the offshore of northeast Indian Ocean

Analysis of the Tortonian sediments in the northeast Indian Ocean from NGHP–01–17A drilled at Andaman Basin shows 22 silicoflagellate taxa belonging to the genera Bachmannocena, Dictyocha, Distephanopsis, Paramesocena and Stephanocha. This study records the first evidence of Tortonian silicoflagellates from the offshore of northeast Indian Ocean. Two Partial Range zones namely Dictyocha varia, and Dictyocha extensa and one Subzone i.e. Bachmannocena elliptica have been proposed. In the present study, it has been observed that the transition of D. varia to D. extensa took place within the Tortonian. Fossil records of double skeletons in the silicoflagellate taxa are rare; however, we have documented double skeletons in three silicoflagellate taxa i.e. Dictyocha calida, Dictyocha fibula subspecies ausonia and Stephanocha speculum (6-sided). Ratio of Dictyocha and Stephanocha based on the combined abundance of all the taxa of the two genera reveals that there was substantial dominance of Dictyocha over Stephanocha throughout the studied sequence. The Dictyocha/Stephanocha palaeotemprature proxy indicated prevalence of warmer climate during the Tortonian (9.86 Ma to 7.39 Ma) with a cooling minimum in the Bachmannocena elliptica Subzone during 8.104 Ma – 8.088 Ma corresponding with nutrient rich condition.