Record of the Miocene Climate Optimum in the northeast Indian Ocean: evidence from the microfossils

In the Havelock Island of the Ritchie’s Archipelago (northeastern Indian Ocean), two lithological successions viz., the Inglis and Long formations, are exposed. Chronostratigraphically, the Havelockian Regional Stage has been named on this island for the sediments of both the formations. Based on previous planktonic foraminiferal biostratigraphy, the sediments of Havelock Island have been dated from early Miocene to late Pliocene. The present analyses on multiple microfossils (diatoms, calcareous nannofossils and radiolarians) have been undertaken on the samples collected from the outcrop exposed in Vijaynagar Village near Kalapathar Beach, Havelock Island. The diatom assemblage is represented by the marker diatom taxon viz., Cestodiscus peplum that indicates the geological age as late early to early middle Miocene. The calcareous nannofossil assemblage also indicates a latest early Miocene to earliest middle Miocene (Burdigalian-Langhian) age based on the presence of characteristic forms of NN 4 Zone. The age has also been reconfirmed more precisely by marker radiolarians from the RN 4 Zone. Together, the microfossils indicate an age range of ~ 16.4 to ~ 15.0 Ma for the section. Most of the recovered microfossils are indicative of warm water. The age of the outcrop derived from multiple microfossils, i.e. calcareous nannofossils, radiolarians and diatoms, is co-relatable with the event of Miocene Climate Optimum (MCO).