Stable δ 13 C and δ 15 N isotope and palynofacies study of the late Miocene to early Pliocene Nicobar Fan sediments, Indian Ocean: implication for organic matter provenance and depositional environment

The sedimentary core (late Miocene to early Pliocene) retrieved from the Nicobar Fan (Site-U1480) was used to determine organic matter provenances, paleodeposition, and paleoclimate conditions on the hinterland. Total organic carbon (C org ), nitrogen (N total ), and C org /N total vary from 0.11 to 0.58%, 0.01 to 0.08%, and 2.5 to 33.0, separately. High C org /N total values (11.0 to 33.0) are observed between 367.53 and 718.05 mbsf depths, suggesting highly land-derived OM. The carbon (δ 13 C org : −22.95 to −27.58‰) and nitrogen (δ 15 N: + 0.40 to + 2.51‰) isotope values suggest that OM was mostly derived from mixed terrestrial C 3 plants and freshwater-phytoplanktons, with some marine OM. Palynofacies analysis also indicates that during the late Miocene, majority of OM is of terrestrial origin with little of marine origin. The spore-pollens are represented by Pinus , Picea , Abies , and Cerdus pollen, as well as trilete, and monolete spores, transported through major rivers like Irrawaddy and Salween from the surrounding Himalayan regions. Two facies assemblages have been recognized. Assemblage I: ~100% opaque phytoclasts at depths of 718.05mbsf, 969.78mbsf, and 1291.77mbsf indicate an oxidizing environment and existence of wildfires in the hinterland, which further point towards increased aridity during the late Miocene period. Assemblage II: non-opaque phytoclasts (avg. ~66.68%) are dominant over opaque phytoclasts (avg. ~25.82%) with few palynomorphs, suggesting enhanced sediment supply into the Nicobar Fan region. The dinoflagellate cysts are low in abundance, mostly represented by Spiniferites spp. and Operculodinium spp. Their productivity is relatively high during the late Miocene and declines in early Pliocene period, indicating a shift in monsoon conditions.