Geochemistry and isotopic study of southern Bay of Bengal sediments: Implications for provenance and paleoenvironment during the middle Miocene

Abstract

The Bay of Bengal (BoB), the largest bay in the world, is known to receive large amounts of fresh water and suspended particulate matter from different sources. The sediments deposited in the bay serve as a repository for paleoenviromental reconstruction studies. The Miocene Epoch is an important interval in geologic time as it is marked by major climatic events that controlled ocean productivity, redox conditions and provenance of sediments. However, limited work has been carried out to establish paleoenviromental changes and provenance of sediments during the Miocene. Here we present detailed geochemical and radiogenic (Sr, Nd) isotopic results from predominantly middle Miocene sediments of Ocean Drilling Program (ODP) Site 758A, located in the southern BoB (Lat. 5 °23′N, Long. 90°21′E; water depth 2925 m), to understand paleoredox conditions, paleoproductivity and provenance. The results suggest that these sediments are strongly linked to climatically modulated surface processes that affected the region.The distribution of redox-sensitive element proxies (U, V, Cr) at interval ~17–8 Ma points to an oxic depositional environment. The major, trace and rare earth elements distribution supports prevalence of the Indian summer monsoon (ISM) at ~12–10 Ma. Productivity proxies (Babio, Porg) show a decreasing trend during colder events (East Antarctic Ice Sheet expansion – EAIS and advent of late Miocene cooling – LMC events). The trace and rare earth element concentrations are generally low, except at intervals where glass shards were found. The 87Sr/86Sr and εNd(0) values indicate that the sediments belong to the High Himalayan sedimentary series (HHSS) throughout the middle Miocene. However, prior to initiation of the ISM, these sediments underwent meteoric alteration in the lowlands before deposition in the BoB. Our records suggest that sediments at ODP Site 758A may have been introduced into the BoB through the Irrawaddy-Yarlung-Tsangpo drainage system during the entire period of this study. The Irrawaddy-Yarlung-Tsangpo system drained the Indus-Tsangpo Suture zone, southern Tibet, the eastern Himalayas and the Arakan coast before debouching in the BoB.