A late Miocene-Early Pliocene biogenic silica crash in the Andaman Sea and Bay of Bengal

Variations in the mass accumulation rate of biogenic silica (BSi) in continental margin sediments can be used to reconstruct relative changes in productivity through time in these settings. In the northern Bay of Bengal a lack of long sedimentary records has historically precluded this type of reconstruction. The acquisition of 21 new long sedimentary records during the 2006 Indian National Gas Hydrate Program (NGHP) Expedition-01 has made it possible for the first time to reconstruct paleoproductivity in this important region of the world that is dominated by intense changes in the geological and biological fluxes largely driven by tectonic and climate related mechanisms. In the research presented here, fluctuations in the mass accumulation rate of biogenic opal during the past similar to 9.4 Myrs are reconstructed using continental margin sediment cores from the Andaman Sea (Site NGHP-01-17A) and the northern Bay of Bengal (Site NGHP-01-19). Within these records, a biogenic silica crash is recorded at similar to 6 Ma and is consistent with previous geotectonic, geochemical and paleontological studies of the southern Indian Ocean and Pacific Ocean that suggest connectivity, and thus exchange of nutrient-rich water masses, between the eastern tropical Indian Ocean and western tropical Pacific Ocean was diminished as a result of the tectonic restriction of the northerly sector of the Indonesian Throughflow (ITF). The biogenic silica crash at Sites 17 and 19 is consistent with a decrease in surface water productivity that may have been driven by the reduction of nutrient-rich Pacific waters delivered to the Andaman Sea and Bay of Bengal via the northerly route of the ITF. Following the BSi crash at similar to 6 Ma, subsequent recovery of the BSi mass accumulation rates at Sites 17 and 19 occurred and was perhaps renewed by an enhanced supply of nutrient-rich freshwater from the nearby Irrawaddy and Mahanadi Rivers, which could have occurred during a documented increase in the intensity of the Indian monsoon at similar to 5 Ma. Although recovery is noted at both core locations, biogenic silica mass accumulation rates did not fully recover in the Andaman Sea. This could be explained by the restricted nature of the Andaman basin and its more distal location from a major source of nutrient-rich freshwater. (C) 2014 Elsevier Ltd. All rights reserved.