Reconstruction of Miocene to Pleistocene sea‐surface conditions in the eastern Indian Ocean on the basis of calcareous nannofossil assemblages from ODP Hole 757B

Abstract

AbstractWe studied the calcareous nannofossil assemblages from Ocean Drilling Program Hole 757B in the eastern Indian Ocean to reconstruct the sea‐surface conditions, especially changes in thermocline/nutricline depth, from the middle Miocene to the Pleistocene. Nutrient levels at the sea surface were reconstructed from the nannofossil accumulation rate, abundance of Discoaster, and coccolith size variations in Reticulofenestra. The sea‐surface conditions were likely to have been oligotrophic with a deep thermocline/nutricline during 11.8–7.5 and 6.7–6.2 Ma. The change from oligotrophic to eutrophic conditions at 7.5–6.7 Ma was caused by an influx of nutrient‐rich terrestrial material or upwelling of nutrient‐rich water induced by intensified southeast trade winds. This conclusion is supported by a marked increase in the magnetic susceptibility of the sediment that is coincident with the change from oligotrophic to eutrophic assemblages. The diminished abundance of Discoaster after ~ 3.0 Ma indicates shallowing of the thermocline/nutricline after that time, which confirms the reduced Indonesian Throughflow resulting from tectonic restriction and intensified cooling of the Benguela upwelling system. Decreases in the modal coccolith size of Reticulofenestra, indicating eutrophic conditions, are observed in three horizons. Two of these events (at 7.5 and 5.0 Ma) correspond to biogenic blooms in the Indian Ocean at 7.6 Ma and 6.0–5.0 Ma, respectively.