Lithology of sediments from the Arabian Sea

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced >4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5-11.5 km. Curiously <400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (>600°C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting >28 m.y., and induced by the steep continent-ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.

Element concentrations and stable isotope record of planktonic foraminifera of sediment cores from the northeastern Brazilian Continental Margin

Detailed 14C AMS data and isotope based stratigraphies from high-resolution paleoceanographic records for the last 22 ka of cores from the upper continental slope off NE Brazil reveal sedimentation rates of up to 100 cm per 1000 yr. Variations in the sediment composition relate to changes in the input of terrigenous material. The sedimentation is controlled by sea level and by the climatic regime of the hinterland. Short-term changes in the tropical wind field may act as a climatic trigger. The zonality of the SE trades was probably increased and the monsoonal activity over Africa reduced during the Younger Dryas period.