High-resolution magnetostratigraphic and biostratigraphic study of Ethiopian traps-related products in Oligocene sediments from the Indian Ocean Yannick Touchard a; Ã , Pierre Rochette a , Marie Pierre Aubry b , Annie Michard a a CEREGE - CNRS, Europo “le Me ¤diterrane ¤en de l’Arbois, BP80, F-13545 Aix en Provence cedex 4, France b Rutgers University, Department of Geological Sciences, Wright Geological Laboratory, 610 Taylor Road, Piscataway, NJ 08854-8066, USA Received 12 August 2002; received in revised form 8 November 2002; accepted 15 November 2002 Abstract Volcanic traps correspond typically to aerial emissions of more than 10 6 km 3 of magma over 1 Myr periods. The potential global impact of such emissions makes the precise correlation of traps with the global magnetobiochro- nologic timescale an important task. Our study is focused on the Ethiopian traps which correspond to the birth of the Afar hotspot at the triple junction between the Red Sea, Aden Gulf and East-African rift. The Ethiopian traps have a significant acidic component (about 10% of the traps by volume) which enables more efficient stratospheric aerosol diffusion than for the main basaltic eruptions. Furthermore, a magnetostratigraphy is well established for the traps: traps activity began in Chron C11r.2r and ended in Chron C11r.1r or C10r, with well clustered 40 Ar/ 39 Ar ages at 30þ0.5 Ma. Four tephra layers, marked by prominent magnetic susceptibility peaks, occur in Oligocene sections of sites from Ocean Drilling Program Leg 115, drilled in the southern Indian Ocean near Madingley Rise, 2600 km away from the Ethiopian traps. In order to demonstrate that these tephra layers are related to the Ethiopian traps, a high- resolution study of sites 709 and 711 was undertaken, involving magnetostratigraphy and nannofossil stratigraphy, together with isotopic and geochemical characterization of the tephra. Geochemical analyses and isotope ratios of the glass shards indicate the same acid continental source for these tephras which is compatible with the Ethiopian signature. Moreover, Hole 711A provides a reliable magnetostratigraphy for the Oligocene (Chrons 13^9). The tephra layers occur in the interval spanning Chrons C11n.2n^C11n.1n which agrees with the positions of acidic layers in the traps. Calcareous nannofossil stratigraphy confirms the magnetostratigraphic interpretation, with the NP23/24 zonal boundary occurring within the interval containing the tephra layers. Hole 709B supports the results from Hole 711A. Thus, the Ethiopian traps can be correlated to the geomagnetic polarity timescale. Our results confirm an earlier proposal that the onset of Ethiopian traps-related emissions is synchronous with the Oi2 oxygen isotope event. Published by Elsevier Science B.V. 0012-821X/02/$ ^ see front matter Published by Elsevier Science B.V. PII:S0012-821X(02)01084-1 * Corresponding author. Tel.: +33-169824362. E-mail addresses: yannick.touchard@lsce.cnrs-gif.fr (Y. Touchard), rochette@cerege.fr (P. Rochette), aubry@rci.rutgers.edu (M.P. Aubry), michard@cerege.fr (A. Michard). Earth and Planetary Science Letters 206 (2003) 493^508 www.elsevier.com/locate/epsl