séminaire du pôle évolution du vivant - vendredi 21 octobre 2011

sulfur-dioxideCycle externe du soufre à l'Archéen : marqueur de l'activité microbienne ou de la croissance crustale ?

Pascal Philippot, équipe géobiosphère actuelle et primitive, institut de physique du globe de Paris

vendredi 21 octobre 2011, 11 heures, amphi Monge

The reports of mass-independent sulfur isotope anomalies (MIF-S) in sediments older than 2.45 Ga have been attributed to photolytic reactions involving volcanic SO2 in an oxygen-poor atmosphere. Photolysis experiments of SO2 coupled with various UV shielding scenarios provided additional links to the early atmosphere. However, no simple model can reproduce the mismatch in the ?33S-d34S relationship between the reference Archaean sulfide array (positive ?33S-d34S correlation) and product sulfate (negative ?33S but positive d34S). The discrepancy in the temporal and spatial record of sulfur isotope anomalies, with three main sulfate horizons deposited within less than 300 Ma compare to a global distribution of sulfide over more than 1,500 Ma, is also unexplained. Here we report a new ?33S-d34S linear trends recovered in two felsic volcanic ash layers of the 3.2 Ga Mapepe Formation in South Africa. This « felsic volcanic array » forms a tight ?33S-d34S linear correlation that is best approximated by SO2 photolysis experiments at deep UV wavelength. The perfect match to the ?33S-d34S values of associated sulfate and equivalent felsic volcanoclastic and sulfate horizons of the 3.5 Ga old Dresser Formation, Western Australia, indicates that the exogenic sulfur cycle that produced this array was linked to felsic volcanism and sulfate precipitation. An emerging scenario for the early Earth atmosphere is a continuous photochemical haze that is perturbed between 3.5 and 3.2 Gyr by massive and optically thick volcanic plumes. This volcanic activity coincides in time with a period of rapid crust formation (Valbaara supercontinent).