Helium transport in sediment pore fluids of the Congo-Angola margin

During the ZaiRov2 cruise of the ZaiAngo project (1998–2000) on the passive Congo-Angola margin, several gravity cores were analyzed for helium isotopic composition of sedimentary pore waters in two cold fluid seepage zones: the Astrid slide area and the Regab giant pockmark. Gas concentration and isotopic composition are presented along with thermal data in terms of the origin and circulation of fluids. Helium isotope data lie on a mixing line between bottom seawater and an almost pure radiogenic. Helium and temperature vertical profiles are well described by the classic diffusionadvection equation. On the basis of He profiles, we estimate the advection rate on the rim of the pockmark between 1.2 and 2.3 mm/a. The He flux derived for a pure diffusive regime (2.4 × 10 −8 mol/m 2 /a) can favorably be compared to literature data and contrasts with the flux computed close to the pockmark center (1.9 × 10 −7 mol/m 2 /a). Helium depth profiles turned to be more sensitive to advection rate than temperature profiles are.