Influence of intense scavenging on Pa-Th fractionation in the wake of Kerguelen Island (Southern Ocean)
Résumé
Dissolved and particulate excess (230)Th and (231)Pa concentrations (noted (230)Th(xs) and (231)Pa(xs) respectively) and (231)Pa(xs)/(230)Th(xs) activity ratios were investigated on and out of the Kerguelen plateau (Southern Ocean) in the framework of the Kerguelen Ocean and Plateau compared Study project in order to better understand the influence of particle flux and particle chemistry and advection on the scavenging of (231)Pa. In the wake of Kerguelen, particulate (231)Pa(xs) is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between (230)Th and (231)Pa (F(Th/Pa) ranging from 0.06 +/- 0.01 to 1.6 +/- 0.2) reflects the domination of the biogenic silica in the particle pool. Along the eastern escarpment of the Kerguelen plateau, the strong (231)Pa(xs) horizontal gradient in the deep waters highlights the intense removal of (231)Pa at depth, as already observed for (230)Th(xs). This local boundary scavenging was attributed to re-suspension of opal-rich particles by nepheloid layers, resulting in fractionation factors F(Th/Pa) <= 1 along the Kerguelen plateau slope. Therefore, both the composition (biogenic opal) and the flux (intense along the margin) of particles control the scavenging of the two radionuclides in the Kerguelen wake. The modelling of (231)Pa distribution with an advection-scavenging model demonstrates that lateral advection of open ocean water on the Kerguelen plateau could supply most of the (231)Pa, which is then efficiently scavenged on the highly productive plateau, as previously proposed for (230)Th(xs). It stresses that lateral advection can play a significant role in the overall budget of particle reactive trace elements in a coastal-open ocean system.
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