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Influence of intense scavenging on Pa-Th fractionation in the wake of Kerguelen Island (Southern Ocean)

Author(s): C. Venchiarutti | M. Roy-Barman | R. Freydier | P. van Beek | M. Souhaut | C. Jeandel

Journal: Biogeosciences Discussions
ISSN 1810-6277

Volume: 8;
Issue: 3;
Start page: 4871;
Date: 2011;
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Dissolved and particulate excess 230Th and 231Pa concentrations (noted 230Thxs and 231Paxs, respectively) and 231Paxs/230Thxs 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 231Pa. In the wake of Kerguelen Island, the relative abundance of 231Paxs in the particles compared to the dissolved phase associated with a low fractionation factor between 230Th and 231Pa (FTh/Pa ranging from 0.06 ± 0.01 to 2.13 ± 0.63) is consistent with particles being dominated by biogenic silica in this area. Strong 231Pa concentration gradients occur on relatively short distances. Along the eastern escarpment of the Kerguelen plateau, an intensive scavenging affects significantly the dissolved distribution of 231Paxs at depth, as already observed for 230Thxs. This local boundary scavenging was attributed to re-suspension of opal-rich particles by nepheloid layers, resulting in fractionation factors FTh/Pa ≤ 1 along the Kerguelen plateau slope. Therefore, these results showed that in the Kerguelen wake, both the composition (biogenic opal) and the flux (intensive along the margin) of particles control the scavenging of the two radionuclides. The modelling of 231Pa distribution with an advection-scavenging model demonstrate that lateral advection of open ocean water on the Kerguelen plateau could supply most of the 231Pa, which is then efficiently scavenged on the highly productive plateau, as previously proposed for 230Thxs. 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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