Abstract
Abstract. 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, particulate 231Paxs is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between 230Th and 231Pa (FTh/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 231Paxs horizontal gradient in the deep waters highlights the intense removal of 231Pa 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, 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 231Pa distribution with an advection-scavenging model demonstrates 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.
Highlights
The Kerguelen plateau (South Indian Ocean) is an ideal laboratory to study the mechanisms of natural iron fertilization in the Ocean
In the wake of the Kerguelen plateau, 231Paxs and 230Thxs distributions in seawater and particles and their fractionation are in the range of those previously observed in the Southern Ocean
This study shows that in the Kerguelen wake dominated by biogenic silica, 231Pa removal is at least as efficient and possibly more efficient than 230Th removal, thereby setting the Pa/Th ratios and fractionation factor (FTh/Pa ≤ 1) in the water column, and in the sediments of this area of the Southern Ocean
Summary
The Kerguelen plateau (South Indian Ocean) is an ideal laboratory to study the mechanisms of natural iron fertilization in the Ocean Better defining these mechanisms was the main aim of the KErguelen Ocean and Plateau compared Study project (KEOPS, Blain et al, 2007). They are both produced at a fixed known rate in the ocean with a production activity ratio 231Pa/230Th of 0.093. Both radionuclides are particle reactive and rapidly adsorbed onto settling particles and removed (scavenged) from the water column to the sediment.
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