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

Abstract. Dissolved and particulate excess ²³⁰Th and ²³¹Pa concentrations (noted ²³⁰Th_xs and ²³¹Pa_xs respectively) and ²³¹Pa_xs/²³⁰Th_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 ²³¹Pa.

In the wake of Kerguelen, particulate ²³¹Pa_xs is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between ²³⁰Th and ²³¹Pa (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 ²³¹Pa_xs horizontal gradient in the deep waters highlights the intense removal of ²³¹Pa at depth, as already observed for ²³⁰Th_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 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 ²³⁰Th_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.