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

• Published in: Biogeosciences
• Main Authors: C. Venchiarutti, M. Roy-Barman, R. Freydier, P. van Beek, M. Souhaut, C. Jeandel
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2011
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Southern Ocean; Kerguelen; Kerguelen Island
• Online Access: https://doi.org/10.5194/bg-8-3187-2011; https://doaj.org/article/1f83d82f7ecd4140b5ba22c78cc9d122

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.