Particle dynamics study in the wake of Kerguelen Island using thorium isotopes

In the context of the KErguelen Ocean and Plateau compared Study (KEOPS, 19 January-13 February 2005), particle dynamics were investigated using thorium isotope measurements over and off the Kerguelen plateau. Dissolved and particulate Th-230 and Th-232 samples were collected at nine stations. Dissolved excess Th-230 concentrations (Th-230(xs)) vary from 0.5 to 20.8 fg/kg and particulate Th-230(xs) concentrations from 0.1 to 10.0fg/kg. Dissolved and particulate Th-232 concentration ranges are 16.8-450.2 pg/kg and 3.8-502.8 pg/kg, respectively. The Th-230(xs) concentrations increase linearly with depth down to the bottom at most of the plateau stations and down to 1000m at the off-plateau stations. This linear trend is observed down to the bottom (1550 m) at Kerfix, the open-ocean "upstream" station located west of the Kerguelen plateau. A simple reversible scavenging model applied to these data allowed the estimation of adsorption rate constant (k(1) approximate to 0.2-0.8 year(-1)), desorption rate constant (k(-1) approximate to 1-8 year(-1)) and partition coefficients (average K = 0.16 +/- 0.07). Calculated particle settling velocities S deduced from this simple model are ca. 500m/year at most of the plateau stations and 800m/year at all the off-plateau stations. The plateau settling velocities are relatively low for such a productive site, compared to the surrounding HNLC areas. The difference might reflect the fact that lateral advection is neglected in this model. Taking this advection into account allows the reconstruction of the observed Th-230(xs) linear distributions, but only if faster settling velocities are considered. This implies that the I D model strongly underestimates the settling velocity of the particles. In the deep layers, the occurrence of intense boundary scavenging along the escarpment due to bottom sediment re-suspension and interaction with a nepheloid layer, yielding a removal of similar to 50% of the Th stock along the northwestward transect, is suggested. (c) 2008 Elsevier Ltd. All rights reserved.