Coppola^1,2, L., M. Roy-Barman^1,3 and C. Jeandel¹

¹LEGOS, 14 av. E. Belin 31400 Toulouse, France, Fax: +46 8 5195 4031, E-mail: laurent.coppola@nrm.se, ²LIG, Swedish Museum of Natural History, Box 50007,104 05 Stockholm, Sweden and ³LSCE, Avenue de la Terrasse, 91198 Gif sur Yvette, France

Dynamics processes of marine particles revealed by ²³⁰Th and a ²³⁰Th-²³⁴Th coupling in the Indian sector of the Southern Ocean

The ²³⁰Th and ²³⁴Th concentrations have been measured in the Indian sector of the Southern Ocean during the French JGOFS ANTARES 4 cruise project in January-February 1999. The aim of this work was to study the dynamics processes which affect the marine particles transport in the water column. From the ²³⁰Th measurements, we were able to constrain the thorium residence time and the horizontal transport of the deep water masses. The results suggest a rapid renewal time (1-15 y) of the deep water column by the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW). The ²³⁰Th and ²³⁴Th are produced in the ocean by the radioactive decay of ²³⁴U and ²³⁸U, respectively, and they have two different half-life. Therefore, the ²³⁰Th-²³⁴Th coupling permitted to improve the accuracy of the dynamic parameters (sinking velocities and dissolved/particle exchanges). In the surface layer (0-100 m), the results show that the large filtered particles had a lower sinking velocity in the north of the Agulhas Front (AF). This is maybe due to a high degradation of the organic matter in this part of the study zone. The small particles had a sinking speed around 1 m/d in the Polar Front Zone (PFZ) and in the Subtropical Zone (STZ). Finally, the ²³⁰Th-²³⁴Th coupling permits to show that the desorption and desaggregation processes of marine particles were rapid. This could be explained by the presence of detrital organic matter and/or an efficient microbial loop limiting the export of organic matter to the deep layers.