Thorium isotopes as tracers of particles dynamics and deep water circulation in the Indian sector of the Southern Ocean (ANTARES IV)
✍ Scribed by L. Coppola; M. Roy-Barman; S. Mulsow; P. Povinec; C. Jeandel
- Publisher
- Elsevier Science
- Year
- 2006
- Tongue
- English
- Weight
- 639 KB
- Volume
- 100
- Category
- Article
- ISSN
- 0304-4203
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✦ Synopsis
Dissolved and particulate samples were collected to study the distribution of thorium isotopes ( 234 Th, 232 Th and 230 Th) in the water column of the Indian sector of the Southern Ocean (from 428S to 478S and from 608E to 668E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess 230 Th ( 230 Th xs ) increases linearly with depth in surface water (0-100 m) and a model was applied to estimate a residence time relative to the thorium scavenging (s scav ). Low s scav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the 230 Th xs scavenging efficiency and explain this difference. An innovative coupling between 234 Th and 230 Th xs was then used to simultaneously constrain the settling velocities of small (0.6-60 Am) and large (above 60 Am) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60 F 2 m d À 1 , station 8) than south of the Agulhas Return Current (119 F 23 and 130 F 5 m d À 1 at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the 230 Th xs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the 230 Th xs concentrations. A specific 230 Th xs transport model is applied in the deep water column and allows us to assess a btravel timeQ of NADW ranging from 2 to 15 years.