The influence of particle composition on Thorium scavenging in the Mediterranean Sea

Sediment trap data are crucial for the study of marine biochemical cycles but they need careful validation by Thorium (Th) isotopes. In the present study, Th-230, Th-232, Uranium (U), Aluminum (Al), Barium (Ba) and Manganese (Mn) were analyzed in sinking particles collected by moored sediment traps in the Ligurian sea (DYFAMED site) in order to determine the collection efficiency of the traps and to constrain which particulate phase(s) carries Th isotopes in a region with strong lithogenic inputs. The Th-230(xs) evaluation was based on the U content of each sample rather than on the Th-232 content of each sample and the average U-238/Th-232 ratio of the continental crust because the latter method introduced too much uncertainty in the calculation. High trapping efficiencies (187 +/- 85% at 200 m and 87 +/- 11% at 1000 m) indicated no evidence of particle under-collection by the traps. The lack of correlation between Th-230(xs) and the carbonate or the organic matter fraction suggests that these phases are not major Th-230(xs)-carrying phases in the deep ocean. For most samples of the time series, the Th-230(xs) concentration is correlated with the lithogenic fraction and the Mn concentration, but pulses of particles with high lithogenic and Mn content and low Th-230(xs) concentration occur in winter. Assuming that Th-230(xs) is only carried by lithogenic particles, we estimate K-d_(Th)(litho) ranges from (0.42 +/- 0.04) x 10(7) ml/g to (0.8 +/- 0.2) x 10(7) ml/g. Assuming that Th-230(xs) is only carried by authigenic Mn oxide precipitates (MnO2), we estimate that K-d_(Th)(MnO2) ranges from (0.6 +/- 0.1) x 10(10) ml/g to (1.1 +/- 0.4) x 10(10) ml/g. The relative variation of K-d_(Th)(MnO2) (a factor 7) between different oceanic sites is significantly lower than the relative variation of K-d_(Th)(litho) (a factor 50), suggesting that Th-230(xs) has a tighter link with MnO2 rather than with lithogenic particles and hence that Th-230(xs) may be more likely scavenged by MnO2 than by lithogenic particles. The unambiguous determination of the particles carrying Th-230(xs) remains to be done. (C) 2009 Elsevier B.V. All rights reserved.