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, 230Th, 232Th, 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 230Thxs evaluation was based on the U content of each sample rather than on the 232Th content of each sample and the average 238U/232Th 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 230Thxs and the carbonate or the organic matter fraction suggests that these phases are not major 230Thxs-carrying phases in the deep ocean. For most samples of the time series, the 230Thxs concentration is correlated with the lithogenic fraction and the Mn concentration, but pulses of particles with high lithogenic and Mn content and low 230Thxs concentration occur in winter. Assuming that 230Thxs is only carried by lithogenic particles, we estimate Kd_lithoTh ranges from (0.42 ± 0.04) × 107 ml/g to (0.8 ± 0.2) × 107 ml/g. Assuming that 230Thxs is only carried by authigenic Mn oxide precipitates (MnO2), we estimate that Kd_MnO2Th ranges from (0.6 ± 0.1) × 1010 ml/g to (1.1 ±0.4) × 1010 ml/g. The relative variation of Kd_MnO2Th (a factor 7) between different oceanic sites is significantly lower than the relative variation of Kd_lithoTh (a factor 50), suggesting that 230Thxs has a tighter link with MnO2 rather than with lithogenic particles and hence that 230Thxs may be more likely scavenged by MnO2 than by lithogenic particles. The unambiguous determination of the particles carrying 230Thxs remains to be done.