The Behaviour of Thorium Isotopes During the Decomposition of Large Marine Particles: In vitro Experiments and Field Data

M. Roy-Barman
1998 Mineralogical magazine  
Thorium isotopes are used as tracers of solution/ particle interactions in the ocean. 232Th is a tracer of lithgenic matter. 23~ is also present in lithogenic matter but it is mainly produced by the in situ radioactive decay of 234U in the water column. In this work, we compare the 232Th flux obtained by in vitro experiments with 232Th flux infered from the 23~ data in the Mediterranean Sea. In vitro experiments The behaviour of organic matter, trace elements and Th isotopes (232Th and 23~ has
more » ... (232Th and 23~ has been studied during the decomposition of large marine particles under laboratory controlled conditions , Arraes-Mescoffet al., 1998. The sampling site (DYFAMED) is located in the Mediterraenan Sea 5 miles off Nice. Large volumes of seawater were filtered through 60 gin grids using in situ pumps at 30 m and 200 m. The particles were then distributed among different batches in sterile (0.2 I.tm filtration) seawater and left for variable incubation times (from 0 to 20 days) in the dark, at the in situ temperature and with oxic conditions. Each incubations was stopped by filtration on a 0.2 ~tm filters yielding a filtered solution and residual particles. Some samples were sterilized by gamma irradiation in order to distinguish between biotic and abiotic processes. We obtained results on the particles collected at 30 m. 23~ and 232Th were analysed on the filtered solutions and some residual particles by isotope dilution and TIMS. After 20 days of incubation, less than 1% of the 232Th initialy present on the particles is dissolved. The temporal evolution of 232Th in solution seems to mimic the temporal evolution of Mn (Arraes-Mescoff et al. 1998, fast initial dissolution and possible biology-related scavenging at the end of the experiment), but some Th contamination of the samples may have occured. The variations of the 23~ ratio of the filtered solutions suggest that during the experiment 23~ and 232Th have identical behaviours. Field data Seawater collected on a vertical profile 30 miles off Nice was analysed for Th isotopes. Surface waters (20 m) have 232Th concentrations of 176 pg/kg (filtered, 0.2 ktm) and 204 pg/kg (unfiltered) and 23~ ratios of 1.77 x 10 .5 (filtered) and 1.41 x 10 -5 (unfiltered). Deep water (1000 m) have 232Th concentrations of 176 pg/kg (filtered) and 222-238 pg/kg (unfiltered) and 23~ ratios of 2.9-3.0 x 10 -s (filtered and unfiltered). Preliminary measurement suggests about 90% of the 232Th present in the 1000 m filtered water is carried on colloids larger than 1000 Daltons. Comparison The 23~ ratio in the western basin of the Mediterranean Sea depends on the flux of Th brought by lithogenic material (FL232 and FL230) , the percentage of dissolution of this material (D) and the production of 23~ by in situ radioactive decay (P230). At steady state, (23~ = (FL230 • D + P230)/(FL232 x D). If 1% of dissolution of large marine particles is the only source of 232Th in the western basin, we should obtain (23~ sea = 10-3. The 23~ ratios of 2.9-3.0 x 10 -5 in deep waters would requeire that D = 60%. Nd isotope data suggest that 30_+ 20% of the Nd carried by lithogenic particles is exchanged with seawater in 1295
doi:10.1180/minmag.1998.62a.3.11 fatcat:6y56n7vltjckvf4a5jffpvxe3e