Thorium isotopes in the western Mediterranean Sea: an insight into the marine particle dynamics

摘要

We present a detailed view of the ~(23)Th-~(232)Th systematics in the western Mediterranean Sea in order to constrain water and particle fluxes. The conclusions obtained at the regional scale are also relevant at a more global scale although they may be more difficult to establish in the open ocean. ~(23)Th and ~(232)Th were analyzed in size-fractionated seawater and marine particle samples collected in the Ligurian, Alboran and Ionian seas. The ~(23)Th_(xs) (~(23)Th produced by the radioactive decay of ~(234)U in seawater) profiles at DYFAMED and in the Alboran sea site are not linear due to the formation of deep waters. The difference of ~(23)Th_(xs) concentration between these two sites is related to the aging of the water masses and to different scavenging conditions. The ~(230)Th/~(232)Th ratio is used as a tracer during mixing processes to provide information on the pathways followed by the matter in the water column. At DYFAMED, filtered (<0.2 μm) and ultrafiltered (<1 kDa) solutions have similar ~(23)Th/~(232)Th ratios, suggesting that an equilibrium exists between truly dissolved and colloidal Th. The change of the Th flux and of the ~(23)Th/~(232)Th ratio of trapped particles observed in August between 200 m and 1000 m is best explained by aggregation of filtered large particles on the trapped particles. The residence time of filtered large particles with respect to aggregation on trapped particles is of the order of 8-80 days. The combined budget of ~(23)Th and ~(232)Th inputs to the western Mediterranean Sea requires the dissolution of 3-5% of the Th associated with all the continental particulate inputs (including particles sequestered in margins) suggesting that eolian inputs are not the only source of refractory elements to the Mediterranean Sea and may be to the whole ocean.