Contrasted release of insoluble elements (fe, al, rare earth elements, th, pa) after dust deposition in seawater: A tank experiment approach

Abstract

Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (232Th and 230Th, given as Th) and protactinium (Pa) are often assumed to be insoluble. In this study, their dissolution from Saharan dust reaching Mediterranean seawater was studied through tank experiments over 3 to 4 d under controlled conditions including controls without dust addition as well as dust seeding under present and future climate conditions (+3C and-0.3 pH). Unfiltered surface seawater from three olig otrophic regions (Tyrrhenian Sea, Ionian Sea and Algerian Basin) were used. The maximum dissolution was low for all seeding experiments: less than 0.3 % for Fe, 1 % for 232Th and Al, about 2 % 5 % for REEs and less than 6 % for Pa. Different behaviors were observed: dissolved Al increased until the end of the experiments, Fe did not dissolve signifi cantly, and Th and light REEs were scavenged back on par ticles after a fast initial release. The constant 230Th/232Th ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite un expectedly, comparison of present and future conditions in dicates that changes in temperature and/or pH influence the release of Th and REEs in seawater, leading to lower Th re lease and a higher light REE release under increased green house conditions.