Contrasted release of insoluble elements (Fe, Al, rare earth elements, Th, Pa) after dust deposition in seawater: a tank experiment approach

Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (inline-formula²³²Th and inline-formula²³⁰Th, 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 (inline-formula+3 inline-formula^∘C and inline-formula−0.3 pH). Unfiltered surface seawater from three oligotrophic 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 inline-formula²³²Th 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 significantly, and Th and light REEs were scavenged back on particles after a fast initial release. The constant inline-formula $M7inlinescrollmathmlchem{}^{\mathrm{normal\; 230}}\mathrm{normal\; Th}{/}^{\mathrm{normal\; 232}}\mathrm{normal\; Th}$ 61pt15ptsvg-formulamathimgc9737dd2eaac67c6418a5cd8a6debca4 bg-18-2663-2021-ie00001.svg61pt15ptbg-18-2663-2021-ie00001.png ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite unexpectedly, comparison of present and future conditions indicates that changes in temperature and/or pH influence the release of Th and REEs in seawater, leading to lower Th release and a higher light REE release under increased greenhouse conditions.