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

<p>Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (<span class="inline-formula">²³²Th</span> and <span class="inline-formula">²³⁰Th</span>, 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 (<span class="inline-formula">+3</span> <span class="inline-formula">^∘C</span> and <span class="inline-formula">−0.3</span> 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 <span class="inline-formula">²³²Th</span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">230</mn></msup><mi mathvariant="normal">Th</mi><msup><mo>/</mo><mn mathvariant="normal">232</mn></msup><mi mathvariant="normal">Th</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="c9737dd2eaac67c6418a5cd8a6debca4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2663-2021-ie00001.svg" width="61pt" height="15pt" src="bg-18-2663-2021-ie00001.png"/></svg:svg></span></span> 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.</p>