Impact of atmospheric cycling on the release of iron and manganese into seawater from Saharan soil particles

Abstract

Atmospheric transported dust is a major source of iron, manganese and other nutrients to the ocean. During atmospheric transport, dust containing iron and manganese undergoes cloud processes that can increase the reactivity and solubility of iron and manganese species when they enter into seawater. Saharan sieved soils were artificially atmospherically weathered with different low pH treatments, using simple dilute acids and also cycling through a typical cloud pH range. Formation of nano-particulate iron (ferrihydrite) occurred, and was identified using high-resolution microscopy, together with a measured increase in the fraction of amorphous iron species that were leachable from the processed soil. Both treated and untreated soils were then mixed with filtered stored North Atlantic seawater in the laboratory and also with freshly collected seawater from the NE Atlantic whilst at sea. Initial release of iron from artificially treated Niger soil reached 6 nM and from artificially treated Niger Mali soil reached 3.5-10 nM (depending on the different experimental variables). This was significantly greater than from un-weathered particles ~1-1.5 nM at the soil concentrations used (10 mg). For Mn all the dissolvable metal was removed during the cloud pH cycling and was not retained on the particles due to slower oxidation and scavenging than with the Fe. However, significantly more manganese ~ 60 nM from acid treated Mali soil and ~30 nM from acid treated Niger soil was released into seawater compared to the manganese released from the untreated soil (30 nM and 22 nM for Mali and Niger soil respectively). Low pH soil treatment led to increased release of Mn in the cloud water that stayed in solution for at least 5 days. This cloud-released Mn will therefore enter the ocean surface in a soluble form. These data show the major impact of atmospheric processing on dissolution of Fe and particularly Mn from Saharan sieved soils that are transported through the troposphere to the surface ocean