Transport phases of transition metals in the Amazon and Yukon Rivers

Abstract

Samples representing yearly averages of material transported by the Amazon and Yukon Rivers were analyzed to separate the transition metals (Cr, Mn, Fe, Co, Ni, and Cu) into the following transport phases: (1) crystalline particles, (2) metal hydroxide coatings, (3) solid organic material, (4) sorbed material, and (5) those in solution. The major transport phases are crystalline particles and metal hydroxide coatings, which, combined, carry 65% to 92% of the transition metals transported. Solid organic material, the next most important phase, transports between 5% and 19% of the total transported. Material carried in solution transports 0.6% to 17% of the total transported. Sorbed transition metals account for between 0.02% and 8% of the total transported. Metal hydroxide coatings represent the major transporting mechanism potentially available to organisms, since, for the Amazon and Yukon Rivers, respectively, 87% and 78% of the Fe, 69% and 73% of the Mn, and 71% and 69% of the Ni are transported in this form. Comparing the concentrations of transition metals carried to the oceans with the concentrations on the continents, a high Cu ratio (5 to 7) indicates continental depletion or river output enrichment; a moderate ratio (1.1 to 1.7) for Ni, Co, and Cr indicates intermediate depletion or enrichment, and a near-unity ratio for Fe and Mn indicates little depletion or enrichment. The sediments transport >97% of the total mass of transition metals to the world9s oceans.