Abstract

Intense chemical weathering in the tropical Amazon River basin strips crustal material of alkali and alkaline earth metals, leaving a residue composed predominantly of oxyhydroxides of Fe, hydroxides of Al, and cation poor clays. This lateritic residue often coats soil particles. Within the marine environment, lateritic particle coatings on sand undergo, or have the capacity to undergo, rapid reaction and mineralogical alteration: 1) at least 20% of Fe in coatings can be readily liberated during short term incubation in anoxic sediment; 2) coatings change color from red/white to green/yellow during burial (implying Fe (super 3+) --> Fe (super 2+) ); 3) average Mg cation content of coatings increases (net addition) from 1.85% near the Amazon's mouth to 4.76% 120 km from the mouth (or with burial); 4) a narrowing of the measured range and a slight increase of Si/Al occurs near-mouth to offshore; 5) Mg-Fe-Al layer silicates are probably forming by transformation of kaolinite, or other cation stripped material of similar composition, in coatings. Mg uptake by lateritic particle coating reactions (assuming comparable involvement of all particle sizes) could account for up to 67% of the Mg discharged in solution by the Amazon River. Evidence for Mg-Fe-Al layer silicate formation supports the hypothesis that ancient ironstone minerals formed in some cases by direct alteration of lateritic material.

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