Long-term vegetation-induced goethite and hematite dissolution-reprecipitation along the Brazilian Atlantic margin

Abstract

Distinctive sediments containing mostly quartz, kaolinite, and both detrital and authigenic hematite and goethite blanket ~5000 km of the Atlantic coast of Brazil from Rio de Janeiro all the way to the mouth of the Amazon River. The sediments represent a significant period of continental erosion followed by renewed weathering. Here we present (U-Th)/He ages of detrital and authigenic goethites and hematites collected from six weathering profiles in Espírito Santo, southeastern Brazil. Detrital goethites are as old as ~26 Ma and the oldest authigenic hematite is ~20 Ma, suggesting that erosion, transportation, and deposition of the sediments occurred in the 26–20 Ma period. Intense post-depositional weathering and ferruginization of the sediments suggest that precipitation-dissolution-reprecipitation of iron oxides and oxyhydroxides were strongly controlled by biologically driven weathering reactions. (U-Th)/He geochronology of 158 grains of authigenic goethite and hematite precipitated during biologically mediated water-rock interaction yield 137 results in the 5–0.6 Ma period, suggesting that tropical climate and abundant vegetation dominated the coast of Espírito Santo since the Pliocene.