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

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 Espirito 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 biologi-cally 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 Espirito Santo since the Pliocene.

Automated summary

Distinctive sediments containing quartz, kaolinite, detrital and authigenic hematite and goethite are present along the Atlantic coast of Brazil. These sediments represent a significant period of erosion and weathering, with strong influence from biologically driven weathering reactions. The ages of detrital and authigenic goethites and hematites suggest that sedimentation occurred between 26-20 million years ago. Furthermore, (U-Th)/He geochronology indicates a period of tropical climate and abundant vegetation dominating the coast of Espirito Santo since the Pliocene.