Iron Loss During Continental Weathering in the Early Carboniferous Period Recorded by Karst Bauxites

Significant iron (Fe) loss can occur during continental weathering and efflux to the ocean via runoff, historically affecting global Fe cycling and marine ecosystems. Here, we report extremely low Fe content in early Carboniferous (ca. 340 Ma) bauxites in southwestern China. These bauxites were formed by redeposition of terrestrial soils along the paleo-continental margin of the western South China Plate in warm climates. The bauxites contain high delta Fe-56 (-0.17 parts per thousand to +1.15 parts per thousand) values with a negative correlation between Fe2O3 and delta Fe-56, indicating that a substantial amount of Fe(III) was reduced to isotopically light dissolved Fe(II) and effuxed to the ocean via reductive dissolution under anoxic conditions. The low C-org content and low Fe-HR/Fe-T, Mo/Al, U/Al, and V/Al ratios of bauxite suggest that this reduction process occurred during the pedogenic (continental weathering) rather than the depositional/diagenetic stage of karst bauxite formation. Most of the dissolved Fe(II) were rapidly re-oxidized to Fe(III) and transported toward the paleo-continental margin forming iron ores with delta Fe-56 values around zero (-0.13 parts per thousand to +0.16 parts per thousand). The negative correlation between Al2O3 and Fe2O3 contents in global karst bauxites suggests common Fe loss processes during continental weathering in geological periods favoring karst bauxite formation, such as during the Carboniferous, Permian, and Cretaceous periods and the Cenozoic era. Karst bauxite may thus provide a record of Fe loss during continental weathering and act as an indicator of enhanced Fe flux to oceans. Bauxite deposits are weathered residues enriched in aluminum (Al). They are of economic importance and record critical information concerning extensive weathering in geological history. Karst bauxite has extremely low iron (Fe) contents, and the mechanism of Fe loss is poorly understood. We observed high delta Fe-56 values in the early Carboniferous period (ca. 340 Ma) karst bauxite in southwestern China, suggesting the bauxite formation under water-saturated anoxic conditions with extensive loss of isotopically light Fe(II). Our results indicate strong decoupling of Fe and Al during the formation of karst bauxite, which may thus act as an indicator of Fe loss during continental weathering.