Topography-dependent formation and transformation of lithogenic and pedogenic iron oxides on a volcano under a tropical monsoon climate

Abstract

Volcanoes are widely distributed on the surface of the Earth and on other planets. Volcanic activities play a key role in the long-term evolution of the planetary atmosphere and geomorphology. Iron oxides on volcanoes often exist as the products of rapid magmatic oxidation driven by earlier lithogenesis or the weathering of primary iron-bearing minerals driven by later pedogenesis. The antiferromagnetic (AFM) and ferrimagnetic (FM) particles associated with the formation and transformation of iron oxides dominate the color and magnetism of soil and can potentially serve as an indicator to trace these processes. In this study, we systematically investigated the subsoils derived from a Pliocene volcano under a tropical monsoon climate. We observed that the crystalline iron oxides are differentiated along the leeward and windward slopes, while amorphous iron oxides are mainly differentiated along the shady and sunny slopes. The AFM hematite is more concentrated on the purplish top and the reddish leeward slopes, while the AFM goethite is more concentrated on the yellowish windward slopes. Coarser FM particles accompanied by lithogenic hematite form via early lithogenesis at the top, while more fine FM particles are concentrated on the leeward slopes with the neoformation of pedogenic hematite. Fewer but finer FM particles form with goethite on the windward slopes. These results suggest that FM particles could coexist with AFM hematite via lithogenesis or pedogenesis and that goethite would be a more reasonable pedogenic indicator in volcanic regions.