Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

Abstract

[1] The iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate. Previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall; the correlation seems to be reversed under extreme climates with high rainfall. Here we present a transect of saprolitic soils from Hainan Island, South China, with high rainfall (1440∼2020 mm/yr), little temperature variation (23∼24°C), and extreme weathering. Along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall, whereas goethite concentration displays a large increase. However, there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect. Goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall-driven processes. These pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite, magnetic susceptibility and rainfall control of an inflection point. This study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range. A conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point.