The formation of iron oxides and magnetic enhancement mechanisms in northern Iranian loess-paleosol sequences: Evidence from diffuse reflectance spectrophotometry and temperature dependence of magnetic susceptibility

Abstract

The magnetic properties of the loess-paleosol sequences from northern Iran provides a model for understanding pedogenic processes and their relationship to paleo-precipitation in this region. For the first time, we present diffuse reflectance spectrophotometry (DRS) analyses, color measurement analyses, and temperature-dependent susceptibility (χ-T) in argon from two late Pleistocene loess-paleosol sequences in northern Iran. Magnetic iron oxide minerals, principally magnetite, maghemite, hematite, and goethite were identified in the study sections. The results of DRS and color measurements show that a* values, the hematite to goethite ratio and the maghemite to hematite ratio, exhibit a significant positive correlation with the pedogenic process and paleosol formation and are superior to low-frequency susceptibility alone for estimating paleoprecipitation. Our data also indicates that pedogenic maghemite-magnetite formation and dissolution/reprecipitation are major contributors to the magnetic enhancement of loess-paleosol sequences and might indicate a seasonal aspect for pedogenic maghemite-magnetite formation. We propose the DRS-derived illite to chlorite ratio as a useful index for assessing the intensity of silicate mineral weathering in the northern Iranian loess-paleosol sequences. In addition, DRS can be used to discriminate between pedogenic and detrital hematite of loess-paleosol sequences. We also propose that the χ-T cooling and heating ratio model is a quantitative proxy for magnetic enhancement in soils and paleosols of loess-paleosol sequences.