Magnetite weathering in a Vertisol with seasonal redox-dynamics

We investigated soil samples from a Vertisol in a flat lying flood plain in the savannah woodland of southern Mali, which is characterized by a water regime governed by regularly occurring wet and dry seasons. The soil was subdivided into four horizons which differed in the content and speciation of the redox-sensitive elements and in the length of waterlogging. Magnetically, the profile could be subdivided into two parts. The magnetic susceptibility, the hysteresis parameters Mr, Ms, and Bc, as well as the coercivity of remanence Bcr, indicate that the upper two horizons differed from the lower two horizons by a higher content of magnetite and a larger contribution of multi-domain size magnetite grains as compared to single-domain size grains. A comparison of the chemical and magnetic data suggests that dissolution of magnetite in our soil is driven by a seasonal change between anoxic and oxic conditions.