Mineral magnetic properties of ultisol profiles from tropical southern India

The magnetic grain size, mineralogy and concentration of five ultisol sequences (lateritic soil profiles) developed on different parent rocks (syenite, dolerite, charnockite, gneiss and sandstone) in southwestern India were investigated to gain a better understanding of the pedogenic processes under tropical climate. Field investigations reveal the presence of various horizons in the lateritic profiles, such as saprolite, saprock, pallid zone, mottled zone, pebble horizon and top soil. These horizons exhibit differences in their mineral magnetic, Fourier Transform Infrared Spectroscopic (FTIR), Diffuse Reflectance Spectroscopic (redness %, amplitudes of goethite (AGt) and haematite (AHm)) and particle size properties. Magnetic susceptibility (χ lf ) values of the ultisol profiles varies from 5.8 to 2858.7 × 10 −8 m 3 kg −1 , whereas percentage frequency dependent susceptibility (χ fd %) ranges between 0.15% and 14.0%. The FTIR spectra reveal the presence of minerals like haematite, goethite, kaolinite and quartz. The upper horizons (top-soil and pebble horizon) are marked by magnetic enhancement by ultra-fine grained superparamagnetic (SP) minerals with low coercivity, most likely magnetite/maghemite. Whereas, the lower horizons (saprock, saprolite, pallid zone horizons) exhibit no magnetic enhancement and are characterized by antiferromagnetic minerals, e.g., haematite/goethite. The unweathered parent rock is characterized by low coercivity minerals of coarser magnetic grain size with a little to no superparamagnetic (SP) grains. However, the magnitude of enhancement of fine grained ferrimagnetic grains in the top soil in relation to bottom horizons varies among the five lateritic profiles. The degree of enhancement and production of the fine-grained superparamagnetic grains in topsoil decreases in the order charnockite > syenite > sandstone > gneiss > dolerite. Because the doleritic parent rock exhibits a high initial Fe concentration, it is difficult to ascertain the degree of pedogenesis, due to a strong lithogenic signature, which decreases towards the profile top. The tropical soils in the region have undergone a higher degree of pedogenesis with increased magnetic mineral concentration compared to temperate soils. However, magnetic grain size appears to be the same in both soil types. The magnetic enhancement in the topsoil may be due to the neoformation of fine-grained SP magnetite, aided by sufficient Fe supply, alternate wetting and drying cycles, dehydration, oxidation and redox conditions.