Topographical influence on pedogenesis – Insights from Rock Magnetism

Topography being a pedogenic factor influences the physico-chemical and biological properties of soil profiles. Besides, the magnetic characteristics of soil profiles may also be varied, as iron minerals in soils are strongly redox sensitive. However, hardly any study has been reported from the topographic implication on the magnetic characteristics of soils to date. Hence, the present study aims to delineate the influence of topography on the in-situ soil profiles developed on hill topography by studying their rock magnetic properties.Two soil profiles each, formed at different altitudes of the two hills named Doddabathi Siddeshwara (14.58° N; 75.66° E) with peak ~650 m above mean sea level (AMSL) and Elimale (12.5° N; 75.5° E) with peak ~200 m AMSL were chosen for the investigation. Rock magnetic measurements were carried out on soil samples; Rock Magnetism is a geophysical technique applied on environmental samples such as rocks, soils, dust and sediments for determining their intrinsic magnetic properties namely, magnetic susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetisation. These intrinsic magnetic properties are the result of concentration, mineralogy and grain-size of magnetic minerals present in the samples that are unique for every environmental condition. Hence, determination of rock magnetic properties of environmental samples leads to the corresponding environmental set-up in which the samples are formed. Particle size and organic matter content were also estimated to support the rock magnetic interpretation. Based on the investigation it was concluded that topography has indeed influenced the magnetic properties of soil profiles of the hills, irrespective of rainfall and organic matter content. Both depletion in magnetic mineral concentration and coarsening of magnetic grains towards the profile-top were observed in all the soil profiles; this indicates the effect of soil erosion. The decrease in the magnetic properties of hill-top soil profiles was determined to be dominantly due to the removal of finer pedogenic magnetic minerals. Whereas the lowered magnetic characteristics of the hill-bottom soil profiles were attributed to the mixing of bulk materials eroded from higher elevations. Meanwhile, the greater difference in magnetic properties of Doddabathi soil profiles than that of Elimale was attributed to the combined effect of slope-angle and lateral-distance between their respective hill-top and -bottom profiles. Thus, exploring the applicability of rock magnetism to address the spatial-spread of magnetic minerals on hill topography, the study confirms the suitability of the technique to be used as a proxy for the same.