Rock Magnetic Characterisation of Tropical Soils From Southern India: Implications to Pedogenesis and Soil Erosion

In this study we report the rock magnetic properties of surface soil samples and their spatial variability from the northernmost district of Kerala to understand pedogenic processes and soil erosion. The magnetic signal is mainly from fine grained pedogenic magnetite as other sources like anthropogenic and lithogenic magnetite, bacterial magnetite and greigite are absent. The surface soil samples were collected from four locations (Aribail, Kodlamogaru, Miyapadavu and Seethangoli). Magnetic properties (c(lf), c(fd,) c(ARM,) IRM's at different field strengths) determined on these samples exhibit significant variations for the four locations. Aribail samples contain a lower concentration of magnetic minerals whereas Miyapadavu samples exhibit higher values. The magnetic mineralogy is softer and the magnetic grain size finer in Miyapadavu samples whereas it is harder and coarser in Aribail samples. The magnetic properties of samples from other locations (Kodlamogaru and Seethangoli) exhibit wide variations. In general, the magnetic concentration increases, grain size decreases and mineralogy become softer as one traverses from NW to SE in the study area. Such variations in soil magnetic properties may be due to the geographical location of sampling sites and vulnerability to erosion, as other factors like parent rock, rainfall, temperature and vegetation are almost similar in all the four locations. Our study also indicates that higher production of pedogenic magnetite occurs in the monsoon season and the time lag between erosion of magnetite during heavy rains and production of 'new magnetite' is not very large. This data would also serve as primary database for future pollution studies.