Relationship between surface-soil magnetic susceptibility and its shallow vertical distribution

Measurements of magnetic susceptibility on the soil surface are routinely used for, e.g., assessment of distribution of atmospherically deposited dust particles rich in iron oxides. In general, the data obtained using Bartington MS2D coil integrate signal from a depth down to about 10 cm, with non-linear response function. Therefore, it is believed that the iron oxides in this layer may be due to atmospheric deposition, weathering of lithogenic rocks, and pedogenic processes occurring directly in the soil. In order to assess the significance of these sources, as well as to observe the stratification of the soil column, shallow vertical soil cores, usually down to 30 cm, are used. However, there is no comprehensive comparison of the vertical distribution of magnetic susceptibility, projected on the surface, with the values actually measured on the soil surface. In our contribution, we use the known response function, defining the weight of magnetic susceptibility with depth, to obtain the total model susceptibility projected on the soil surface, and compare it with the real data measured in the field. Our results show that good agreement between the measured and modelled surface values is not a general rule. Thus, the use of shallow vertical distribution of susceptibility in terms of interpreting the data measured on the surface is subject to ambiguities and doubts.