Soil formation and mineralogy of a Rhodic Luvisol - insights from magnetic and geochemical studies

Relict terra rossa soil from the most south-eastern part of Bulgaria, characterized by transitional Mediterranean climate, has been comprehensively studied by integrating magnetic, geochemical and spectroscopic methods to reveal the origin, pedogenic processes and phases in soil development of this particular soil type. The red colored Rhodic Luvisol is developed on metamorphosed Triassic limestones. Magnetic methods, which include thermomagnetic analysis of susceptibility, isothermal remanent magnetization (IRM) acquisition and thermal demagnetization, IRM component analysis, hysteresis measurements, low-temperature (down to 10 K) IRM behavior, anhysteretic remanence and frequency dependent susceptibility, indicate the presence of three major magnetic phases - maghemite, hematite (Hm) and goethite (Gt). Hematite and goethite are identified also by diffuse reflectance spectroscopy (DRS). Depth variations of the ratio Hm/(Hm + Gt), deduced from the DRS spectra show higher hematite content in the upper soil horizons (A + B), while goethite's contribution is enriched in the lowermost part of the profile. A similar ratio, based on the established magnetic proxies for hematite and goethite, was constructed and its variations were compared with the DRS data. The magnetic proxy for Hm/(Hm + Gt) reflects the variations in the remanence-carrying mineral fraction of hematite and goethite and the obtained difference with the DRS data are ascribed to the presence of the paramagnetic (or superparamagnetic) goethite in the A and illuvial Bt1 and Bt2 horizons. The low ratio Feo/Fed between dithionite (Fed)- and oxalate (Feo)-extractable iron, and the large proportion of extractable iron with respect to total iron (Fed/Fetot) indicate an advanced degree of weathering. Depth variations of magnetic parameters and ratios (χlf, χfd, S-ratio) suggest magnetic enhancement with SP-SD maghemite grains, accompanied by magnetically stable magnetic carriers in the soil, while the parent material is magnetically depleted. Based on grain-size variations of the pedogenic maghemite phase, deduced from a χARM vs. χfd plot, and variations in the content of hematite and goethite in depth, in the uppermost part of the profile, aging of the pedogenic iron oxides is assumed (grain growth from SP to SD and larger), while in the bottom part of Bt1 and the Bt2, changes only in the concentration of maghemite are inferred. The presence of goethite in different parts of the profile and the mineralogy of Fe-Mn nodules are linked to more recent (Holocene) pedogenic changes in the profile. Analyses of trace and rare earth element content and magnetic data suggest possible contributions of aeolian dust flux from Sahara during the soil formation.