Soil development in a Maar lake basin in central Mexico based on micromorphology and magnetic mineralogy

Abstract

The weathering and diagenesis processes in the soils of a basin usually produce secondary minerals, which can be transported to the lake basins as part of the detrital fraction. The occurrence of magnetite in lake sediments, or the maghemitization of (Ti)-magnetites, may indicate pedogenic activity and weathering before transporting them to lakes. This study analyzes the differences in soil development in the Laguna de Majahual Maar Lake catchment soil profiles under a tropical-humid climate. Three soil profiles were sampled at different depths in the rim lake basin of Laguna de Majahual (MJ1, MJ2, and MJ3). Magnetic mineralogy and micromorphology analyses were completed, complemented by color, grain size, and mineralogy from the bulk sample and the clay fraction by X-ray diffraction (XRD). The basin's local topography variability influences the Laguna de Majahual soils' magnetic properties and degree of pedogenesis. The two soils in the upper part (MJ1 and MJ2) are formed mainly by the A, AB, and C horizons, suggesting several sedimentations and pedogenesis cycles. At the same time, the MJ3 profile close to the lake's surface shows well-developed Bt and Btg horizons. The micromorphology of the A and AB horizons of MJ1 and MJ2 showed abundant fresh plant fragments and a granular structure, while in the Bt and Btg horizons of MJ3, illuviation of clays and accumulation of Fe oxides was identified. The identification of clays in the MJ1, MJ2, and MJ3 horizons revealed the presence of halloysite. In the MJ1 and MJ2 horizons, this is likely due to hydrothermal alteration of volcanic materials. However, in the MJ3 horizon, halloysite formation is more likely the result of extensive leaching in soil profiles under tropical humid climates. Regarding their magnetic mineralogy, titanomagnetite (soft detrital minerals), and magnetite (soft pedogenic minerals) were found, whereas hematite (hard pedogenic mineral) was found exclusively in the MJ3 profile. It is now clearer that the contribution of titanomagnetite and magnetite in the MJ1 and MJ2 profiles is due to the incorporation of new volcanic material deposits, and hematite in the MJ3 profile related to pedogenic processes.