Rates of pedogenic processes in volcanic landscapes of late Pleistocene to Holocene age in Central Mexico

Abstract

The Transmexican Volcanic Belt and its many volcanic fields of different ages offer good opportunities to study soil development on volcanic tephra of intermediate to basaltic composition. We studied a soil chronosequence within the Sierra Chichinautzin volcanic field (SCVF), south of the basin of Mexico, and aimed to establish the rates of pedogenic processes. This field has been active for around 50,000 years, and produced 221 cinder cones with their respective lava flows. We selected 11 sites located on 14C dated lava flows of ages between 1800 and 30,500 BP, at 3100–3200 m above sea level, covered with pine–fir forest, with an ustic soil moisture regime and an isomesic soil temperature regime. We also included a younger site (1000 BP) and three older sites (>100,000 years), two at 3100 masl and one at 2600 masl, from nearby volcanic fields to widen the time frame of the chronosequence. Soil profile samples were analysed for total organic carbon as well as for mineral neoformations related to clay contents, selective chemical extractions, and X-ray diffraction analyses.Within the SCVF the total soil thickness, carbon accumulation and Al, Si and Fe extracted with acid ammonium oxalate increased linearly with age on surfaces up to 10,000 years old at rates of 19 cm ky−1, 4.1 kg C m−2 ky−1, 4.6 kg Al, 2.7 kg Si and 2 kg Fe m−2 ky−1, respectively. Crystalline clay and iron oxide formation reach a maximum at the oldest site located at 2600 masl of 1650 kg clay m−2 and 70 kg Fed m−2. Their increase is linear up to ages of 10,000 years at rates of 22 kg clay m−2 ky−1, and 2.8 kg Fed m−2 ky−1. Thereafter the rates are much slower. Allophane and allophane-like minerals dominate in the clay fraction at all sites of the chronosequence, and small amounts of halloysite can be identified in soils older than 6200 years, while kaolinite was only identified at the three oldest sites (>100,000 years).The linear increases of all indicators of pedogenesis in the first 10,000 years are presumably driven by recurrent tephra deposition during this time frame. Thereafter, erosion and colluviation processes seem to disturb pedogenesis, changing its rates and redistributing its products in the landscape.