Rates of Pedogenic Processes in Volcanic Landscapes of Late Pleistocene to Holocene Age in Central Mexico.

The Transmexican Volcanic Belt offers good opportunities to study soil development on volcanic tephra. We studied a soil chronosequence within the Sierra Chichinautzin volcanic field (SCVF), south of the valley of Mexico, and aimed to establish the rates of pedogenic processes. We selected 11 sites located on ¹⁴C dated lava flows of 1835, 3800, 6200, 8000, 10000 and 30500 years BP, at 3100 – 3200 m above sea level, covered with pine-fir forest, with ustic and isomesic soil moisture and temperature regimes. We also included a younger site (1000 years BP) and three older sites (100000 years), two at 3100 masl and one at 2600 masl, from nearby volcanic fields. At each site we described a soil profile, and collected samples from each horizon. We analyzed the accumulation of organic carbon, and mineral neoformations on behalf of clay contents, selective chemical extractions, and X-ray diffraction analyses. Within the SCVF the thickness of the solum, carbon accumulation and Al, Si and Fe extracted with acid ammonium oxalate increased linearly until ages of up to 10000 years by rates of 19 cm ky^-1, 3.4 kg C m^-2 ky^-1, 4.4 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 in the solum. Their increase is linear up to ages of 10000 years at rates of 19 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 (100000 years).