Polygenic soils in the southern central Ecuadorian highlands as the result of long-lasting pedogenesis, geodynamic processes and climate change
Résumé
In the high altitude (>3000 m asl) grasslands of northern Ecuador, the P ' aramo ecosystem, soils generally formed on recent volcanic ashes being <10,000 years old in the northern and central part of the cordilleras. With time these soils evolved in to non-allophanic Andisols with hydric properties such as evidenced in profiles from the Cajas massif located in the western central Cordillera. However soils at the Fierro Urcu massif and at the On similar to a plateau at the eastern Cordillera formed on a non pyroclastic indurated parent material. Compared to non-allophanic Andisols from the Cajas massif, they present similar morphological properties with a dark 50 cm thick carbon-rich (80-120 g kg-1) topsoil with a very low bulk density (<0.65 g cm-3) and high water retention. These top soils appear to have originated from fall-out, derived from close and still active volcanoes of Sangay and/or Tungurahua, both located in a distance of 200-250 km NNE of the On similar to a Massif, respectively. Geochemical determinations of trace and rare earth elements, however, confirm the lack of volcanic ash contribution in the Saraguro and On similar to a samples based on Eu/Eu* and Ce/Ce* anomalies relative to the lack of such anomalies in the Cajas samples. The On similar to a and Saraguro soils appear to be the result of a long-lasting double pedogenic process. The basement of the Saraguro soil is a Ferralsol with gibbsite and kaolinite and the On similar to a soil is a 5 m thick kaolinitic Ferralsol. The upper part of these profiles are the result of a major climatic change with increasing colder conditions, generating an increase in organic matter content and a very acidic environment leading to the instability of kaolinite and gibbsite, thereby liberating large amounts of free aluminium and iron oxides. The evolution of these very highly evolved Ferralsols into soils with Andic properties is proposed to be the result of rapid tectonic uplift calculated at a rate exceeding 0.6 mm/year in the region leading to a general decrease in temperature limiting carbon mineralization and favouring the formation of organo-metallic complexes. The soils studied here reveal compelling new evidence and information about the uplift of a part of the Ecuadorean Andes.