Paleoclimatic implications of micromorphic features of a polygenetic soil in the Monegros Desert (NE-Spain) .

Abstract

Pedofeatures can be repositories of information about soil forming factors such as climate. The aim of this work is to provide a model of interpretation of a polygenetic soil in the Monegros desert (Ebro Basin, NE-Spain) and its relationship to environmental changes during the Quaternary. To achieve this goal, the physical, chemical, mineralogical and especially the micromorphic pedofeatures of this profile were studied. Carbonate accumulations extend into all of the horizons of the profile. The paleosol has a thick petrocalcic horizon at the top, with a massive-laminar structure comprising layers of micrite and sparite that sometimes form pendants. Towards its base, the petrocalcic horizon contains a spaced framework of orthic micrite nodules packed between relatively pure micritic laminar bands. Below the petrocalcic horizon, coatings and infillings of microcrystalline calcite occur in old channels, and soft concretions (some of them geodic) indicate an in situ accumulation process (Bkc, calcic horizon). Another calcic horizon with orthic nodules of calcite, impregnative and diffuse (Ckc), is present at the bottom part of the profile. Between the two nodular calcic horizons, two recarbonated argic horizons are found (Btkc and Btk) with coarse orthic nodules of dense micrite superimposed on textural pedofeatures. These textural micromorphic pedofeatures are: (1) interbedded microlaminated clay pockets not associated with current or past pores and (2) microlaminated clay and silt (dusty clay) present as weakly oriented coatings on channel walls. Reduction pedofeatures are associated with textural ones: (1) coatings of manganese oxides around pore channels and cracks, and (2) nodules of manganese and iron oxides within the peds. The presence of calcic horizons alternating with argic horizons, all positioned below the petrocalcic horizon, confirm fluctuations in paleohydrological conditions in the Pleistocene. Its presence indicates that the oldest soil corresponds to a Calcic Luvisol-like pedotype, which is overlain by an Haplic Calcisol-like pedotype and this, in turn, by a Petric Calcisol-like pedotype. This superposition of profiles indicates, within the mentioned climatic changes, a tendency towards increasing dryness during the Pleistocene in the semiarid Ebro Valley.