Génesis de las Toscas de Suelos Pampeanos.

BUSCHIAZZO, D. E.

Acumulaciones calcáreas de suelos pampeanos

AACS-INTA

Lugar: Buenos Aires; Año: 2014; p. 1 - 2

Hardened lime accumulations are common in soils of arid and semiarid regions. Such accumulations receive different names in different parts of the world: Calcretes in USA, Nari in Israel, Caliche in Spain and Tosca in the Argentinean Pampas. The genesis of Tosca is not clear. Open questions are whether they were formed by infiltrating (pedogenesis) or by ascending waters and if they were formed in a continental or marine environment. In this chapter several geo-, macro- and micromorphological features, and some geochemical indexes were analyzed and CaCO3 balance sheets were performed in order to determine if the CaCO3 of Toscas of Pampas soils was accumulated by ascending or descending sea or continental waters. With this purpose 16 profiles distributed in three different climatic regions of the Pampas (Balcarce: humid region, Bahia Blanca: semiarid region and Algarrobo: arid region) were analyzed. The occurrence of thicker and harder Toscas in hills and plains than in slopes and particularly basins in all three analyzed sites, were geomorphological features that indicated a per descensum origin of the lime. In the case that Toscas were formed by ascending waters, thicker and harder accumulations should be expected close to water-tables, i.e. in the lowest relief positions, rather than in the highest ones. The following macromorphologial features were indexes of a pedogenic genesis of the CaCO3: - Laminar structures at the upper- and nodular at the deeper Tosca parts. When Toscas are formed by descending waters they growth from the bottom to the top. At the first forming stages CaCO3 forms nodules at the bottom. At advanced forming stages the infiltrating waters found a barrier formed by the previous precipitated CaCO3, making the new CaCO3 to distribute laterally. - Sharp limits between the Tosca and the overlying soils and diffuse between the Tosca and the underlying sediments. - The lack of reductimorphic features (iron or manganese concretions or flakes) below the Toscas. If CaCO3 was accumulated by ascending waters, water-tables should have been frequently close to the lower Tosca limits, producing the reduction and mobilization of Fe and Mn oxides. - The occurrence of ?carbonate beards? (CaCO3 accumulations below stones or aggregates). At a micromorphologial level, the following evidences speak for a per descensum movement of the lime: - A matrix formed mainly by micritic calcite. It is known that vadose environments (ascending waters) have better conditions for the growth of calcite crystals than pedogenic environments. Therefore, micrite diameters between 1 and 4 µm) is more common than microsparite (4 to 20 µm in diameter) under pedogenetic forming conditions. - ?Microstalactites? in the upper Tosca pores, which genesis is similar to that of ?carbonate beards?. - Ooids and lamellae in laminar structures of Toscas. These are indexes of a lateral movement of calcite above preexisting hardened lime accumulations - Calcified algae hyphen in the upper Tosca portions. This is an index of biological activity under pedogenetic conditions. Similar B:Ga and Mg:Ca quotients in Toscas placed at different sea distances indicated a formation of the Tosca under continental rather than under sea water conditions. As B and Mg are more abundant in marine waters and Ga and Ca in continental sediments, an increase of these quotients was expected in Toscas formed under sea water conditions. A tentative model of Tosca formation was presented. This was based on cyclic sequences of loess sedimentation, soil formation and soil erosion events, occurred under variable climatic loess sedimentation, soil formation and soil erosion events, occurred under variable climatic conditions of the Pleistocene. CaCO3 balance sheets indicated that in order to accumulate the current CaCO3 amounts of the Tosca, the pedogenesis of a 3 to almost 16 m thick sediment was needed. Current climatic conditions are suitable for Tosca dissolution in the humid Balcarce region and for further CaCO3 accumulation in the other two dryer regions.