Phytolith Analysis as an approach to comprise the genesis of soils in the SW of Entre Ríos province (Argentina).

PATTERER, N.I.*; ZUCOL, A.F.* AND PASSEGGI, E.*

Mar del Plata

Congreso; 7th International Meeting on Phytolith Research and 4th Southamerican Meeting on Phytolith Research; 2008

In the south west of Entre Ríos province (Argentina), most of the well-developed soils are originated from eolian Quaternary deposits, which are known in the region as the Tezanos Pinto Formation. These soils were developed under herbaceous vegetation and a large part of this area has been replaced by agricultural practices and also by human settlement. Although at present several pedological studies have been carried out on these productive soils, their siliceous microremains are scarcely known, principally in reference to their distribution and origin. In the present contribution, the presence and distribution of siliceous microremains in characteristic soil types of the Diamante Department (La Curtiembre soil-series) are analysed. These soils are aquic Argiudols, characterized by dark brown colours in the superficial horizon, silty clay loam texture, good structure (granular and in block), good provision of organic matter and easiness to work (Plan Mapa de Suelos, INTA, 1991). Two representative profiles were selected from this experience: 2m thick “Cantera Pre-delta” (32° 06´29.4´´S and 60° 37´54.6´´ W), and 1.5m thick “Curva Strobel” (32° 03´32´´S and 60° 36´50.1´´W). Both profiles -with an A horizon (30cm of average thickness), B horizon (40cm of average thickness) and C horizon (105cm of average thickness) - are characterized by silty clay loam or silty loam texture, and granular, in block and mass structures of the dependent type horizon. The phytolith extraction was made following Zucol and Osterrieth’s (2002) methodology that comprises the elimination of soluble salts, the remission of carbonates and organic matter and the desegregation with sodium hexametaphosphate (0.1 N). Size separation was made in three fractions: the fine fraction (- 5 µm), the middle fraction (5-250 µm) and the coarse fractions (+ 250 µm). The middle fraction was used for densimetric separation with sodium polytungstate solution (2.3 g/cm-3). The counting was made in a sample of 400 phytoliths per slide and each phytolith was assigned to a particular morphotype based on the classification and descriptors developed by Twiss et al. (1969), Bertoldi de Pomar (1971), Twiss (1992), Zucol (1996) and ICPNWG (2005). The information was analysed by multivariate numerical analysis to describe phytolith assemblages variations across each profile. The results show that the phytolith assemblages are dominated by graminoid phytoliths, associated in less abundance with ciperoid and arecoid ones and scarce stomatocysts, spicules and diatoms. The graminoid types are characterized by the presence of prismatic elongated phytoliths with smooth or undulated contour, fan-shaped and polyhedrical phytoliths, between among the biggest components in size. Among the smallest phytoliths in size, bilobate and saddle types are present, especially in the upper and lower samples; although in the middle levels, roundel or truncated cones are more abundant. Echinate globular phytoliths are present with scarce variations across the studied profiles. The quantitative analysis of type abundances allows to interpret the presence of three phytolith sources in soil profiles. The first one is the parental material, the Tezanos Pinto Formation, principally abundant in the lower levels of the profiles. The second one originated in a paleocommunity that was present in early stages of the soils development, represented in the middle samples of the profiles that clearly show evidence of material mobility along each profile. The third one is the phytolith assemblage of the upper levels, with a composition mainly linked to the current vegetation. These analyses demonstrate that these soils developed in a mixed community in association to the grassland dominated by panicoid and arundinoid affinities, jointly with palms.