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

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

Mar del Plata

Encuentro; 7th IMPR – 4th SMPR; 2010

Society for Phytolith Research GEFACS

In the southwest of the province of Entre Ríos(Argentina), most of the well-developed soils areoriginated from eolian Quaternary deposits, whichare known in the region as the Tezanos PintoFormation. These soils were developed underherbaceous vegetation and a large part of this areahas been replaced by agricultural practices and alsoby human settlement. Although at present severalpedological studies have been carried out on theseproductive soils, their siliceous microremains arescarcely known, principally in reference to theirdistribution and origin. In the present contribution,the presence and distribution of siliceousmicroremains in characteristic soil types of theDiamante Department (La Curtiembre soil-series)are analysed. These soils are aquic Argiudols,characterized by dark brown colours in thesuperficial horizon, silty clay loam texture, goodstructure (granular and in block), good provision oforganic matter and easiness to work (Plan Mapa deSuelos, INTA, 1991). Two representative profileswere 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 -withan A horizon (30cm of average thickness), Bhorizon (40cm of average thickness) and C horizon(105cm of average thickness) - are characterized bysilty clay loam or silty loam texture, and granular, inblock and mass structures of the dependent typehorizon. The phytolith extraction was madefollowing Zucol and Osterrieth’s (2002)methodology that comprises the elimination ofsoluble salts, the remission of carbonates andorganic matter and the desegregation with sodiumhexametaphosphate (0.1 N). Size separation wasmade in three fractions: the fine fraction (- 5 μm),the middle fraction (5-250 μm) and the coarsefractions (+ 250 μm). The middle fraction was usedfor densimetric separation with sodiumpolytungstate solution (2.3 g/cm3). The countingwas made in a sample of 400 phytoliths per slideand each phytolith was assigned to a particularmorphotype based on the classification anddescriptors developed by Twiss et al. (1969),Bertoldi de Pomar (1971), Twiss (1992), Zucol(1996) and ICPNWG (2005). The information wasanalysed by multivariate numerical analysis todescribe phytolith assemblage variations across eachprofile. The results show that the phytolithassemblages are dominated by graminoid phytoliths,associated in less abundance with ciperoid andarecoid ones and scarce stomatocysts, spicules anddiatoms. The graminoid types are characterized bythe presence of prismatic elongated phytoliths withsmooth or undulated contour, fan-shaped andpolyhedrical phytoliths, among the biggestcomponents in size. Among the smallest phytolithsin size, bilobate and saddle types are present,especially in the upper and lower samples; althoughin the middle levels, roundel or truncated cones aremore abundant. Echinate globular phytoliths arepresent with scarce variations across the studiedprofiles. The quantitative analysis of typeabundances allows to interpret the presence of threephytolith sources in soil profiles. The first one is theparental material, the Tezanos Pinto Formation,principally abundant in the lower levels of theprofiles. The second one originated in apaleocommunity that was present in early stages ofthe soils’ development, represented in the middlesamples of the profiles that clearly show evidence ofmaterial mobility along each profile. The third oneis the phytolith assemblage of the upper levels, witha composition mainly linked to the currentvegetation. These analyses demonstrate that thesesoils developed in a mixed community inassociation with the grassland dominated bypanicoid and arundinoid affinities, jointly withpalms.