Micromorphology and phytoliths study in coastal dunes of southeastern Pampas Plains, Argentina.

OSTERRIETH, MARGARITA; FERNÁNDEZ HONAINE, MARIANA

Cambridge, Inglaterra

Otro; 4th International Meeting on Phytolith Research; 2002

The evolution of the coastal dunes during the late Holocene (1000 to 500 years BP) in the Pampean Plains of Buenos Aires Province, involves the combination of : marine erosion, Aeolian action, dynamics of vegetation cover, soil development and human activities. Results of phytoliths, micromorphological and mineralochemical analysis are presented . The interrelation between biomineralization, weathering rate of non biogenic and biogenic silica and soil profile development are performed. The amorphous silica fraction of the disturbed samples was concentrated with sodium polytungstate at a density of 2.3. The undisturbed samples were impregnated in synthetic resin for the thin section description. The semi-quantitative studies of the elementary components were made by means of EDAX and X ray diffraction. The dominant soils are typical Quarzipsaments and Entic Hapludolls which present scarce development (C-2AC-2C); alkaline pH (8-9, 2) and low content of organic matter (0.4 to 4 %). Under SEM, many phytoliths exhibited weathering features like dissolution micro-cavities. These soils present a single grain structure to a bridge grain structure, and the related distribution pattern of a coarse and material is Monic to Gefuric. The skeleton grains are similar in size (fine sand) shape, and mineralogy. No cutan or subcutan were observed, but a little plasma separation existed around some skeleton grains. Cements constitute small adhesive bridges at grains points in a skeleton matrix. Allophane was detected in the AC horizon. The material coating and bridging the frame work grains is enrich in silica, and also contains calcium, iron, sodium and chlorine. The alteration levels in the silicobioliths are not related with temporary events , but rather they are related with the biogeochemical, and with the organic components. These interactions allow the formation of organo-mineral complexes starting from biogenic silica, like part of the matrix of the soils. It favors the development and persistence of structure peds, and also the resistance to the erosion of these soils located in highly unstable coastal environment.