SILICA BIOMINERALIZATIONS IN HOLOCENE PEATLAND ENVIRONMENTS, TIERRA DEL FUEGO, ARGENTINA

BENVENUTO, MARIA LAURA; FERNÁNDEZ HONAINE, MARIANA; CORONATO, A; RABASSA, JORGE; OSTERRIETH, MARGARITA

Mar del Plata

Otro; 7th International Meeting on Phytolith Research; 2008

The peatlands of Isla Grande de Tierra del Fuego (52-55º S), Argentina, have preserved peat sequences and clastic sediments of up to 11 m. They have been investigated for many palynological studies because they keep very important records of paleoenvironmental information since the Late Glacial (15-10 ka A.P.). In this paper, the first results of ongoing research are presented, aiming to evaluate the potential of Fuegian peatlands as reservoirs for silica biomineralization as indicators of paleoenvironmental conditions. Two sedimentary sequences were studied: 1) the Río Turbio minerotrophic mire, developed in a deltaic plain generated in the headlands of Lago Fagnano; and 2) stratified deposits along the fluvial banks of Arroyo Catalanes, north of Lago Fagnano. Firstly, samples were described by binocular magnifier. Due to differences in the materials, two different treatments were used. Phytoliths from Rio Turbio samples were extracted using the Labouriau’s (1983) calcination technique and silica content was calculated based upon the differences of weights. Phytoliths from Arroyo Catalanes samples were extracted using Na politungstate (Madella et al.,1998). At least 200 phytoliths were counted and the percentage of diatoms, sponge spicules, Crysophyceae stomatocysts and phytoliths were calculated. Phytoliths were classified according to ICPN and phytolith morphotype percentage was calculated. Mesoscopic observation of the Río Turbio samples revelead that over 90% of the material is composed by vegetal remains (leaves, glumes, rootlets and unidentified material). The silica content values vary between 1 and 21%. Phytoliths represent the largest percentage (50-97%) of the biogenic silica fraction, except in two levels. Four levels contained sponge spicules (<2%). Stomatocysts percentages varied from 3.54 and 42.56, whereas diatom values ranged between 0 and 36.62%. Intermediate levels (1-1.3m) would have developed in the largest saturation conditions, probably in temporary ponds and small lakes formed by subsidence processes or changes in stream bed orientation. The dominant morphotypes are elongate phytoliths and grass phytoliths (rondels, bilobates and trapeziform crenate). In level 0.2-0.3m the largest percentage of articulated phytoliths (7%), the largest percentage of elongate >30μm phytoliths and the absence of degraded elongate, would indicate that this level have evolved under important environmental stability conditions, which would have permitted the preservation of articulated and larger phytoliths. It also could indicate a fast establishment of different plant species. The mesoscopic study of the Arroyo Catalanes samples showed differences with Rio Turbio samples: only the AC4 level showed some plant remains (70%) consisting in leaves, rootlets and material strongly mineralized and/or unidentified. The phytolith content is low, between 4-5%, probably indicating that it was a poorly vegetated alluvial plain. The highest phytolith content (23%, AC4 level) would indicate a thicker plant cover which could be related to a modification in stream pattern. The dominant phytolith morphotypes are elongate <30μm phytoliths. Elongate >30μm phytoliths and prickles abundances increase with depth, meanwhile the unidentified phytoliths and trapeziform crenate decrease. The latter ones are associated to Poaceae, subfamily Pooideae, and are very abundant in the AC2F sample. This would reflect a thicker plant cover, consequence of a modification in stream pattern or disminution of environmental moisture, or stream recession.