Silicophytoliths in sedimentary sequences in the Laguna Potrok Aike, Santa Cruz, Argentina

OSTERRIETH, M; ALVAREZ, MF; BUEZAS, G; ROJO, J; BORRELLI, N; FERNÁNDEZ HONAINE, M AND L BENVENUTO

Buenos Aires

Workshop; 1er Workshop Argentino PASADO; 2010

The biomineralization process is the precipitation of a mineral or mineraloids as a result of the metabolic activity of organism (Jahren, 1996; Osterrieth, 2004). Phytoliths are biomineralizations deposited in plant tissues, most of them compounded by hydrated SiO2 (Parry and Smithson, 1964; Blackman, 1971; Piperno, 1988). Following degradation of the tissue, phytoliths are incorporated into the soil, where they can be preserved for thousands of years in depositional environments that do not have high alkalinity. Thus, it is possible to use these microfossils as indicators of the plant communities of the past, in palaeoenvironmental, palaeobotanical or archaeological studies (Twiss et al., 1969; Rovner, 1971; Piperno, 1988; Fredlund and Tieszen, 1994; Alexandre et al., 1997; Barboni et al., 1999; Carter, 2000; Osterrieth, 2008; 2009). The aim of this study is to analyze the silica biomineralizations, particularly the phytolith morphotypes present in sedimentary sequences of the Laguna Potrok Aike (PTA) located in southern Santa Cruz, Argentina. PTA is one of the very few locations that are suited to reconstruct the paleoenvironmental and climatic history of southern Patagonia. In the framework of the multinational ICDP deep drilling project PASADO several long sediment cores to a composite depth of more than 100 m were obtained. According to Roig et al. (1985) and León et al. (1998), the vegetation surrounding the PTA is the xeric grass steppe. In this first stage to the subsequent sampling of PASADO, we worked with surface sediment samples of PTA. These samples were selected for pollen studies (Quintana, 2008). These samples also were included in the SALSA project (South Argentinean Lake Sediment Archives and modeling). In addition, we worked with some of the treated samples for the study of diatoms provided by Dr. Maidana, all they included in the ASADO project (Analysis of Sediment Distribution in Laguna Potrok Areal Aike). In the sediment samples of SALSA, organic matter was oxidized with 30% hydrogen peroxide at 70 °C. Biomineralizations were extracted by repeated centrifugation at 1000 rpm for 3 min (Alvarez et al. 2008). Sediment samples of ASADO (sequences of 0-1m depth) were analyzed along four transects in N, S, E and W directions (Figure 1). The samples (SALSA and ASADO) were mounted with immersion oil and 400 minerals grains were counted in each slide under optical microscope for the quali-quantitative analysis. The phytolith morphotypes were described according to ICPN descriptors (Madella et al. 2005). Preliminary results of SALSA showed a predominance of phytoliths with respect to other biomineralizations in the Laguna Potrok Aike samples (Figure 2). These were mostly represented by rondel and elongated morphotypes, other phytolith morphologies (unciform and cuneiform), chrysophytes and zooliths were also observed. The results of ASADO showed a predominance of diatoms (entire and broken) with respect to other biomineralizations in the four transects (Figure 3). Phytoliths were observed in low percentages, and were mostly represented by rondel and elongated morphotypes, all corresponding to the Pooide. A few bilobates phytoliths were observed and they were more weathered. Due to the presence of vegetation around the wetland, phytoliths percentage was higher in lower depths of it. In relation with phytolith distribution within the lagoon, higher values were detected in the northern sector (transect 1) while the lower ones were in the southern sector (transect 3) (Figure 3). Nowadays, wetland samples are still being collected in order to obtain a better understanding of the phytolithic assemblage in relation to the vegetation of the area. These studies are relevant and, through phytolith study, it is possible to make inferences about taphonomic, environmental and paleoenvironmental processes in the core samples of PASADO project.