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

OSTERRIETH, M. M.F. ALVAREZ; J. ROJO; M. FERNÁNDEZ HONAINE; N. BORRELLI; L. BENVENUTO; N.I MAIDANA; B. ZOLITSCHKA; SALSA AND PASADO SCIENCE TEAMS

Congreso; 8th International Meeting on Phytolith Research; 2011

The circular maar lake Laguna Potrok Aike (PTA; 51"58' 5,70"23'\W) is a 100 m deep crater lake, with a maximum lake diameter of 3.5 km and a volume of 0.41 km3, located in southern Santa Cruz, Argentina, in the Patagonian steppe. The climate in the region is very and semi-arid and windy. The aim of this study was to analyze the silicophytoliths present in actual soils corresponding to the most representative plant communities from W-SW sector of Santa Cruz province and from soils surrounding Laguna Potrok Aike (PTA). Also, phytoliths were described from 46 samples from cores obtained with the framework of SALSA (South Argentinean Lake Sediment Archives and modeling) and ASADO (Analysis of Sediment Distribution in Laguna Potrok Areal Aike) projects; and 40 samples from cores coming from Potrok Aike Sediment Archive Drilling Project (PASADO). Around 2-5g of soil and sediments samples were taken from each level and analyzed following routine techniques. At this stage, we analyzed the total sample, determining the percentage of phytoliths, diatoms, spicules, and Chrysophyceae cysts and scales, related to the total mineralogical components in each level. The samples were mounted with immersion oil, and 400 mineral grains were counted in each slide under a light microscope for the quali-quantitative analysis. The phytolith morphotypes were described according to ICPN descriptors (Madella et al. 2005). The results of modern soils in the main plant communities had a predominance of phytoliths with respect to other biomineralizations. They were mostly represented by rondel and elongated morphotypes, and other phytolith morphologies such as unciform and cuneiform. The results of the 46 samples of the different cores from PTA showed a predominance of diatoms (entire and broken) with respect to other biomineralizations. Phytoliths were observed in low percentages, and were mostly represented by rondel and elongated morphotypes, all corresponding to the Pooideae. A low number of bilobates and weathered phytoliths were also found. Due to the presence of vegetation around the lake, the percentage of phytoliths in samples from shallow areas was higher. In relation with phytolith distribution within the lake, higher values were detected in the northern sector, while the lower ones were in the southern sector. The quantities, morphology and the preservation state of the phytoliths found in these samples involved taphonomic aspects that reflected their transport from the surrounding soils. Also, reworking occurred through morphodynamic processes of the lake, incoming streams, and waves associated with the predominant and strong westerly winds. The preliminary exploration of the upper sector of the PASADO composite core showed that almost all the analyzed levels were sterile, with very low content (< 10%) of largely weathered and broken elongates and rondels, and eroded diatoms. Therefore, these studies made possible to make inferences about taphonomic, environmental, and paleoenvironmental processes in the Potrok Aike maar lake region.