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The preliminary phytolith studies from the Potrok Aike Lake sedimentary sequence were made with SALSA core samples (Zolitschka and Schabitz, 2007), which included 9 samples from across this drilling section (Table 1). These samples were sent for analysis by Dr, Nora Maidana (PI of the PASADO project) for the purpose of verifying the presence of siliceous microremains and particularly phytoliths, in order to evaluate the potential information that might be obtained using this approach in the analysis of the Potrok Aike Lake sedimentary sequence. In these preliminary analyses, the phytoliths became the main emphasis and therefore the study of other types of microremains (Maidana et al., 2005 ; Wille et al. , 2007) was not included. The samples had been processed previously in the Laboratorio de Diatomeas Continentales (FCEyN, USA), according the diatom protocol (Schabitz and Wille, unpublished) of this research group, with warm hydrogen peroxide treatment followed by washes with distilled water. No separation of the components or concentration of any type took place (Maidana com. pers.), and because of this these samples contained both microremains and clastic materials. Although counting levels in these preliminary analyses did not reach those of the minimal representative sample , a variety of phytolith types could nevertheless be observed and identified in the sequence. The results of the preliminary analysis show the presence of various types of silica microremains in all of the analyzed samples (Fig.1). Phytolith distributions show a marked abundance of non-diagnostic elements such elongate prismatic, fan-shaped, and pointshaped phytoliths, as well as a clear abundance of diagnostic elements linked to the microthermic grass lineages (such as pooids, festucoids, stipoids, and danthonioids). These were represented mainly by festucoid boats, crescents, truncated cones, and stipa-type morphotypes. The majority of these grass groups are typical of the Patagonian steppe. For other types of grasses, diagnostic phytolith morphotypes such dumbbell shapes are found in these samples, with highest abundances in the upper samples of the sequence , Panicoid elements are not characteristic of the region´s floral composition , and the presence of these megathermic grasses can perhaps be related to an antropical factor. Other diagnostic grass types such as collapsed saddle phytoliths, which can be related to the bambusoid groups, are considered as Andean forest elements, as are some nothofagoid and gymnosperm morphotypes. These distributions, although not considered securely comparable because of the insufficient counting levels applied during this preliminary analysis, do create a pattern that resembles the pattern seen in the paleopalynological records (Wille et al., 2007), with Patagonian steppe in the lower core section and Andean forest in the upper section . Current studies As part of the PASADO project. the study of phytoliths from samples from the most recent drilling has begun, and this work is now being carried out by the Diamante and Mar del Plata research groups in a coordinated manner. However, the newer PASADO drilling sequence contains a more extensive record than the previously analyzed SALSA sequence. For this 89 reason , as well as the fact that there are both undisturbed and redeposited sediments in the sequence, two research approaches have been initially planned: 1) a detailed analysis of the upper 2500 cm section (Fig. 2), whic includes the material equivalent to the sequence already analyzed for the SALSA project: and 2) a low-resolution analysis of the middle and lower sections. The preliminary analysis of these samples will allow evaluation of the future potential for performing high-resolution studies of both sections, which are characterized by the presence of many redeposited samples. Also included in this approach will sections expected to reflect a low level of biotic activity corresponding to glacial periods. In addition to these low-and high-resolution studies that are now being implemented, members of the Diamante research group in particular are also carrying out complementary analyses with the goal of establishing modern reference patterns of phytolith production/ incorporation system . These are focused on the main ecological communities found in the region: Patagonian steppe, Patagonian ecotone, and Andean forest. These studies are being developed in part as the subject of a doctoral thesis by one of the present authors (MGFP), who is now performing analyses of phytolith production and incorporation in various plant communities from the ecotonal region of Tierra del Fuego. These studies have been designed to complement the phytolith analysis of various species of the Andean forest that have also been carried out, specifically in the case of some Bambusoideae, Nothofagaceae, Asteraceae and Gymnosperm species.

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