Recent environmental changes at the end of the world: the diatom-based record of Lago Fagnano, Tierra del Fuego

RECASENS, C.; ARIZTEGUI, D.; MAIDANA, N. I.; WALDMANN, N.; MOY, C.; DUNBAR, R.; AUSTIN JR, J.; ANSELMETTI, F.

Barcelona

Congreso; 4th International Limnogeology Congress(ILIC2007); 2007

International Association of Limnogeology

The diatom stratigraphy of well-dated lacustrine sediments is an established approach to reconstruct past environmental changes. We use variations of diatom assemblages blended with geophysical, sedimentological and geochemical datasets to reconstruct former environmental changes in Lago Fagnano (54°S, Tierra del Fuego, Argentina). Climate in the southernmost Patagonia area is highly influenced by the westerly winds, the Southern Ocean circumpolar flow and the South Pacific Gyre, which makes of it a key site for paleoclimatic and paleoecological reconstructions.More than 800 km of geophysical data were acquired and up to 90 m of sediment cores were retrieved. Water and surface sediment samples were also collected in order to constrain the modern limnology. Preliminary results of the physical and chemical parameters of the water column reveal a well-mixed lake with no evidence of water stratification. The surface sediment samples were used for grain size distribution analysis and for the identification of the different diatom assemblages, with the aim to document the actual flora of the lake. The taxonomy and quantitative analysis carried on show a highly diverse diatom community, though species from the genus Discostella are visibly dominant. These results, along with the water data, are essential to characterize the present limnological conditions of the basin and for further interpretation of the sedimentary record. A 160 cm gravity core retrieved from the central area of the lake presents alternating light and dark laminae with an abundant content of diatoms and amorphous organic matter. Sediment density and magnetic susceptibility profiles show relatively uniform values occasionally interrupted by peaks associated with turbidites. Analogously, rock-eval pyrolisis reveal a relatively uniform distribution of organic and inorganic carbon along the core. Ultra-high resolution X-ray fluorescence micro-profiles show fluctuations at mm scale in major and trace elements. Sediment samples collected at decadal frequency along the core allow the characterization of the sediments as well as diatom identification and counting. The combination of the different diatom assemblages found in the sedimentary record, and their calibration with modern limnological data, provides very valuable paleoecological information. This dataset is a key component for the development of diatom-based transfer functions, a final goal of this study that will be further applied at different temporal scales.