5th Swiss Geoscience Meeting, Geneva 2007
Late Holocene ecological response to environmental
changes in Lago Fagnano, Tierra del Fuego
Cristina Recasens*, Daniel Ariztegui*, Nora I. Maidana**, Nicolas Waldmann*,
Christopher M. Moy***, Robert B. Dunbar***, James A. Austin Jr. ****, Flavio S.
Anselmetti*****
*Section des Sciences de la Terre, Rue des Maraîchers 13,1205 Geneva, Switzerland
(recasen9@etu.unige.ch)
**Depto. de Biodiversidad y Biologia Experimental, Fac. de Cs. Exactas y Naturales,
Universidad de Buenos Aires, Argentina
***Department of Geological and Environmental Sciences, Stanford University, CA,
USA
****Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences,
University of Texas at Austin, TX, USA
*****EAWAG, Überlandstrasse 133, 8600 Dübendorf, Switzerland
A high-resolution diatom stratigraphy of a sediment core from Lago Fagnano
has been used to document the paleoecological changes of this lake during the
late Holocene. Located at 54°S in Tierra del Fuego, Argentina, Lago Fagnano
occupies a tectonic depression along the Magallanes-Fagnano fault system.
Climate in this area is highly influenced by the westerly winds, the Southern
Ocean circumpolar flow and the South Pacific gyre, which makes of Tierra del
Fuego a key site for paleoclimatic and paleoecological reconstructions in the
Southern Hemisphere.
Water and surface sediment grab samples were collected in the lake 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. Taxa identification and counting of diatoms from
sediment samples of the modern lake bottom show a highly diverse diatom
community, though species from the Discostella genus are visibly dominant.
These results, along with the water data, are essential to characterise the
present limnological conditions of the basin and for further interpretation of the
sedimentary record. The 160 cm gravity core used for the high-resolution
diatom analysis exhibits alternating light and dark laminae, which have been
assimilated to varves, allowing to estimate an age of 200 years for the base of
the core. Ongoing radiocarbon dating in terrestrial macrofossils may confirm this
hypothesis. Sediment density and magnetic susceptibility profiles show
relatively uniform values occasionally interrupted by peaks, which are
interpreted as turbidites. Rock-eval pyrolysis reveals a relatively uniform
distribution of organic and inorganic carbon along the core. Sediment samples
collected at estimated decadal to sub-decadal frequency were used for diatom
identification and counting.
Combining the different diatom assemblages found in the sedimentary record
with the modern limnology will provide very valuable paleoecological
information. A good interpretation of this dataset is important to develop diatombased transfer functions, a final goal of this study that will be further applied at
various temporal scales.