José María Guerrero

Silvia E. Sala, Sonia L. Fontana y Thomas Giesecke, División Científica Ficología, Facultad de Ciencias Naturales y Museo, Univ. Nacional de La Plata, Argentina []

Lacustrine diatom stratigraphy and environmental changes during the Holocene in the Andes of northern Patagonia, Argentina

The eastern flanks of the Andes in north Patagonia exhibit a marked vegetation gradient from temperate forests to woodlands and steppe, which is mainly determined by the eastwards diminishing precipitation. Altitudinal and latitudinal variations in temperature also contribute to generate diverse forest ecotones. The region has been affected by intense volcanic activity and remained relatively undisturbed by human activities. This contribution is part of a multi-proxy study aiming to reconstruct Holocene vegetation history and climate variability of the region, as recorded in sediments from small lakes located along a forest-steppe gradient. We present results of diatom analysis from Lake Torta (39°06’S, 71°21’W), located in a mixed forest of Nothofagus antarctica, N. obliqua and Araucaria araucana. The 10 m sediment sequence recovered spans the last 10,000 years, indicating a relatively late deglaciation for the area. Numerous tephra layers and redeposited material caused by floods of the nearby river Pilhue, are discernible throughout the core. Immediately after deglaciation diatom abundance is low and assemblages are dominated by colonizing, epiphytic and aerophilic taxa. The dominance of planktonic taxa during the earlyand mid Holocene indicates a relatively greater water depth, a turbulent water column and low to moderate nutrient concentrations, consistent with a more open landscape. Small fragilarioid diatoms, generally associated to shallow environments, are dominant during the late Holocene evidencing a decrease of the water depth. A recent rise of Asterionella formosa, a diatom associated to high content of lake nutrients, might be related to human activities such as animal farming in the lake catchment, which is in agreement with the palynological record of the core. Disturbances caused by major flooding events seem to have impacted on the diatom community, whereas the deposition of tephra layers did not result in substantial changes in long-term trends.