Late pleistocene and early Holocene ecological response of Lake El Trebol (Patagonia, Argentina) to environmental changes

Stratigraphy of Lake El Trebol (41 degrees S 71 degrees W, 758 m a.s.l.) de scribed from an 11 m long sediment core provided a continuous record of pat ecenvironmental changes occurred during the last 14,000 years in southern S outh America east of the Andes. Located in an ecotonal area within a strong humidity gradient, this lake offers a unique opportunity to develop paleol imnological studies. Using a geochemical approach that includes elemental a nalysis, bulk organic matter, major nutrients and pigments combined with ch ironomids and pollen analysis, three zones were identified in the sedimenta ry record of this take. The oldest one (between 680 and 1,100 cm) correspon ds to the late Glacial period. During that time, clayish laminated sediment s were formed in a proglacial environment. Low organic matter, biogenic sil ica and nitrogen contents and the absence of pigments, pollen and chironomi ds characterise this period. At the end of this period, ca. 14,000 yr BP, a sharp increase in phosphorus concentrations predates major changes in limn ological parameters. The transition zone (between 560 and 680 cm) is relate d to major changes in the drainage system that started soon after ca. 14,00 0 yr BP and continued to ca. 10,500 yr BP. The chemical composition of the sediments was affected by an increase in physical weathering mechanisms alm ost coincident with the increase in the regional volcanic activity that pro duced suspended volcanic detritus. The increase of mainly coarser detritica l components may indicate a reorganisation of the fluvial system by that ti me. Simultaneously, an increase in the productivity of the lake accompanied by a gradual replacement of open vegetation with Nothofagus, Poaceae and E ricaceae, by Nothofagusforest, was observed. In the last zone (between 0 an d 560 cm) the productivity of Lake El Trebol achieves maximum values. The t rends in geochemical and biological indicators observed at Lake El Trebol c an be up to some extent, correlated to those registered in Lake Mascardi. T he results support earlier paleoctimatic scenarios from southern South Amer ica.