Preliminary study of subfossil chironomid community at a lagoon of northern Patagonic steppe

RIZZO A, RIBEIRO GUEVARA S, DUTRÚS S, ARRIBÉRE M., MASSAFERRO J.

Miramar, Laguna Mar Chiquita

Otro; Holocene environmental catastrophes in South America: From the lowlands to the Andes; 2005

ICSU Dark Nature and IGCP 490

Chironomids are widely used in paleolimnology as sensitive indicators of environmental changes. Their narrow ecological tolerances and habitat preferences are valuable tools for reconstructing past environmental fluctuations and disturbances. El Toro is a steppe lagoon located in a small closed basin in NW Patagonia (40º14’S-70º22’O). Records of subfossil chironomid assemblages from short sediment cores, covering the last ca 200 yr, were studied in order to obtain a scenario of short-term changes occurred in the lagoon related to natural and anthropogenic events. The 32 cm core activity profiles of 137 Cs were measured to estimate the age. Chironomids taxa were sampled and identified. Organic matter (LOI) and biogenic silica were measured for each layer. Concentrations of rare earth elements (La, Ce, Yb, Lu, Sm, Eu, Tb) and others elements (Al, As, Ba, Br, Ca, Ce, Co, Cr, Cs, Fe, Hf, Mn, Na, Rb, Sb, Sr, Sc, Ta, Ti, Th, U and V) were measured in all layers. The mean sedimentation rate estimated for the upper layers is 0.111 g.cm -2 y -1 (0.27 cm.y -1 ). Chironomids sub-fossils are only abundant in the upper ten layers. Organic matter fluctuates between 9 and 21 % along the core, the lower values corresponding to layers with volcanic ashes. Four taxa were identified corresponding to Subfam. Chironominae and Orthocladiinae and their relative composition were analyzed. Polypedilum sp. is the main taxon. The maximum abundance index (82 individuals/grams of sediment, ind.g -1 ) is coincident with the maximum values of biogenic silica (7.35 mg.g-1). There is a strong association between Br concentrations and organic matter. In the ten uppermost layers, chironomid abundance and biogenic silica are also associated with organic matter. These results show a strong change in the environmental conditions in time period studied.