MODERN AND FOSSIL DIATOMS AS PALEOENVIRONMENTAL INDICATORS ALONG CHUBUT RIVER, PATAGONIA, ARGENTINA

ESPINOSA, M. A; ESCANDELL ALEJANDRA; ALONSO; ISLA, F.

Natal

Congreso; Quaternario: processos naturais e antropicos; 2013

ABEQUA

The main goals of this study are to (a) analyze the distribution pattern of diatom assemblages in a flowing freshwater system from Patagonia, Argentina, (b) investigate the environmental factors affecting the species distributions and (c) reconstruct Holocene paleoenvironmental evolution in the area. Samples for modern diatom analyses were collected by triplicate with 20 mm diameter x 100 mm length plastic tubes from the littoral zone of each site. Physical and chemical variables were quantified in seven sites of the middle and lower course of the Chubut River. Turbidity, salinity, conductivity, pH and summer temperature (February) data were obtained. In addition, a surface sediment sample was taken for grain size analysis and organic matter content. The diatom content of modern sediment samples were studied in order to achieve ecological information useful as modern analogous for Patagonian rivers. Fossil diatom assemblages were studied from two sequences that spanned between ca. 8000 years BP and present, the former 0.40 km from the inlet (Magagna core; 43° 19? 54.1" S; 65° 04? 04.5" W) and the other 36 km upstream (Gaiman core; 43°17?17.6´´ S; 65°28?12.4´´ W). Environmental variables significantly influencing diatom assemblage composition was identified through canonical correspondence analyses (CCA). In order to compare modern and fossil assemblages, Detrended Correspondence Analysis (DCA) was applied. A total of 47 samples (modern and fossils) were analyzed and 135 taxa were identified. Diatom assemblages showed distinct abundance patterns in relation to salinity with a shift from communities dominated by estuarine taxa in lower sites to communities dominated by freshwater taxa in the middle course of the river. The comparison of modern diatom assemblages with fossil sequences allowed reconstructing the evolution of estuaries, marshes and brackish/freshwater vegetated habitats. At Magagna site, a marsh evolved into marine coastal environment (estuary) at 4,376±69 14C years BP. Then, a salt marsh settled about 672±39 14C years BP, where it remains today. Gaiman core analysis showed the dominance of freshwater taxa since the base to the top (8020±110 14C años AP to present) represented by the planktonic and epiphytes diatoms Aulacoseira granulata, Cocconeis placentula and Ulnaria ulna. The results obtained from modern assemblages allowed to interpret the paleoenvironmental evolution of two holocene sequences in Patagonia and provide useful analogues for future paleoenvironmental reconstructions in coastal and associated fluvial systems in southern South America.