Recent environmental changes inferred from sediments in a shallow lake of the Argentinian pampas

Abstract

The Pampas are fertile lowland plains that extend across east-central Argentina and are renowned for their agricultural importance. The low geomorphologic relief of the region accounts for the occurrence of numerous shallow lakes whose sediments constitute paleoenvironmental archives of the outcomes of natural processes and human activities in the Pampas. We identified the main forcing factors behind hydrological shifts during the past ca. 200 years in La Barrancosa, a shallow lake located in the southeastern Pampas. The data provide a historical context to better understand environmental changes in the area during the twentieth century. Variations in biological (ostracods, chironomids and pigments), geochemical and sedimentological variables were interpreted in terms of lake level and trophic state changes. The low diversity of biological assemblages and the autoecology of the ostracod and chironomid taxa suggest that during parts of the record assigned to the nineteenth century, near the end of the Little Ice Age, La Barrancosa was a shallow, ephemeral, subsaline wetland, indicating drier-than-present conditions. Thereafter, a hiatus from ca. AD 1860–1940 is attributed to denudation processes during the Pampas Dust Bowl drought of the 1930s. The onset of wetter conditions around 1940 brought higher lake levels and establishment of a perennial shallow lake, inferred mostly from shifts in sedimentological and geochemical variables. Marked variations in fossil pigments and in ostracod and chironomid assemblages reveal that the basin shifted from being endorheic to arheic around 1970, in spite of increased precipitation, forced by changes in global circulation patterns. This shift coincided with an inferred increase in primary production and was coeval with profound land use changes and technological innovations in the region. Further changes in sedimentological, geochemical and biological variables indicate that eutrophication accelerated from 1990 onwards, most likely as a consequence of the increase in planted area around the lake, massive application of fertilizer, and cultivation of pesticide-intensive transgenic soybeans. This study provides an historical perspective into ongoing environmental deterioration of shallow lakes in the Argentinian Pampas.