Hydrological changes in a small lake at the Pampean region of the South American central plains (Argentina) during the last centuries

Abstract

Paleolimnology studies are extremely useful in the argentine Pampas because they represent the main source of hydrometeorological reconstructions in this mid-latitude South American temperate plain. The upper 30 cm of a short sediment core (81,5 cm) obtained from Laguna La Barrancosa (37º19?S / 60º06?W) were studied for estimating hydro-ecological variations based on fossil ostracod assemblages. These results were considered jointly with variations in fossil pigments content (total carotenoids -TC- and chlorophyll derivatives-CD) and grain-size sediment distribution. A preliminary age model was established based on stratigraphic correlation with dated cores from nearby Pampean small lakes (lagunas). The core, which spans approximately 300 years, shows centimeter scale lamination and frequent textural variations. Consideration of statistically significant ostracod zones determined by numerical zonation of the assemblages allows recognition of five stages in the hydrological evolution of the studied laguna. Zone A (30-26cm) is characterized by low abundance of ostracods -Potamocypris villosa and Limnocythere solum?-, high index CD/TC, low water content and predominance of sand and silt. These features are consistent with sedimentation in a small seasonal stream with abundant allochthonous organic matter. Zone B (25-23cm) is characterized by a rise in ostracod abundance and the appearance of Heterocypris incongruens, as well as an increase in water content, TOC (Total Organic Carbon), TC and CD. In this phase, the system would have evolved to a grassy stream which periodically drained shallow seasonal pools, which would indicate a positive precipitation/evaporation balance. In Zone C (22-18cm), Cypridopsis vidua appears and H. incongruens is replaced by Eucypris sp., while both CD and the CD/TC ratio increase notably. These variations allow inferring the establishment of a shallow temporary pool stream in the early 1800?s. In Zone D (18-15cm) P. villosa disappears, pointing to the establishment of a shallow temporary pond. The increase in the CD/TC ratio indicates a productive water body and authochtonous origin of organic matter. Zone E (15cm-top) is characterized by the total replacement of L. solum by Limnocythere sp. nov. Ramón Mercau et al. (this volume) and Eucypris sp., pointing to the establishment of an oligohaline permanent shallow pond around 1870 AD. The marked increment both in the absolute amount of pigments and the CD/TC ratio in the top core would reflect enhanced anthropic activity in the area. To sum up, the sedimentary record of La Barrancosa traces the gradual establishment of a permanent pond at the coring site, indicating an amelioration of the hydrological balance in the Pampean region since the end of the Little Ice Age, around the mid-1800s. Besides this long term trend, certain decadal-scale events can be tracked in the record as well. In particular, at 7-8 cm depth the occurrence of macrophytes remains, a peak in CD and in the index CD/TC, and an impoverishment in ostracod absolute abundance seem to indicate retraction of the water body, which according to the preliminary age model could be coincident with the 1930´s-1940´s megadrought.