Late Holocene environmental history of a shallow pampean lake based on fossil pigments and ostracods

M.S. PLASTANI, C. LAPRIDA, J. MASSAFERRO, A. LAMI.

Gral Roca

Congreso; Reunión Anual Comunicaciones Asociacion Paleontologica Argentina; 2012

APA

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 pigments (total carotenoids -TC- and chlorophyll derivatives-CD), ostracod fossil assemblages and grain-size sediment distribution. Based on a preliminary model age, variations in these parameters allow recognition of hydrometeorological changes occurring in the last 300 years. The core shows centimeter scale lamination and frequent textural variations. The system evolved from a small seasonal stream to a grassy stream which periodically drained shallow seasonal pools at around late 18th century. Thereafter, a shallow temporary pond was established until mid-late 19th century. The establishment of an oligohaline permanent shallow pond coincides with the end of the Little Ice Age in the region and related precipitations increase that allow the development of near-present conditions. A remarkable level with macrophytes, a peak in CD and in the index CD/TC, and the impoverishment in ostracod absolute abundance seem to indicate the water body retraction and an allochtonous origin of the organic matter related with the 1930?s-1940?s megadrought. The inclusion of fossil pigments together with other proxies has improved the understanding of the environmental history of this small laguna in the Pampas plain.