The recent climatic record of the Lagunas Encadenadas del Oeste, Argentina: implications for paleoenvironmental and paleoclimate reconstructions

CÓRDOBA, F.; PIOVANO, E.; MULSOW, S.; ZÁRATE, M.

Punta del Este

Conferencia; Shallow Lakes Conference 2008: "Structure and function of world shallow lakes"; 2008

The Lagunas Encadenadas del Oeste (LEO) are a chain of shallow lakes (from 2 to 6 m deep) located in the southwestern part of the Pampean plains of Argentina (36º S - 62º W). The LEO complex system includes five main interlinked lakes with salinities ranging from 0.72 g/L (Laguna Alsina) to 54,5 g/L (Lago Epecuén). Variations in the subregional hydrological budget during the 20th century have triggered marked water-level fluctuations and salinity across this chain of lakes. The LEO location at the transition between two climatic domains (Pampean Plains/Patagonia) highlights the potential of these lakes for unraveling the mechanisms behind changes in the atmospheric circulation in southeastern South America. Field observations, instrumental data and historical documents show that the LEO system has been particularly sensitive to interseasonal, interannual and interdecadal hydrological variability. Their ecosystems as well as their sedimentary dynamic have responded markedly to recent environmental changes. In order to analyze the LEO sedimentary archive, a multi-set of sedimentary cores was retrieved at different water depths. The chronological framework was developed by radiometric dating (210Pb y 241Am). The multi-proxy analyses include sedimentology (textural description: colour, grain size, structure, contact, and microscopic analysis: smear slides), mineralogy, grain size, loss-on-ignition and magnetic susceptibility. The chronological framework was used for calibrating the sedimentary record of the lakes system to the last 110 years of instrumental record and historical data, as well as to correlate the records of each lake. The LEO cores comprises three main lithological units: A) Banded to massive organic-carbon rich muds, showing a distinctive dark colour associated with organic matter enrichment and indicating periods of high lake levels (e.g., ?wet? phase between 1915-1922 and the ?hyper-wet? since 1970s; B) Massive to banded greenish to grey clay muds with evaporites, corresponding to the record of extremely low lake levels (e.g., end of the Little Ice Age (LIA) and the drastic drought during the 1930 decade); and C) Banded to massive, exceptionally laminated, dark to light grey muds, matching the transition from low to highstands. Our results show that sedimentary facies variations reflect changes in the hydrological balance of the lakes that in turn follow the regional climatic variability. The analyzed sediments contain the record of dry and wet phases from the end of the LIA until the present. Ongoing research will provide important clues on the past South America monsoonal activity at its southernmost influence as well as new elements for a better depicting of the antiphased hydrological balance at both sides of the austral part of ?South American Arid Diagonal?. The analyses of longer cores will provide new elements for reconstructing the regional Holocene climate history at different time-windows in this portion of South America.