Reconstitution of Holocene El Niño Cycles in Southern America from High-Resolution Lake Sediments.

BELLA workshop Productivity of large lakes

RECONSTITUTION OF HOLOCENE EL NIÑO CYCLES IN SOUTHERN AMERICA FROM HIGH-RESOLUTION LAKE SEDIMENTS.

X. Boës1_{, M.F. Loutre}2_{, M. De Batist}3_{, N. Fagel}4

1_{Royal Observatory, Ringlaan 3, 1180 Brussels, Belgium}

2_{Institut d'astronomie et de géophysique G. Lemaître, Université catholique de Louvain,} Louvain-la-Neuve, Belgique

3_{Ghent University, Department of Geology, Renard Centre for Marine Geology Krijgslaan} 281-S8, 9000 Gent.

4_{University of Liège, Geology Department, 1000 Liege,Belgium.}

The aim of this communication is to show how laminated lake sediments driven by winds can be utilized for age modeling and for tracking El Niño Cycles. In South America, the lake systems located between 40° to 55° S are particularly sensitive to atmospheric air-humidity changes driven by the Southern Westerlies, El Niño, and the polar front. For example, Lago Puyehue (40°S) is stratified in summer and mixed by the stronger austral winter winds (i.e., monomictic temperate lake). The autumn/winter atmospheric conditions involve water circulation and nutrients (phosphorus and nitrogen) increase in the lake (Campos, 1989). The seasonal thermal lake cycle and nutrients increase lead to annual blooms of phytoplankton (diatoms). A varved micro-structure (varve couplets) is formed every year at the floor of the lake. The recurrence of the distinct light layers produced by the mass flocculation of diatoms blooms after winter nutrient turn-over therefore provides an annual temporal resolution (by varve counting).

We present chronologies obtain over the “Little Ice Age” (LIA) and the Last Deglaciation. The comparison between radionuclide (210_Pb,137_{Cs) and} 14_{C sedimentation rates} estimate validate the annual character of the laminations over thousand years, making this lake one of the few resolved varved record in southern hemisphere. The evolutive MTM and wavelet analyses of the varve thickness allow the detection of some dominant climate periodicities. For the AD 1400-2000 time window, our results shows that the 15, 9, 4.4, 3.2 and 2.4 years represent the most robust periodicities, and that the typical El Niño 4.4 periodicity is dominant during the Little Ice Age in South America.