Pleistocene climate proxies from landslide impounded lakes

MOREIRAS S.M.

Mendoza

Congreso; 18th International Sedimentologist Congress; 2010

IAS

Dammed lakes in mountain areas are characterised by a very short life as rarely they survive more than one day. Nevertheless, long life impounded lakes such as the Negra lake dammed by the Holocene Meson Alto rock avalanche (Yeso River, Chile) and the Inca lake dammed by Portillo rock avalanche have been studied for understanding paleo-climate conditions. Despite of this, climatic proxies from Pleistocene paleolakes are occasional in the literature. Herein, three palaeolake sequences were studied in the northern extreme of the Cordon del Plata, Central Andes (32º 30´ S) in order to assess the paleoclimatic history of the region.Analysed lake sequences, reaching 30 meters in thickness and extensions of 0.4 square kilometres, were impounded by huge rock avalanches with volumes in the order of one million cubic meters, called as TD, PA, and PB, respectively. These diamictons resulted older than an ash level dated by tephrochronology 350±80 Ky (Ar39/Ar40 method on biotite) (Moreiras, 2006, 2010). However, new ages obtained for the lake sequences of PA and TD by thermoluminescence technique reveals 47,440±3700 ky and 43,950±3895 Ky, respectively. Thus, dental remains found in the paleo-lake related to the PB rock avalanche were identified as Hippidion devillei (Cerdeño et al, 2008). This endemic species appearing in South America around 2.5 My is restricted to the Andes mountains and associated to Early-Middle Pleistocene sediments (Ensenadense) (Alberti and Prado, 2000).Sedimentary sequence collected from the palaeo-lake related to the PB rock avalanche (22 samples) shows a very fine grain size distribution with more than 75% of silts-clay content. Scarce organic matter and no palinological content were determined for these levels. However, abundant carbonate nodules were observed at 17 m deep from the top of the sequence. As well diatomite levels, rich in carbonate and with high volcanic glass content, were identified in the PA paleo-lake. Plant species identified in diatomites are cosmopolitan (Amphora sp., Achnantes longipes, Central, Cymbella af. cistula cymbiformis, Cymbella ventricosa, Denticula, Fragilaria brevistriata var. Trigona, Gomphonema sp., Navicula sp., Neidium sp.?, Nitzschia sp.) not being a good climate proxy. Both sections are not varved. Moreover, the PB lake sequence is disturbed by liquefaction processes.Existence of the long life paleo-lakes with calcareous layers and very low matter content could roughly implicate dry-warm climate conditions. However, δ13C values range (-8.5 and -9.3 PDB) obtained from the tooth enamel carbonate of the horse remains indicate a regime diet based on C3/C4 plants. The while the oxygen isotope values range between -4.5 and -8.3 (PDB) may indicate a similar climate to the Present as -8 %0 VSMOW has been measure in water stream at 2000 mean elevation (Hoke et al., 2009). However, differences in δ18O values of meteoric water are suspected according to rain source (Pacific vs Atlantic).