Stable Carbon Isotopes from Bulk Organic Matter as Indicators of Paleoenvironmental Changes in Arroyo La Estacada, Mendoza (Argentina) Leandro D.

ROJO, L. D.; MEHL, A. E.

Vista Flores, Tunuyàn, Mendoza

Conferencia; 4th Southern Deserts Conference; 2014

Laboratorio de Paleoecologìa Humana - Universidad Nacional de Cuyo

Late Quaternary sedimentary sequences outcropping at Arroyo La Estacada (33°29?15?S, 69°01?30?W, 890 masl) are mainly formed by alluvial deposits, encompassing the last 50,000 BP, which have been subjected to pollen analyses And have contributed to the knowledge of past vegetation dynamics on the eastern Andean piedmont. However, the information is scanty and constrained to a few temporal windows due to some poor pollen-preservation intervals that coincide with deposits of dominant fluvial genesis. Stable carbon isotopes (􀀂13C) of bulk organic matter of the Arroyo La Estacada deposits, which include paleosoils and limnic layers, provide a useful proxy that documents the presence of different plant functional types (C3 vs. C4 photosynthetic pathways). Its analysis complements the information of past changes on vegetation derived from previous pollen evidence. According to the relationship between the current setting for phytogeographical desert plants and C3?C4 plant distribution in the area, we hypothesize that late Quaternary conditions values of 􀀂13C < -19? represent cold desert vegetation (C3, Patagonian and Altoandean phytogeographic formations) and values of 􀀂13C > -19? were linked to hot desert vegetation (C4, Monte phytogeographic formation). Three lithostratigraphic sequences from Arroyo La Estacada outcrops that encompass the last ~35,000 BP where chosen to test the hypothesis, based on the study of the 􀀂13C variation in the bulk organic matter from its sediments: La Bomba (LB; 35,000?31,000 BP), Puente El Zampal (PEZ; 17,000?3000 BP), and Puesto Ramírez (PR; 5000?0 BP). Preliminary results suggest that between ~35,000?30,000 BP (LB profile) cold desert vegetation developed, with a progressive increase of more humid and colder conditions. The 􀀂13C record during late Pleistocene and early Holocene (PEZ profile) indicates the development of hot desert vegetation, particularly in the intervals recording relative stabilization of the floodplain environment surfaces, i.e., paleosoil development. During middle to late Holocene (since 5000 BP), the alluvial sedimentary sequence (PR profile) shows 􀀂13C values that represent cold desert plants and only in the last 700 yrs BP a change is observed toward a predominance of hot desert vegetation. These results are discussed and contrasted with previous pollen analyses of these outcrops in order to assess their accuracy and contribute to a more robust paleoenvironmental and paleoclimatic framework about past changes in the eastern Andean piedmont region between 33?34ºS.