The Mid- to Late Holocene Paleoenvironmental Record at El Peñón Glacial Valley (~35º15? S, 70º31?W), High Andes Cordillera

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

Vista Flores, Tunuyàn, Mendoza

Conferencia; 4th Southern Deserts Conference; 2014

Laboratorio de Paleoecologìa Humana - Universidad Nacional de Cuyo

In the highest mountains of Mendoza Province,the Valenzuela River basin reveals evidence of past fluctuations of the El Azufre and El Peñón glaciers. The moraines there record middle and late Holocene glacier advances and, the reported ages reported may be minimum ages. Due to the lack of maximum age constraints, a likely late glacial age for the oldest moraine in each valley is possible. The onset of peat growth in the valleys could have been delayed by early Holocene arid conditions and not been directly correlated with neoglacial advance. The region,located on the western side of a sensitive climatic area known as the South American Arid Diagonal, is a valuable location to look for paleoenvironmental archives. This poster provides new information (integrated analyses of pollen, geomorphology and sedimentology) on the evolution of the high-altitude mid-Holocene lithostratigraphic sequence of the Vega El Peñón (VEP) profile (􀀃2445 masl) in the El Peñón glacial valley. The aim is to improve the accuracy of the paleoenvironmental and paleoclimatic reconstructions in South America, particularly those at high altitudes. The sedimentological record of the VEP profile developed during the middle and late Holocene, as a result of fluvial and growing-peat aggradation processes. Fluvial processes were responsible for the aggradation of silty sand levels formed by coarse sand in the lower part of the profile and mainly fine sand in the medium and upper parts. The sediment inputs were likely driven by wash out processes occurring in the lateral moraine slope as well as fluvial inputs that reworked glacial sediments from the upper glacial valley sector. Low-energy and low-depth environments with aggradation of the finest clastic fractions (sandy silt and silt layers), mainly from suspended loads, generated some deposits that carry high relative organic content (OC). They resemble the development of relative abundant vegetation in peat-growing environments. These deposits are inferred here as peat layers, whereas other levels that have similar features but do not carry such a high OC could be considered paludal-like or overbank deposits. These fluvioglacial deposits started to develop at ~5691 cal BP. The formation of peatlike deposits seems to be mainly restricted to 1221?594 cal BP. Currently, the glacial valley surface is a peatland area. The hydrophilous vegetation is mainly composed of sedges (Cyperaceae), rushes (Juncaceae), and humidadapted grasses (Poaceae). Cushion plants form the peat deposits favored by the compact and rhizomatous growing of its roots, e.g., Oxychloe sp. and Deyeuxia sp., among others. Regional plant communities are part of the Altoandean phytogeographic unit, a grassy steppe associated with cushion plants and herbs adapted to strong winds and high solar radiation. Dwarf shrubs as Adesmia hemispherica and A. subterranea dominate; there are other shrubs such as Berberis empetrifolia and herbs such as Viola, Calandrinia, Barneoudia, Tropaeolum, Nassauvia and Mutisia. Preliminary pollen results indicate that when the VEP sedimentary sequence started to form,vegetation was mainly composed of grasses (Poaceae) while sedges and rushes were rare. The presence of highly abundant Pteridophytes (􀀁20%) is an unexpected result because these taxa are not typically described in literature. After 􀀁3500 cal BP, the pollen record seems to exhibit a higher fluctuation in herbs (i.e., Oxalidaceae, Malvaceae, Cariophyllaceae), shrubs (i.e., Nassauvia, Senecio, Menonvillea) and hidrophylous taxa, probably influenced by volcanic and/or glacifluvial disturbances. In the last 􀀁600 cal BP, Poaceae dominates again and Pteridophytes are still recorded but with very low values. Obtaining further pollen results (in progress) will make it possible to asses external driving factors that could have influenced changes in vegetation belts, as glacier fluctuations or volcanic activity evidenced by the sedimentology and the geomorphology of the area. The study of the VEP sedimentary sequence helps reveal the evolution of this setting from non-biological (sedimentology, geomorphology) and biological (pollen record, present-day vegetation) perspectives. The mid- and late Holocene at this high Andean location, immersed in a glacial valley context, was dominated by fluvial aggradation with the development of peatland environments. Neoglacial advances have been inferred for the Little Ice Age and are recorded by moraine deposits upstream from the VEP profile. These environmental fluctuations in the glacial valley are also reflected in modifications to the vegetation recorded in the VEP pollen record, reinforcing the potential of pollen as a sensitive indicator of past environmental changes.