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

ROJO, L. D.; MEHL, A. E; 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 ofpast fluctuations of the El Azufre and El Peñónglaciers. The moraines there record middle andlate Holocene glacier advances and, the reportedages reported may be minimum ages. Due to thelack of maximum age constraints, a likely lateglacial age for the oldest moraine in each valleyis possible. The onset of peat growth in thevalleys could have been delayed by earlyHolocene arid conditions and not been directlycorrelated with neoglacial advance. The region,located on the western side of a sensitiveclimatic area known as the South American AridDiagonal, is a valuable location to look forpaleoenvironmental archives. This posterprovides new information (integrated analyses ofpollen, geomorphology and sedimentology) onthe evolution of the high-altitude mid-Holocenelithostratigraphic sequence of the Vega El Peñón(VEP) profile (∼2445 masl) in the El Peñón glacialvalley. The aim is to improve the accuracy of thepaleoenvironmental and paleoclimaticreconstructions in South America, particularlythose at high altitudes. The sedimentologicalrecord of the VEP profile developed during themiddle and late Holocene, as a result of fluvialand growing-peat aggradation processes. Fluvialprocesses were responsible for the aggradationof silty sand levels formed by coarse sand in thelower part of the profile and mainly fine sand inthe medium and upper parts. The sedimentinputs were likely driven by wash out processesoccurring in the lateral moraine slope as well asfluvial inputs that reworked glacial sedimentsfrom the upper glacial valley sector. Low-energyand low-depth environments with aggradationof the finest clastic fractions (sandy silt and siltlayers), mainly from suspended loads, generatedsome deposits that carry high relative organiccontent (OC). They resemble the development ofrelative abundant vegetation in peat-growingenvironments. These deposits are inferred hereas peat layers, whereas other levels that havesimilar features but do not carry such a high OCcould be considered paludal-like or overbankdeposits. These fluvioglacial deposits started todevelop at ~5691 cal BP. The formation of peatlikedeposits seems to be mainly restricted to1221?594 cal BP. Currently, the glacial valleysurface is a peatland area. The hydrophilousvegetation is mainly composed of sedges(Cyperaceae), rushes (Juncaceae), and humidadaptedgrasses (Poaceae). Cushion plants formthe peat deposits favored by the compact andrhizomatous growing of its roots, e.g., Oxychloesp. and Deyeuxia sp., among others. Regionalplant communities are part of the Altoandeanphytogeographic unit, a grassy steppeassociated with cushion plants and herbsadapted to strong winds and high solarradiation. Dwarf shrubs as Adesmia hemisphericaand A. subterranea dominate; there are othershrubs such as Berberis empetrifolia and herbssuch as Viola, Calandrinia, Barneoudia,Tropaeolum, Nassauvia and Mutisia. Preliminarypollen results indicate that when the VEPsedimentary 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 thesetaxa are not typically described in literature. After∼3500 cal BP, the pollen record seems to exhibita higher fluctuation in herbs (i.e., Oxalidaceae,Malvaceae, Cariophyllaceae), shrubs (i.e.,Nassauvia, Senecio, Menonvillea) andhidrophylous taxa, probably influenced byvolcanic and/or glacifluvial disturbances. In thelast ∼600 cal BP, Poaceae dominates again andPteridophytes are still recorded but with very lowvalues. Obtaining further pollen results (inprogress) will make it possible to asses externaldriving factors that could have influencedchanges in vegetation belts, as glacierfluctuations or volcanic activity evidenced by thesedimentology and the geomorphology of thearea. The study of the VEP sedimentary sequencehelps reveal the evolution of this setting fromnon-biological (sedimentology, geomorphology)and biological (pollen record, present-dayvegetation) perspectives. The mid- and lateHolocene at this high Andean location,immersed in a glacial valley context, wasdominated by fluvial aggradation with thedevelopment of peatland environments.Neoglacial advances have been inferred for theLittle Ice Age and are recorded by morainedeposits upstream from the VEP profile. Theseenvironmental fluctuations in the glacial valleyare also reflected in modifications to thevegetation recorded in the VEP pollen record,reinforcing the potential of pollen as a sensitiveindicator of past environmental changes.