Late Pleistocene Puna Glaciation, Deglaciation and High Lake-Levels during the Transition Wisconsin-Holocene, and Postglacial Megafauna Refuges in the Atacama Desert and Puna Regions

Abstract

Ochsenius, C. (1986). "The Late Pleistocene Puna Glaciation, Deglaciation and High Lake-Levels during the Transition Wisconsin-Holocene and Postglacial Megafauna Refuges in the Atacama desert and Puna regions''. Geoscientific data (Geotectonics/Volcanology} support and important uplift of the Andean Cordillera during the Quatemary ("Junge Hebung"). Thisphase ofuplift (still continuing in someareas)was in an extension of more than 8,000km not isometric but differential and reached an average of ca. 3,000 m in altitude. This last phenomena would be the cause for the discontinuous track and stratigraphical hiatus left by earlier glaciations (cf. "glaciation islands" during the Kansan/ Mindel/EIster, etc.). Towards the Upper Pleistocene the uplift trend increased progressively permitting an extensive alpine glaciation during the wisconsin along the whole Andes. In this context, ancient glaciations in the Andean region wereltmited, its are aldepending lesson the atmospheric cooling than on the preexisting topography (except, of course, southernmost South America]. The geomorphological features of the Puna Glaciation indicate a maximum advance ofglacier tongues to 4,200 m, the regional average (between 17º-240 Lat. S) being at ca. 4,400 m.a.s.l. Radiocarbon datings oflacustrine sediments of basins with outlet (at 2,500 m) reveal the importance of the predomain effect of this glaciation [or the evolution of perennial shallow lakes 27,000 y.B.P. Their water budgest were more determined by emissary rivers (nivo-glacial regime}, originated at the foot of glaciers, and not by a typical pluvial climate. High lake-levels (100-160 m depth] in closed basins were caused by the melting an drapid recession of glaciers (Deglaciatíon} atap proximately 13,000-10,000 y.B.P. During the full-glacial the lacustrine status of these basins was much lower. Thus for example, the Carcote Basin(southernmost waterbody ofthelacustrineTaucaPhase oftheAltiplano)hadanormallake-level ofabout 2m similar to the present Poopó Lake. During the transition Wisconsin-Holocene an apparently contradictory ecological phenomena turned up: High lake-levels were contemporary with an increase of aridity in the non-lacustrine areas because of a major evaporation rate resulting of the postglacial trend of temperatures. This fact would also explain the drastic contraction of the prehistoric human settlement distribution pattern appearing the archaeological evidence (basical/y lithic workshops] in abundance around these paleoenvironments which at the present are filled with evaporites.

Stil/ unknown is how long the deep fresh-water lakes resisted the worldwide Holocene climatic amelioration. However, it is probable that some reached ca. 9,000 y.B.P. After that time the whole region was submitted to an irreversible desertification.

In sum, during the last 30,000 y .B.P. two extreme situations characterized the stand of the lake-levels near the Occidental Cordillera, in the Atacama and Altiplano region: lower and middle status dominated during the full-glacial and exceptionally high levels (more than 100 m deep) during the drastic glacial recession (Eo-Holocene Deglaciation and very probably during the Last Interstadial before 27,000 y.B.P.). Former paleoclimatological models, supposed a horizontal shift ofthe standard climatic zones to explain the major humitity of the region during the Last Glaciation (southem shift of the Equatorial Low Pressurearea andlor northem drift o] the Polar Front activity by 60 Lat.). The present study supports however that the atmospheric circulation pattern has remained constant . A proof is given by the total correspondence between the maximum in altitude, at the same latitude, of the firn snow line during the Puna-Glaciation and the present firn snow lineo This would not only implicate that thís region was one of the driest domains along the Andes but also the persistence in latitude of the Subtropical Low Pressurearea.

Final/y, the comparatíve analysis and reínterpretation ofradiocarbon datings, on continental and locallevel, suggest the probability that isolated megafauna groups (Megatheriidae (Megatherium) Mylodontidae (Glossotherium) would have survíved in the Atacama Desert and other regions of South America until far into the Holocene. The latest fauna findings show a typical refugial dístribution partern and are concurrent with the existence of oases in the Atacama (Pampa del Tamarugal). The edentata refuges (only 0,5% of a total of 500 paleo-ontological localities in South America) modífy ín no way the notable mass extínctíon of lhe South American megafauna towards the end of the Last Glaciation and its succession, in lhe Atacama and Puna region, by large camelíd populatíons (Tribe Lamíní) , which occupíed the vacant ecologícal niches left by the giant grazers.