LATE QUATERNARY HISTORY OF LAGUNA LLANCANELO, MENDOZA, ARGENTINA.

GARCIA, ADRIANA; CHIVAS, ALLAN; D'AMBROSIO, D. SABINA; PAUL CONOLLY

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

LagunaLlancanelo (35°38´S, 69°08´W) is an ephemeral saline lake located ~400 km southof Mendoza, and ~70 km east of the Andes, developed in the Huarpes depression.The lake is within the Llancanelo and Payunia back-arc volcanic fields, an areacharacterized by ~800 volcanic eruption sites which have been active between 2Ma and 7 ka years ago, perhaps also, in more recent times. A multidisciplinarystudy of sediment cores from the lake is under way, with nine undisturbed longcores up to 10 m long collected in 2010 from the eastern and western margins,and representing at least the last 30 ka (the onset of Marine Isotope Stage 2)of the lake?s history. Grain size analyses, X-Ray Diffraction/X-RayFluorescence analyses (at least three explosive volcanic events are clearlypreserved in the lake) and micropalaeontological analyses (charophytes andplant remains) were performed on three long cores on both margins of the lake.The charophyte association before the Last Glacial Maximum (LGM)(~ 20 ka ago)is represented by oospores and gyrogonites of Nitella sp. 1, Characontraria, Chara sp. 1 and 2, and oospores of Sphaerochara (=Tolypella sect. Rothia); this section also holds excellently preservedorganic remains of emergent aquatic plants - probably Cyperaceae). During theLGM the species present are almost the same, albeit in different abundances;changing after the LGM to an association of Nitella sp. 1 and 2, Charavulgaris, Chara contraria and Chara sp. 1. For the pre-LGM, a largerfresh waterbody is indicated by abundant freshwater taxa, and particularlylarge numbers of Sphaerochara sp.; conditions continuing during the LGM.The charophyte association changes before the start of the Holocene with thepresence of saline-prone taxa - Chara halina, C. hornemannii and Lamprothamniumsp. -, and the presence of a few Chara contraria gyrogonites,indicating more saline and warmer conditions. The Holocene is characterized byincreasing aridity, until reaching the present state, with abundant Charahalina, C. hornemannii and Lamprothamnium sp., and few Charasp. 1. The Holocene section contains at least two large evaporative cycles.The hypothesis about the past larger size of the lake, has been proved for atleast its pre-LGM stage with the identification of active palaeodrainagechannels on the eastern side of the lake (on the western side, the watercoursesare still active), while for the Holocene, the lake became substantially drierand subject to periodical subaerial exposure (increasing deflation, gypsumprecipitation and higher salinity).