LATE HOLOCENE ENVIRONMENTAL EVOLUTION OF NAHUEL RUCÁ FRESHWATER SHALLOW LAKE, SE PAMPA GRASSLANDS, ARGENTINA

STUTZ, S., BOREL, C.M., DEL PUERTO, L., INDA, H., GARCÍA-RODRÍGUEZ, F. AND FONTANA, S.

Punta del Este, Uruguay

Congreso; VI SHALLOW LAKES CONGRESS; 2008

Late Holocene environmental evolution of Nahuel Rucá freshwater shallow lake, SE Pampa grasslands, Argentina S. Stutz1, C.M. Borel2,, L. del Puerto3, H. Inda3, F. García-Rodríguez3 and S. Fontana4. 1. Lab. de Paleoecología y Palinología, FCEyN, UNMdP, Mar del Palta, Argentina. CONICET. 2 Lab. de Palinología, Depto. de Geología, UNS, Bahía Blanca, Argentina.. CONICET 3. Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay. 4. Dept. of Geography, University of Liverpool, Liverpool, United Kingdom.  We present a multi-proxy reconstruction of environmental evolution from a sediment sequence of Nahuel Rucá freshwater shallow lake, SE Pampa grasslands, Argentina during Late Holocene. This reconstruction is based on analysis of pollen, non-pollen palynomorphs (NPPs), diatom, chrysophyte cysts, ophal phytolits, ostracods, plant macrofossils and organic matter. The chronology is derived from three AMS 14C dates, 3480±70 yr BP, near the base of the sequence, 871±35 yr BP at 71cm depth and 332±34 yr BP, at 50cm depth. Three stages in the evolution of the lake were registered. Before ca. 3500 yr BP, only pollen, NPPs (dinoflagellate cysts and acritarchs) and ostracods  were recovered, suggesting brackish/saline conditions in the water and in the surrounded area. However freshwater conditions were also indicated by Myriophyllum and NNPs, mainly Chlorophyta (Pediastrum and Zygnemataceae). This brackish/saline environment could be explained because of the intrusion of salt water into the lake during transgressive-regressive cycle, which affected the area ca. 6000 yr BP and also reached a contiguous shallow lake, Laguna Hinojales (1). Between ca. 3500 and 332 yr BP, macrophyte pollen and plant macrofossils indicate increasing freshwater conditions in the lake and in the adjacent area. On the other hand, diatoms and ostracods suggest brackish and oligothrophic conditions becoming freshwater and meso-euthrophic toward the end of this stage. Organic matter values are consistent with this trend. The relationship between submerged macrophyte (Myriophyllum, Potamogeton, Ceratophyllum, Chara) and planktonic algae (Chlorophyta and diatoms), suggests the shift of the system from a clear state to a turbid one. This turbid state became more evident after 332 yr BP. The high values of planktonic chlorophycean algae (Pediastrum, Scenedesmus) and diatoms (Cyclotella meneghiniana, Aulacoseira granulata, A. muzzanensis) observed during this stage could have produced a reduction in light penetration, with consequent loss of submerged plants because of shading. Ostracods indicate a eutrophic water body with nutrient enrichment. Pollen and macro remains of the top 20cm indicate a freshwater shallow lake similar to the present one, whereas the increase of brackish/freshwater diatom suggests that the system might have increased in salinity levels. This could be related to periodic droughts. Organic matter shows similar values to those observed for the beginning of the turbid state. Opal phytolits, although there was difference in composition throughout the sequence, indicate that climatic conditions were most probably always cool and very humid to humid within the area. (1) Stutz, S., Prieto, A.R., Isla, F.I. 2006. Holocene evolution of the Mar Chiquita coastal lagoon area, Argentina, indicated by pollen analysis. Journal of Quaternary Science, 21: 17-28.