Late Holocene climate and environment of the SE Pampa grasslands,Argentina, inferred from biological indicators in shallow,freshwater Lake Nahuel Rucá |
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| Authors: | Silvina Stutz C Marcela Borel Sonia L Fontana Laura del Puerto Hugo Inda Felipe García-Rodriguez Marcela S Tonello |
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| Institution: | 1.Laboratorio de Paleoecología y Palinología, FCEyN,Universidad Nacional de Mar del Plata,Mar del Plata,Argentina;2.Laboratorio de Palinología, INGEOSUR-Depto. de Geología,Universidad Nacional del Sur,Bahía Blanca,Argentina;3.Department of Palynology and Climate Dynamics,University of G?ttingen,G?ttingen,Germany;4.Department of Evolutionary Functional Genomics, EBC,Uppsala University,Uppsala,Sweden;5.Facultad de Ciencias,Universidad de la República, Montevideo, Uruguay,Montevideo,Uruguay;6.CONICET,Buenos Aires,Argentina |
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| Abstract: | We analyzed pollen, non-pollen palynomorphs (NPPs), calcareous microfossils, plant macrofossils, diatoms, chrysophyte cysts,
opal phytoliths and organic matter content in a 123-cm sediment sequence from Nahuel Rucá Lake, a shallow, freshwater system
in the southeastern Pampa grasslands, Argentina. Three stages in the lake evolution were identified. Before 3,680 cal year BP,
only pollen, NPPs (dinoflagellate cysts and acritarchs) and ostracods were recovered, suggesting brackish/saline conditions
in the lake and nearby areas. Freshwater conditions are, however, indicated by Myriophyllum, Pediastrum and Zygnemataceae. The brackish/saline conditions could have been caused by marine influence during a Holocene sea level
high stand that affected the area ca. 6,000 year BP. Between 3,680 and 390 cal year BP, macrophyte pollen and plant macrofossils
indicate increasingly freshwater conditions in the lake and the adjacent area. Diatom and ostracod assemblages, however, suggest
brackish and oligotrophic conditions, giving way to freshwater and meso-eutrophic conditions toward the end of this period.
The relationship between submersed macrophytes (Myriophyllum, Potamogeton, Ceratophyllum, Chara) and planktonic algae (Chlorophyta and diatoms), suggests a shift in the lake from a clear to a turbid state. This turbid
state is more evident after 390 cal year BP. High values of Pediastrum, Scenedesmus and diatoms (Cyclotella meneghiniana, Aulacoseira granulata, A. muzzanensis) observed during this stage could have reduced light penetration, with consequent loss of submersed plants. Pollen and plant
macrofossils in the uppermost 20 cm indicate a shallow, freshwater lake similar to present, though an increase in brackish/freshwater
diatoms suggests an increase in salinity, perhaps related to periodic droughts. Opal phytoliths yield a regional paleoclimatic
reconstruction that agrees closely with inferences made using pollen, mammals and sediment characteristics. |
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