Fossil and modern diatom assemblages from the Savanna lake El Pinal, Colombia:: An environmental reconstruction

In this paper we present an environmental reconstruction for the north-eastern part of Colombia known as the Llanos Orientales (4 degrees 8'N, 70 degrees 23'W) based on diatom and sediment analyses of a core drilled in Lake El Pinal and complementing published information from pollen analysis carried out for the same core. The diatom record obtained from the core was interpreted using modem diatom samples from El Pinal and surrounding lakes. The dominant diatom assemblage in the core is composed of several species of Aulacoseira including A. species I that could not be identified and could be a new species. The environmental reconstruction indicates that between 18,290 and 9020 C-14 yr BP the lake was part of a river system where periods of fluvial sediment accumulation alternated with periods of erosion. The pollen record shows dry savanna conditions for this time, with a slight change towards wetter conditions after 10,690 C-14 yr BP. Between 9020 and c. 1260 C-14 yr BP incipient lake conditions are indicated, but the presence of at least one hiatus indicates that the river was still present, It is possible that these conditions correspond to an incipient stage of an oxbow lake. The lake was shallow, with acidic water, low in nutrient concentration. Savanna herbs dominated the surroundings. Between c. 1260 and 1065 C-14 yr BP the lake finally became isolated from the river and conditions similar to those of today were established. Savanna is still the dominant vegetation. The last 1065 C-14 yr BP are missing from the record. The diatom analysis provided additional information to pollen-based reconstructions and gave a clearer picture of the hydrological evolution of the area, for instance, the decrease in Isoetes and increase in Cabomba was intepreted as indicating higher lake levels, whereas diatoms suggest quasi-lacustrine conditions caused by the isolation of the lake from the river and not an increase in water level. This has potential bearing on regional multi-site reconstructions.