Palaeogeography, Palaeoclimatology, Palaeoecology

Texto completo: acesso restrito. p.127–136Aquatic environments in the high-Andean may be quite vulnerable to climatic conditions and hydrological processes (precipitation/evaporation) that have the potential to alter water levels and chemistry. This study reconstructs the paleohydrological history of Lake Tota, Colombia, to provide a record of environmental changes in the eastern flank of the Colombian Andes during the late Holocene. A 54 cm core was collected at the margin of the lake at a depth of 1.5 m, and the paleohydrological reconstructions were based on changes in diatom assemblages, granulometry, and bulk organic geochemical parameters. The core was subdivided into 5 biozones by assessing the significance of the variance among the CONISS clusters. Biozone T5 (base, 4000 calibrated years BP) was dominated by the planktonic diatom Discostella stelligera and was deposited in a period of high-water level. Sedimentation during this period was rich in carbon due to phytoplanktonic production; the granulometry data suggests that the lake was under high to moderate hydrological dynamic conditions, indicating active deposition at the river delta front. Biozone T4, beginning at 3600 cal. yrs BP, was dominated by three species D. stelligera, Cocconeis placentula var. lineata and Fragilaria sensu lato, and this association could indicate that the lake was under the influence of large fluctuations in water level. This interpretation is supported by granulometry and geochemical data, particularly by the low levels of total organic carbon (TOC). In biozone T3, corresponding to 3000 cal. yrs BP, we found a lowering of the water level, suggested by low levels of TOC. Poor granulometric sorting and higher carbon/nitrogen (C/N) ratios suggest a macrophyte influence on the sedimentation, avoiding direct deltaic influence. In this biozone the benthic diatom F. sensu lato group was dominant. In biozone T2, starting 2400 cal. yrs BP, the proxies indicate that the lake experienced increasing water levels and that stronger circulation patterns occurred in the water column than in the preceding periods, as inferred by the presence of Aulacoseira granulata a ticoplanktonic species. The sediment in this biozone is well sorted and finer, indicating a greater depth. Biozone T1 at the top of the core and since 2000 cal. yrs BP to the present, evidence for a recent reduction in water level of Lake Tota can be deduced by the presence of Achnanthidium minutissimum var. minutissimum