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mostly barren of diatoms but containing pollen of Al-ternanthera, a herbaceous saline marsh plant (Leyden, 1985), characterized the lower 2.5 m and indicated that Lake Valencia was shallow and variably saline before 10,000 14C B.P. (Bradbury et al., 1981). Saline planktonic diatoms (Cyclotella quillensis) appeared at 10,000 14C B.P. and corresponded with the initial filling of Lago Valencia by 8700 14C B.P. Thereafter, the lake periodically drained and became fresh as indicated by the diatom Staurosira construens. Abundant Moraceae pollen indicates the replacement of dry scrub vegetation by forests in the early Holocene period (Leyden, 1985). The late glacial and early Holocene paleolimnologic changes at Lago Valencia resemble those at Laguna Quexil.

16.2.12. Laguna de Fúquene: Valle de Ubaté, Colombia (5.450N, 73.770W; 2580 m)

The poorly dated Fúquene record from the Colombian Andes shows that dry full glacial climates supported páramo (grass-dominated) vegetation (Van

Geel and Van der Hammen, 1973). Myriophyllum and Isoetes indicate cold, shallow, moderately alkaline, but oligotrophic, lacustrine conditions during that time (Fig. 13). By 11,000 14C B.P., warmer and moister climates promoted a change to Quercus-rich Andean forest vegetation in the drainage basin. Aquatic vegetation dominated by Potamogeton suggests a possible increase in lake level and productivity.

At the higher elevation site of La Laguna in this area (4.92°N, 74.33°W, 2900 m), palynological studies from a wetland core (Helmens et al., 1996) document cold, oligotrophic conditions (Isoetes) between ~ 15,000 and —8000 14C B.P., with an intermediate, undated interval of less Isoetes, but high Poaceae pollen content assumed to correlate with the Younger Dryas episode. Increased Botryococcus at 8300-8000 14C B.P. may suggest conditions of deeper, more eutrophic, and warmer water. This record resembles the Fuquene core in character, although available chronological control indicates differences in timing of climate-related paleolimnological changes. The higher elevation of La Laguna may explain the more persistent record of Isoetes (to 8000 14C B.P.) than was observed in the Fuquene core. Increased Botryococcus correlates with pollen evidence for a rise in

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