This thesis presents an environmental and climatic reconstruction for Colombia (South America) since the Lateglacial based mainly on changes in lake level obtained on the basis of diatom analysis. Five lakes were analysed from four different climatic and ecological settings of Colombia in order to ensure that the research encompassed a wide variety of ecosystems and climates and a diverse assortment of diatoms. The choice of lakes took into account the diversity of their morphology, depth and extension. The sites range from the very wet western lowlands (rain forest), crossing relatively dry lowlands (dry forest), up to the highlands of Andes (montane ecosystems) and finally the eastern lowlands (savanna ecosystems).

The western most site, lake El Caimito, records periods of higher river influence on the lake that could be related to periods of increased precipitation. The Intertropical Convergence Zone (ITCZ) and the Chocó jet stream are mainly responsible for the amount of precipitation and seasonality in this area and the main disturbing factor is the El Niño Southern Oscillation (ENSO). The effect of ENSO in this area is mainly felt during phases of La Niña that strengthen the effect of the Chocó jet stream causing even more precipitation. The sediments from lake El Caimito seem to be sensitive to changes in precipitation (increase) possibly related to La Niña-type events; studies of high resolution in this area may shed some light about this phenomenon. An accurate climatic reconstruction for the Patía site (between the Western and Central Andean Cordilleras) may be hindered by the fact that more than one factor, such as climate change, ecological processes, hydrological changes and human impact may be responsible for generating the changes in the data. The clearest climatic signal is an increase in humidity between 7670 and 6860 14C BP. Agricultural activities in the Patía area started around c. 2200 14C BP indicated by the presence of Zea mays. The record from Fúquene lake (highlands of Eastern Andean Cordillera) shows Glacial conditions of dry and cold climates that prevailed between 19,700 to 14,200 14C BP. Interglacial climates, warmer and more humid conditions, are recorded after 14,200 14C BP. A return to glacial conditions is indicated by a hiatus between 13,110 and 8680 14C BP covering the full El Abra Stadial (the equivalent to the Younger Dryas in Colombia). Warm and more humid climates return after 8680 14C BP and prevail until 7070 14C BP. In this period the lake reached its maximum extension, which suggests an increase in precipitation. After 7070 14C BP drier conditions became established. The climatic reconstruction from lake Las Margaritas (eastern lowlands) shows dry climates between 11,200 and 8130 14C BP and a general tendency towards wetter climates after 6470 14C BP (c.f. Fúquene). This tendency in the mid Holocene can be explained by the southward migration of the ITCZ that increased the amount of precipitation in the Amazon region and this increase in humidity reached Las Margaritas. The record from the lake El Piñal (eastern lowlands) shows the history of the area since 9020-c. and documents the gradual isolation of the lake from the main fluvial system after 1260 14C BP. This close connection between the lake and the river makes to obtain an accurate climatic reconstruction.

Concluding remarks

During the early Holocene the more northerly position of the Intertropical Convergence Zone (ITCZ) brought more precipitation to the north of South America, including Fúquene. During the mid-Holocene it shifted southwards, causing drier climates in the north of South America and more humidity in the Amazon region. The record from Las Margaritas appears to reflect this Amazonian pattern. The El Patia record seems to be responding a range of forcing factors, but increased moisture in the mid Holocene is consistent with the record from Fúquene.

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