Integrated diatom-pollen based Holocene environmental reconstruction of lake Las Margaritas, eastern savannas of Colombia

A multiproxy analysis that includes diatoms, pollen, element geochemistry, carbon content analysis, and radiocarbon dates was carried out on a core from Las Margaritas (3 degrees 23'N, 73 degrees 26'W) in order to reconstruct environmental and climatic change. This lake is hydrologically closed and is located in the transitional area between the Amazonian rainforest and the savannas of the eastern part of Colombia. It therefore offers an excellent opportunity to study climatic change as expressed by competition between the biomes and changes in lake level caused by changes in water balance. Dry conditions, with a landscape covered by savanna and an incipient lake still influenced by the river system, characterized the period between 11 150 and 9100 cal. yr BP (11 190-8130 C-14 yr BP). More humid conditions prevailed between 9100 and 7330 cal. yr BP (8130-6470 C-14 yr BP), during which the forest advanced over the savanna and a swampy, isolated lake was formed. From 7330 to 1870 cal. yr BP, generally humid conditions prevailed, except for two dry periods between 5700-5200 cal. yr BP (4990-4620 C-14 yr BP) and 2750-1870 cal. yr BP (2640-1920 C-14 yr BP) that caused a marked decrease in lake level. A period of frequent shifts between drier and wetter periods occurred between 4180 and 2750 cal. yr BP (3810-2640 C-14 yr BP) and caused dynamic competition between the savanna and the forest and frequent oscillations in lake level. During dry periods lake level dropped and the water had higher conductivity and was richer in nutrients. Swamps with palms (Arecaceae) were formed. During humid periods, the lake had high water levels, the water was acidic, of low conductivity and poor in nutrients; Mauritia swamps surrounded the lake. Modern climates and limnological conditions have been established since 1870 cal. yr BP. The change to more consistently humid conditions observed after 9100 cal. yr BP and comparison with other environmental and climatic reconstructions from northern Colombia, the Llanos Orientales, and the Amazon allow us to conclude that humidity in this zone has been driven by millennial shifts in the Inter Tropical Convergence Zone and is mainly dependent on moisture coming from the Amazon region.