Abstract
Long pollen records from two sediment cores of the basin of Bogotá (Colombia) are presented, reflecting the dynamic history of environmental and vegetation changes in the tropical high Andes during the Quaternary. An astronomically tuned age model has been developed by using the visual correlation of the temperature-constrained record of
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Alnus with the benthic δ18O stacked record of ODP Site 846 and 849 for the last 1 million year (Ma). In addition, spectral analysis of the arboreal pollen percentages (AP%) prior to 1 Ma shows two consistent peaks at 7.6 and 9.5 m, which could either be interpreted as reflecting the 19 and 23-kyr components of the precession cycle or the 41-kyr cycle of obliquity. Evidence for precession forcing comes from the sum of the filtered 7.6 and 9.5 m cycles, revealing a long-term modulation that can be linked to the short-term (100-kyr) and long-term (400-kyr) eccentricity cycles. A precession-forced scenario, however, is not consistent with zircon fission-track ages and is difficult to explain in terms of climate forcing mechanisms. On the contrary, an obliquity control of the AP% record is consistent with the fission-track ages and with glacial-interglacial-bound temperature changes within the study region. Accordingly, it appears that the Funza09 record extends back to ∼2.25 Ma during which four periods with distinct different depositional environments could be identified, following a chronological poorly constrained interval of fluvial and fluvio-lacustrine sediments. From ∼2.25 to ∼1.47 Ma sediment deposition occurred in an area of wetlands dissected by fluvial channels and swamps. The strongest subsidence of the basin occurred between ∼1.47 and ∼1.23 Ma when a proper lake developed. Lacustrine sediments accumulated in water up to 50 m deep between ∼1.23 and ∼0.86 Ma. Hereafter, water depth was generally lower than 50 m, but fluctuated in conjunction with the 100-kyr dominated glacial-interglacial variability during the middle and late Pleistocene. The evolutionary changes of the páramo and montane forest biomes are described in terms of five characteristic stages. Most of the Pleistocene vegetation has no analogue to modern assemblages principally due to the late immigration events of Alnus (1.01 Ma) and Quercus (0.43 Ma) and we conclude that forest composition similar to modern was not established until the Last Interglacial. However, modern ecological constraints of suites of taxa that formed the vegetation of the pre-MIS 5 part of the record allow a reconstruction of environmental and climate change.
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