Late Holocene environmental history of southern Choco region, Pacific Colombia; sediment, diatom and pollen analysis of core El Caimito

We present a multi-proxy study of pollen, diatoms, sediment characteristics and major elements of a 610-cm sediment core from lake El Caimito, located in the humid rain forest of southern Choco, Pacific Colombia. We propose a n integrated reconstruction of the local basin development and of the regio nal vegetation which is possibly related to the tectonic activity of an uns table coastal area. Time control is based on 7 AMS C-14 dates that show tha t the record represents the last 3850 cal yr BP. From 3850 to 2700 cal yr BP sandy deposits, low carbon content, absence of diatoms, and low diversity of the pollen spectra indicate that the site was under the influence of the fluvial system. Erosive event(s) removed pan of the sediment record and we observed a hiatus representing 700 years. After that, the basin became more isolated from the river drainage system. From 2010 to 1430 cal yr BP mainly clay was deposited and repeatedly interrupted by river pulses that left sandy and silty horizons in the record. Benthic and littoral-benthic diatom species indicate a shallow water body and a sta ble water chemistry. Mangrove forest was close to the lake, apparently grow ing along the close-by inlets. Regionally, the main vegetation elements wer e palms, and taxa in the families of Moraceae-Urticaceae, Melastomataceae, Fabaceae and from a number of other families and genera, characteristic of tropical lowland rain forest. From 1430 to 810 cal yr BP the river impact gradually diminished. Each fluv ial event that affected the local forest is shown in the pollen record by a n expansion of Cecropia dominated pioneer forest. Decreasing intensity of f orest disturbance coincides with an increase in the diversity of fossil pol len taxa, possibly reflecting an increasing plant diversity of the forest. Mangrove pollen declined, indicating that the coastline moved seaward and s uggesting tectonic uplift of the coastal area. Between 810 and 580 cal yr B P mangrove forest was closer to the lake again, reflecting an inland migrat ion of the coastline, suggesting tectonic subsidence. From 580 to 300 cal y r BP the last fluvial events were recorded. Diatom associations indicate ol igotrophic and acidic water. The mangrove belt moved seaward again, suggest ing tectonic uplift. Palms and Cecropia became more abundant, suggesting in creased human impact in the near shore lowland forest. During the last 300 years, stable lacustrine conditions and lowland rain forest with the highes t floral diversity is registered. (C) 2001 Elsevier Science B.V. All rights reserved.