Late Holocene environmental history and tephrostratigraphy in northwestern Costa Rica : a 4000 year record from Lago Cote

Paleoecological studies in the neotropics are revealing changes in past climate regimes and human use of the landscape that were previously unknown. Few paleoecology studies have been conducted in Costa Rica and, in most of these studies, strong evidence of prehistoric human impacts overwhelmed evidence of climate signals. Lago Cote, in the Cordillera de Tilarán, is the largest natural lake in Costa Rica (198 ha). Because of its size, Cote sediments reflect a more regional record of pollen and charcoal accumulation. The sediments also preserve tephra layers from past eruptions of nearby Arenal volcano. In this thesis, I present an environmental history and tephrostratigraphy based on pollen, charcoal, and tephra preserved in a 3.6 m sediment core from Lago Cote. High amounts of grass charcoal and grass pollen from the lower part of the core indicate a drier climate regime with frequent landscape fires from ~4000 to 2600 cal. yr BP. Many or most of these fires were likely of human origin. The occurrence of a maize pollen grain in this zone documents agriculture at ~3900 cal. yr BP. After 2600 cal. yr BP, a dramatic decrease of grass pollen and charcoal reflects wetter conditions that limited landscape fires and agriculture. This climate change occurred simultaneously with an archaeological phase change documented in the nearby Arenal basin by the Proyecto Prehistórico Arenal. Small peaks in pollen of lower montane pollen taxa at the base of this zone may indicate forest regeneration. Comparison of Cote tephra to an earlier tephrochronology at the nearby El Tajo site was not straightforward. Cote tephras, analyzed by X-ray fluorescence, show differences in mineral composition when compared to the more proximal El Tajo tephras, and have lower levels of silicon dioxide than found at El Tajo. Because of major differences in tephra chemistries, visual comparison was a more effective way to compare tephra layers between the sites. Three tephra layers reported from El Tajo are not present in the Cote sediments, including the most recent Arenal eruption in 1968. Two tephra layers in the lower part of the Cote core are not part of the El Tajo tephrochronology, and represent earlier eruptions from Arenal or the nearby and older Chato volcano. In the Cote sediments, peaks in Cecropia pollen follow at least two tephra layers, most likely indicating vegetation regeneration following eruptions of Arenal volcano