Date of Award


Degree Type


Degree Name

Doctor of Philosophy



Major Professor

Sally P. Horn

Committee Members

Kenneth H. Orvis, Henri D. Grissino-Mayer, Randall L. Small


This dissertation presents evidence of long-term changes in climate, fire, and vegetation in northwestern Costa Rica, as revealed by sediment profiles from lakes on the seasonally dry, lower Pacific slope of Miravalles volcano in Guanacaste province (10.7o N, 85.2o W). Sediment cores were recovered for pollen and charcoal analysis from six lakes formed by volcanic activity approximately 8200 years ago.

Six regional pollen zones were delineated based on changes in pollen and sediments, and dated by AMS radiocarbon determinations on macrofossils. The basal pollen zone (Zone 6, prior to 8000 cal yr BP) comprises elastic sediments that correspond to the interval of lake basin formation. Zone 5 (8000-7 500 cal yr BP) represents a transition to lacustrine sedimentation and shows increasing tree pollen percentages and abundant microscopic charcoal. During Zone 4 (7500-6800 cal yr BP) pollen assemblages indicate drier conditions, and the development of Myrica bogs in two basins; charcoal concentrations are highest in this zone. In Zone 3 (6800-5500 cal yr BP), tree pollen generally increases, but there is evidence of an episode of drier climate prior to 5500 cal yr BP. Zone 2 (5500-3700 cal yr BP) represents a period of prehistoric agriculture, punctuated by a dry period around 4000 cal yr BP. Maize pollen first appears at the base of the zone and is present throughout. Herbaceous pollen increases at the expense of tree pollen, and charcoal concentrations are high. Zone 1 (3700 cal yr BP to the present) also contains maize pollen, but charcoal concentrations decrease, and tree pollen remains low until modem times. Sediment characteristics indicate a drought interval at 2500 cal yr BP.

The analysis of cores from multiple lakes made it possible to interpret regional as well as local changes, including shifts in climate and several drought episodes. Maize pollen grains deposited at 5500 cal yr BP constitute the earliest evidence of agriculture in Costa Rica, and document human occupation of the region 1500 years earlier than previously known. Charcoal profiles show that natural and anthropogenic fire has been part of this ecosystem for the past 8000 years.

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