Date of Award

5-2011

Degree Type

Thesis

Degree Name

Master of Science

Major

Geography

Major Professor

Sally P. Horn

Committee Members

Carol P. Harden, Kenneth H. Orvis, Chad S. Lane

Abstract

Lago de las Morrenas 4 (9.498056° [degrees] N, 83.486111° [degrees] W, 3466 m elev.) is the lowest lake in a chain of glacial lakes located in the Valle de las Morrenas, a valley facing almost due north from Cerro Chirripó, the highest peak in the Cordillera de Talamanca in Costa Rica. Coarse resolution analyses of pollen, microscopic charcoal, and loss-on-ignition of a ca. 10,000 year sediment record from Lago de las Morrenas 4 was carried out to complement and extend previous research on the environmental history of the Chirripó highlands and to provide context for high-resolution sampling and analysis of early Holocene sediments. The focus of the high-resolution analysis was the period around 8200 cal yr BP, the time of a prominent Holocene climatic event recognized at many northern hemisphere sites between ca. 8000 and 8400 cal yr BP. The high-resolution sampling of sediments encompassing the time period of the 8200 yr BP event revealed a sharp increase in microscopic charcoal indices coincident with a shift from fibrous silty organic sediment to granular peat. These shifts, which occurred ca. 7900 cal yr BP, are interpreted to reflect an interval of drier climate and lower lake level. A decrease in Poaceae pollen percentages and an increase in Alnus pollen from montane forests may reflect reduced pollen production within the páramo at this time. The peak charcoal concentration and influx value at ca. 7800 cal yr BP in the Morrenas 4 record may match a similar peak in the macroscopic charcoal record from nearby Morrenas 1 that occurred ca. 8000 cal yr BP. Both peaks may correspond to the 8200 yr BP event.

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