New Proxy Analyses from Laguna Martinez, Costa Rica

Authors: Sally P. Horn*, University Of Tennessee, Kurt A. Haberyan, Northwest Missouri State University , Mathew S. Boehm, University of Tennessee, Matthew T. Kerr, University of Tennessee , Wesleigh J. Wright , University of Tennessee, Emma E. Reed, University of Tennessee, Chad S. Lane, University of North Carolina Wilmington, Martin R. Arford, Saginaw Valley State University , Jason E. Graham, University of Tennessee
Topics: Paleoenvironmental Change, Biogeography
Keywords: Costa Rica, environmental history, pollen, fire, maize, sediments
Session Type: Paper
Day: 4/4/2019
Start / End Time: 1:10 PM / 2:50 PM
Room: Coolidge, Marriott, Mezzanine Level
Presentation File: No File Uploaded

Laguna Martínez (10.6405N, 85.1961W; 340 m elevation) is a small (1.5 ha) lake on the lower slope of Míravalles volcano in northwestern Costa Rica. This lake and a dozen lakes nearby occupy undulating topography created by a volcanic debris avalanche about 8400 years ago. Previous pollen analysis of a 5.8 m-long sediment core from the lake revealed the earliest botanical evidence of maize cultivation in Costa Rica. Dated to ~5500 cal yr BP, the early maize pollen occurs during a transition in pollen assemblages from dominance by trees to dominance by grasses and weedy herbs. High-resolution macroscopic charcoal analysis documented recurrent local fires through the record, with early fires attributable to the volcanic eruptions that formed the lake, and later fires of probable human origin. In new work presented here we examined diatoms, bulk stable carbon and nitrogen isotopes, magnetic susceptibility, and elemental composition of the sediments. Diatoms show marked variation across the profile, but changes in assemblages do not align strongly with terrestrial proxies. Stable carbon isotope values show increased dominance of C4 plants beginning ca. 5000 yr BP, tracking increased grass pollen percentages. The most positive δ13C values are in sediments deposited from 3500-1000 cal yr BP that also show peak values for grass pollen and charcoal influx. Within this interval magnetic susceptibility and Mn/Fe ratios peak at ~2400 and 1200 cal yr BP. These sedimentary proxies suggest drier conditions at Laguna Martínez during times of late Holocene drought evident in other records from Central America.

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