Holocene Fire History at Laguna Martínez, Costa Rica, Based on High-Resolution Macroscopic Charcoal Analysis of an 8400 cal yr BP Sediment Record

Macroscopic charcoal proxy records from sediment profiles from lakes and swamps provide valuable information on the role of fire in the world’s varied ecosystems. Macroscopic charcoal is not normally transported long distances, making it a good indicator of local fire history. This study focused on the macroscopic charcoal in an 8400 cal yr BP sediment record from Laguna Martínez, located in northwestern Costa Rica on the Pacific slope of the Miravalles volcano. Prior study of pollen assemblages in the Martínez sediment core revealed the earliest evidence of maize agriculture in Costa Rica at about 5500 cal yr BP. I sampled the core for high-resolution macroscopic charcoal analysis using a 4 cc sampler at contiguous 1 cm intervals, and quantified charcoal fragments in two size classes (250–500 μm and >500 μm) by sieving following treatment with 3% hydrogen peroxide. Charcoal concentration and influx were very high in the deepest portion of the sediment record, prior to about 7500 cal yr BP, when fires were likely associated with volcanic activity. Charcoal values were moderate at the time of the earliest evidence of maize agriculture, but increased in the later Holocene, with charcoal influx indicating elevated fire activity from about 3200–2600 cal yr BP, near 2000 cal yr BP, and near 1200 cal yr BP. Experimental use of the charcoal analysis program CHAPS revealed some trends and events not apparent in the raw charcoal influx data, including evidence of a fire event about 260 years prior to the first evidence of maize agriculture at Laguna Martínez.