Spatial Variation in Organic Carbon and Stable Isotope Composition of Lake Sediments at Laguna Zoncho, Costa Rica

Lake sediments are valuable paleoenvironmental archives that provide information on past climate and land-use change. Most lake sediment studies rely on a single core, usually recovered from the center of a lake, and do not consider spatial variability in the lake basin. My dissertation presents a spatially-explicit record of prehistoric agriculture from Laguna Zoncho, Costa Rica and evaluates spatial variability in lake sediment proxies based on a network of five sediment cores. Results extend earlier proxy analyses of a single core collected near the center of the lake, which documented prehistoric agriculture and forest clearance from 3000 to about 500 years ago, followed by strong forest recovery at the time of the Spanish Conquest. Analyses of the new suite of cores show that agricultural activities increased erosion in the watershed, which lowered organic content from 16% to 5%, and resulted in a shift in bulk sediment stable carbon isotope values from –27 ‰ to –23 ‰ VPDB due to forest clearance. Agriculture made the lake slightly more productive, shown by a decrease in carbon/nitrogen ratios from 16 to 13 and an increase in stable nitrogen ratios from 1 to 3 ‰. Basinwide trends in organic matter and stable carbon isotopes ratios show two distinct periods of agricultural decline (1150–960 and 840–650 cal yr BP) that coincide with intervals of drought detected in regional paleoclimate records. This finding suggests that climate change, not the Spanish Conquest, was the driving force of site abandonment at Laguna Zoncho, and by extension throughout the region.

Inter-core variability in proxies for agricultural activity reveals that crop cultivation may have continued longer in some portions of the watershed, and highlights the influence of sediment-focusing processes on proxy signatures of agriculture in lake basins. Maize pollen concentrations in the sediment cores did not correspond to geochemical and isotopic agricultural indicators, suggesting a need for caution in using the abundance of maize pollen to infer the scale of agriculture in neotropical watersheds.