AN ARIDIFICATION EVENT BEGINNING ABOUT 1600 AD IN CENTRAL AMERICA: EVIDENCE FROM A LAKE SEDIMENT CORE IN LAGUNA ZONCHO, COSTA RICA AND EFFECTS ON AGRICULTURE AND POPULATION

Recent studies note the effect that climate had on indigenous populations of Central America and southern Mexico. Several dry periods over the last millennia have been recorded in lake and ocean records from this region, with possible implications for pre-Columbian agriculture and population. To assess the impacts of climate on soil development (and thus agricultural potential), we examined soil nutrient status in association with pollen profiles from Laguna Zoncho, a small lake in southwestern Costa Rica with a record of agricultural and population from pre-Columbian times through the present.

We used a detailed analysis of phosphorus (P) geochemistry related to soil development on sediment samples from a core taken from Laguna Zoncho. These P fractions, which include mineral P, occluded P (oxide-bound), and organic P, are used to assess soil nutrient status; we use lake sediment records of these fractions to infer past soil nutrient dynamics in the local watershed. The Laguna Zoncho record reveals two interesting features of nutrient cycling in this region. First, despite pollen evidence showing clear and profound shifts in plant species related to maize agriculture in the pre-Columbian record, the net soil nutrient status has remained stable, indicating that even a substantial population base and ~1000 years of agricultural activities did not significantly affect soil nutrient dynamics. Second, a strong transient shift toward higher occluded P occurs around ~400 years ago, coinciding with the re-advance of native forests and nearly complete abandonment of maize agriculture, and presumably population, around 1600 AD, following the Spanish Conquest. This short-term (~100-200 year) interval of arid conditions resulted in more occluded P on the landscape, and is characteristic of low precipitation and the build-up of oxide-associated P in soils (as seen in sites from arid southern California). Aridification may have played a role in the sharp decline in maize cultivation at the site, although the links between precipitation and maize agriculture in this rainy area are still unclear. Whether or not drought played a role in site abandonment, the P geochemistry of the Zoncho core suggests that regrowth of forests that began about 1600 AD occurred in the context of perhaps the most arid conditions that this area has experienced over the past ~3000 years.