STABLE CARBON ISOTOPE RATIOS AT LAGUNA VUELTAS, COSTA RICA, AS INDICATORS OF LAND-USE HISTORY AND PALEOCLIMATE

Stable carbon isotope ratios of total organic carbon in neotropical lake sediments complement and extend vegetation histories derived from the study of pollen and charcoal assemblages. Neotropical forest species predominately utilize the C3 photosynthetic pathway, while shade-intolerant herbs, cultigens, and agricultural weeds employ the C4 pathway. Due to the wide separation in δ¹³C values of these plant types, changes in lake sediment δ¹³C profiles can indicate changes in forest cover within lake basins resulting from changes in agricultural land use by prehistoric and historic human populations. Combined examination of pollen, charcoal, and stable carbon isotopes may in some cases improve the ability to distinguish changes in vegetation resulting from human activity within watersheds from those driven by climate change.

Laguna Vueltas is a small (0.3 ha) lake located at 270 m elevation in the Coto Brus valley of southern Costa Rica. We investigated the carbon isotope record of a 5.4 m long sediment core from the lake as a complement to prior pollen analysis. Fifteen archaeological sites have been identified near the lake. Prehistoric agricultural activity is evident in the lower portion of the sediment record, which corresponds to the Chiriquí archaeological period from ~700 AD–1500 AD. The δ¹³C values are most positive in the later part of the Chiriquí period, coincident with high percentages of maize pollen (Zea mays subsp. mays) and agricultural weeds. These agricultural indicators are also present earlier in the Chiriquí period, but this part of the record differs from the later period in showing a high rate of sediment accumulation and high concentrations of fern spores, along with peak values of microscopic charcoal. We interpret the sediment and microfossil evidence to indicate lowered lake level at ~1180 yr BP, concurrent with dry periods throughout the Caribbean region. The drought interpretation is also supported by the δ¹³C values in this section, which may reflect a combination of moisture stressed C3 vegetation, an increase in C4 vegetation, and maize agriculture occurring in the basin. δ¹³C values reach their most negative values during the post-Conquest period, when pollen evidence shows strong forest recovery associated with regional depopulation.