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We analyzed the δ2H composition of n-alkanes isolated from Laguna Zoncho, a small lake in southern Pacific Costa Rica, to reconstruct paleohydrology. Using a core that spans the past 3600 years, we found evidence of dry periods, most notably during the Terminal Classic Drought (TCD; ~1200 cal yr BP) and the Little Ice Age (~400 cal yr BP). Previous work at Laguna Zoncho, using bulk sedimentary δ13C and geochemical analysis, found that agriculture began to decline during the TCD. Our δ2H records confirm the occurrence of arid conditions coincident with the TCD at Laguna Zoncho and show that, despite receiving more than 3000 mm of precipitation per year, this region is susceptible to multidecadal droughts. [ABSTRACT FROM AUTHOR]

We examined intra-basin spatial variability of sedimentary geochemical and isotopic signals using a network of five sediment cores from a small lake in southern Costa Rica with a history of prehistoric maize agriculture in its watershed. All cores show a similar pattern of agricultural activity (2000-1000 cal yr BP), a transitional period of forest recovery (1000-675 cal yr BP), and a period after forest reestablishment (675-400 cal yr BP). During the agricultural period, bulk sediment stable carbon isotope ratios (δC) indicate significant forest clearance, percent total organic carbon (%TOC) is low due to accelerated erosion and dilution from mineral inputs to the lake, and carbon/nitrogen (C/N) ratios are consistent with increased productivity. At the conclusion of the agricultural period, δC and %TOC indicate rapid forest recovery and reduced mineral inputs and C/N ratios suggest lower lake productivity. There is little between-core variation in the magnitude of the agricultural signal for the four cores taken near the shore, but these cores indicate different timing for the end of widespread agriculture in the watershed. Three of these four cores indicate nearly all agriculture ended by 1000 cal yr BP, but in the fourth core agricultural indicators persist until 675 cal yr BP. The core from the center of the lake shows a gradual decline in proxies indicating agriculture from 950 to 650 cal yr BP, which suggests sediment-reworking processes are integrating material from the entire basin as it is transported to the deeper portions of the lake. Differences between the records from the cores recovered near the shore illustrate the potential of multiple core studies to create spatially explicit paleoenvironmental reconstructions, while the delayed and less abrupt changes in the core from the center of the lake highlight the importance of sediment reworking of paleoenvironmental indicators. [ABSTRACT FROM AUTHOR]

Analyses of pollen and other terrestrial microfossils in sediment profiles from neotropical lakes can complement and extend archaeological studies by documenting the timing of prehistoric human disturbances within watersheds. However assessing the scale of prehistoric human impact from sedimentary microfossil assemblages alone is often difficult. We explore here the utility of combining stable carbon isotope (δ13C) analyses of lake sediments and isotopic mixing models to improve our ability to gauge the extent of prehistoric human disturbance recorded in sediment profiles. Our test case involves the analysis of a sediment core from Laguna Bonillita on the central Caribbean slope of Costa Rica that spans approximately the last 2,700 calendar years. Variations in the δ13C values of the Laguna Bonillita sediments suggest that human population growth and environmental impacts in the watershed were at their maximum ∼cal yr 300 B.C. This finding is in keeping with archaeological evidence of rapid regional population growth at this time but differs from initial interpretations of the sediment record that were based on pollen and charcoal analyses alone. We believe that the use of stable carbon isotope data from sediment profiles can improve estimates of the scale of prehistoric human impact and in doing so improve the contributions of paleoecological research to archaeology. [ABSTRACT FROM AUTHOR]