HIGH RESOLUTION ANALYSES OF SEDIMENT CHARACTERISTICS, STABLE CARBON ISOTOPE VALUES, AND MICROFOSSILS IN LATE HOLOCENE SEDIMENTS FROM LAGUNA CASTILLA, DOMINICAN REPUBLIC

Pollen grains, charcoal fragments, and other terrestrial microfossils in lake sediment profiles document prehistoric forest clearance and agriculture at sites throughout the world.  However, quantifying the scale of prehistoric human impact within watersheds based on sedimentary microfossil assemblages alone is often difficult.  Our recent work with lake and swamp sediments in Costa Rica demonstrates that the stable carbon isotope ratio of sedimentary organic matter (d¹³C_SOM) may provide at least a rough estimate of the scale of prehistoric forest clearance and agriculture in neotropical settings.  To further test the sensitivity and utility of this proxy we conducted high-resolution (~5 year sampling interval) analyses of sediment, pollen, and d¹³C_SOM of a lake sediment core from Laguna Castilla (18°47'51” N, 70°52'33” W, 976 m) in the Dominican Republic.  The Castilla sediment record extends back some ~3000 cal yr B.P., but our focus here is the interval from ~1100 to 600 cal yr B.P., when microfossil assemblages, sediment characteristics, and d¹³C_SOM values indicate prehistoric agriculture.  Comparisons between concentrations of maize (Zea mays subsp. mays) pollen and d¹³C_SOM values indicate that the stable carbon isotope signal of sedimentary organic material is sensitive to the scale of prehistoric maize agriculture and responds rapidly to agricultural expansion.  However, the d¹³C_SOM record may underestimate the scale of prehistoric human disturbance if erosion rates in the watershed decrease or agricultural plots are shifted away from the lake.  We apply a two end-member mixing model to the Laguna Castilla d¹³C_SOM record to explore the changing scale of prehistoric agriculture around the lake.  The mixing model indicates that the percentage of organic carbon originating from C₄ plants deposited from ~1000 to 600 cal yr B.P. varied between ~20% and ~40%.  While our mixing model results depend upon some variables for which we cannot fully account, such a large shift in the percentage of organic matter originating from C₄ plants indicates that a relatively large fraction of the Laguna Castilla watershed was, at times, under maize cultivation.  This finding is at odds with archaeological studies in Hispaniola that suggest maize played a very minor role in the diet of native prehistoric populations.