CONNECTIONS BETWEEN CLIMATE AND LIMNOLOGIC CHANGE DURING THE HOLOCENE IN A VERMONT POND

Deep- (7m) and shallow-water (0.78 m) sediment cores from Twin Ponds in Brookfield, VT provide a suite of fossil, lithologic, and geochemical data used to reconstruct limnological change during the Holocene. Paleolimnologic data was compared with published pollen-inferred climate reconstructions and known climatic forcings. The warmest and driest interval of the Holocene (10-8 ka cal BP) shows evidence for low total aquatic productivity and minerogenic erosional inputs (XRF Ti and K counts) and the preservation of calcite-iron layering. These results suggest prolonged stratification and low-nutrient input during the highly seasonal climate of the early Holocene. An increase in total productivity and erosion at 8.3 ka cal BP coincides with a regional increase in precipitation linked to the collapse of the Laurentide Ice Sheet. Between 8.3 and 5.5 ka cal BP, phytoplankton productivity was dominant relative to macrophytes and zooplankton. Core lithologies indicate faint layering and XRF counts show high sulfur and iron, and declining calcium, all indicating a more anoxic hypolimnion caused by increased productivity and prolonged stratification. Lithologic and geochemical data show a transition at 5.5 ka cal BP when nitrogen isotope values, magnetic susceptibility, and sulfur values all decline, while iron counts increase. Collectively, these data suggest longer periods of seasonal overturn as peak seasonal insolation values shifted towards autumn. Total productivity declines between 5.5 and 3.5 ka cal BP, a fluctuation which coincides with the mid-Holocene decline in hemlock pollen. The period from 3.5 to 1.5 ka cal BP is marked by high plankton productivity and peaks in erosion indicators. Fluctuations between phytoplankton and zooplankton during this time indicate centennial-scale trophic level interactions driven by increased fluxes in nutrients. Between 1.5 ka and 200 cal yr BP evidence for both erosion and phytoplankton decrease, while evidence for zooplankton and macrophytes increase suggesting a shift towards macrophyte-dominance. Decreases in iron, calcium and a small increase in sulfur indicate longer periods of stratification. At ca. 200 yr BP a sharp increase in erosion and phytoplankton coincide with the pollen-confirmed European settlement horizon.