SPATIAL AND TEMPORAL VARIATIONS IN SEDIMENTATION AT TWIN PONDS, CENTRAL VERMONT

Three lake sediment cores taken from across the basin of Twin Ponds in Brookfield, VT were analyzed for stratigraphic changes in macrofossils, magnetic susceptibility, grain-size, carbonate, and organic carbon. Twin Ponds is located within the carbonate-rich bedrock of the Waits River Formation. Stratigraphy of the deep-water core begins with basal clay overlain by laminae of marl which transitions up core to gyttja. The shallow, far from the inlet core also shows basal clay but the rest of the core is comprised of marl/organic marl. The shallow core from adjacent to the current inlet, contains predominantly gyttja, however magnetic susceptibility measurements indicate several intervals of increased minerogenic deposition. The deep-water core contains few marcofossils, with the exception of the egg case of the Cladoceran, Daphnia sp. which peaks at 250 cm depth. The shallow, inlet core is dominated by fluctuations in terrestrial macrofossils, mostly wood; however, the top meter shows an increase in aquatic, submergent flora and fauna (Chara sp. oospores, and bivalve and gastropod shells). In the shallow, far from inlet core, aquatic, submergent flora and fauna macrofossils are dominant throughout the core but peak at 200 cm depth.

These results indicate that sediment deposition close to the inlet is controlled by allochthonous processes, while autocthonous processes control deposition further from the inlet. Peaks in terrestrial macrofossils in the shallow, inlet core correspond to periods of increased minerogenic deposition and suggests stream inflow fluctuations that may reflect changes in precipitation. The post-glacial period of marl deposition seen at Twin Ponds has been observed in other records from the region and is likely caused by groundwater pumping of Ca-rich water into the lake, resulting in inorganic precipitation of CaCO₃. The transition from marl to gyttja in the deep basin suggests the establishment of a forest on the surrounding landscape. The persistence of marl deposition within the littoral zone, along with high concentrations of aquatic, submergent flora and fauna macrofossils, indicate high lake productivity and biologically-induced precipitation of CaCO₃. Intervals of high lake productivity during the middle of these records may reflect warmer conditions within the Holocene.