REFINING THE LAKE LEVEL HISTORY OF SLUICE POND, MA USING DINOFLAGELLATE CYST TAPHONOMY

Palynological samples from a meromictic lake in NE Massachusetts illustrate the importance of dissolved oxygen (DO) on dinoflagellate cysts. Measured DO was ~ 7 – 9 mg/L in the epilimnion of Sluice Pond but <1 mg/L below the chemocline (11 – 16 m below lake level/ mbll) where stable isotope and elemental analysis in sediments reflect bottom water anoxia. The resulting enhanced preservation of susceptible palynomorphs produced exceptionally diverse dinocyst assemblages in the deep basin of Sluice Pond – twelve cyst morphotypes were differentiated, most of which are rarely reported by palynologists. Palynological preparations from sediments above the chemocline, in contrast, are dominated by large, thick-walled cysts of Peridinium willei (relative abundance correlation with water depth: R² = 0.8452). This taphonomic bias was exploited to refine lake level reconstruction, capitalizing on the basin morphology that produces large variations in affinity for meromixis in response to minor changes in water depth. Assemblages strongly dominated by oxidation-resistant cysts of Peridinium wisconsinense and P. willei were deposited when the lake transgressed low-relief parts of the basin (ca.18 – 14 mbll and the upper 6.5 mbll) promoting mixing of the water column. Short-lived regressions (illustrated by the dark blue; ca. 9ka, maroon; ca. 7.6 ka and green; ca. 5.6 ka reflectors) isolated the steep-sided basin, allowing a more diverse cyst assemblage to preserve in core SP09 KC2. The warm, wet climate that allowed the establishment of the mixed hardwood forest produced rapid water level rise between the arid early Holocene pine zone and mid-Holocene “hemlock minimum”. Basin geometry promoted meromixis when lake levels were between 14 and 6.5 mbll, and the eutrophication recorded by abundant cysts assigned to Peridinium gatunense enhanced seasonal anoxia through increased biochemical/ sedimentary oxygen demand (BOD/SOD) over the last 5,000 years.