PALEOLIMNOLOGICAL STUDY OF LAKE CHAMPLAIN: SHALLOW BAY SEDIMENT DOCUMENTS THE TRANSITION FROM CHAMPLAIN SEA TO LAKE CHAMPLAIN

Sediment accumulated on a lakebed archives information about past climate and changes in the regional environment. Two sediment cores were retrieved from Missisquoi and Saint Albans Bays, two shallow bodies of water located along the northeast arm of Lake Champlain, VT. The transition from Champlain Sea to Lake Champlain was captured in both bays, thus providing the opportunity to investigate the gradual ecological shift from a marine to lacustrine water body. The end of the Champlain Sea stage and the onset of Lake Champlain were controlled by isostatic rebound, and accompanied by significant changes in water level.

This study aims to uncover the past environmental conditions of Missisquoi and Saint Albans Bay in order to better estimate the timing and duration of the marine to freshwater transition and the magnitude of the subsequent change in lake water level. Organic matter records (%C) were used to match and extend proxy records from previous studies in the same area. New radiocarbon dates on macrofossils show that new cores contain a record dating back at least 9600 cal yBP. Lithological analyses demonstrate that these two shallow water bodies responded differently to water level change, specifically during the low-stand of the Champlain Sea. The Saint Albans Bay record encompasses a 58 cm thick peat layer displaying evidence for a wetland occupying the bay during the earliest Lake Champlain phase. Based on the location of the peat in the core, we estimate a 7-8 m water level rise in the bay since 9600 cal yBP. Around the same time, the Missisquoi Bay core shows proof of an erosional unconformity in the form of a 2 cm thick deposit of sub-angular shale pebbles and sand, possibly alluding to a partial drying of the Missisquoi Bay basin. The lithology of the Lake Champlain phase is recorded as olive-green silty clay in both bays from the transition to the present.

The Saint Albans Bay peat layer is being investigated for a shift in diatom assemblages and microfossil analysis of Missisquoi Bay sediment is underway. Both of these methods are used to examine in detail the progressive freshening over the course of transition from marine to lacustrine environment.