Lifting the Champlain Sea: A CAS Record of the Transition to Modern Lake Champlain
- Author(s): Robinson, Andrew
- Advisor(s): Lyons, Timothy
- et al.
Between about 9 and 13 ky bp, the isostatically depressed area of the Champlain, Ottawa and St Lawrence valleys were engulfed by the Atlantic Ocean. The resulting Champlain Sea had two distinct sources of water: fresh water sourced from large proglacial lakes fed by the collapsing Laurentide Ice Sheet and North Atlantic marine waters sourced via the contemporaneous eustatic rise. This study focuses on the most recent transition from the Champlain Sea (CS) to modern Lake Champlain when the fluxes between the two water sources were in a state of transition. Carbonate-associated sulfate (CAS) offers proxy evidence for the δ34S of ancient waters, a key piece in our interpretation of both flux dynamics and primary redox conditions within the basin. Previous research into CAS has shown that sulfate can be structurally substituted into authigenic and biogenic carbonates. Following wide ranging efforts to calibrate and validate this proxy, it has been demonstrated that the isotopic composition commonly reflects the ambient waters at the time of carbonate precipitation. Because of relatively low rates of bacterial sulfate reduction, the δ34S of CAS in the restricted CS can be used to approximate salinity relationships over the interval of interest. These data suggest the Champlain Sea was subject to four flood events from Lake Agassiz dropping salinity (measured in Practical Salinity Units) throughout the lifetime of the marine incursion. Splits taken from the CAS solution yield Mg, Fe, & Sr concentrations; unique to each water source, these data reflect the marine to lacustrine change in aqueous metals and identify the source of the proglacial flood source as Lake Agassiz. Due to the low concentrations of both CAS and calcium carbonate in our samples, we are in a unique position to assess the lower limits of the CAS proxys utility in studies of marine to lacustrine transitions. By capturing the transition from the Champlain Sea to Lake Champlain I detail the dynamics of this evolution as both unrelenting isostatic rebounding and diminishing flood sizes repeatedly lowered the salinity (PSU) of the CS. Moreover, low pyrite and iron monosulfide concentrations within the sediments reveal that aqueous sulfate must have been delivered back into the North Atlantic.