Isotopic evolution of Glacial Lake Agassiz: New insights from cellulose and porewater isotopic archives

Significantly differing estimates of the oxygen-isotope composition of Lake Agassiz have been obtained from two co-existing isotopic archives within a sediment core originating from Montcalm, Manitoba, in the southern basin of the ancient proglacial lake. Oxygen-isotope analysis of cellulose extracted from the sediments, which originated during the Lockhart phase ∼ 11,700–11,000 14C yr BP, suggests that phytoplankton lived in surface waters having δ18O around − 18 ± 1‰ VSMOW, substantially enriched relative to connate porewaters in the same core, which indicate bottom waters had much lower values of around − 24.5 ± 0.5‰ VSMOW. This difference may be attributable to seasonal isotopic stratification of the upper part of the water column in the 250 m-deep lake. Modern observations from analogous environments in northern Canada suggest that inflow of evaporatively enriched runoff from the large area of deglaciated terrain contributing to Lake Agassiz, possibly enhanced by evaporation from the surface of the lake itself, could readily account for sufficient seasonal 18O enrichment in the epilimnion. Sediment porewaters, in contrast, have preserved the isotopic signature of hypolimnion waters supplied by a mixture of glacial meltwater and precipitation-derived runoff from the Laurentide Ice Sheet, and lack discernable isotopic alteration by evaporation. These new estimates are combined with inferred lake water compositions from other isotopic archives to develop a speculative framework for the isotopic evolution of the lake, providing improved constraints on the probable isotopic composition of Lake Agassiz outflow over time, which has important implications for efforts to trace and model its changing discharge.