POSSIBLE EVIDENCE FOR ISOTOPIC STRATIFICATION OF GLACIAL LAKE AGASSIZ, 11,000-9,900 14C YR BP

A significant difference exists between the isotopic composition of glacial Lake Agassiz during the interval 11,000-9,900 ¹⁴ C yr BP, as inferred from the oxygen isotope composition of algal cellulose preserved in a sediment core from the south basin of the lake (d¹⁸O_{lake water} = –15 to –19‰ VSMOW), versus that of connate porewaters extracted from the same core (d¹⁸O = -24‰ VSMOW). This substantial discrepancy may reflect the existence of isotopic stratification in the south basin of Lake Agassiz during this time of rapid detrital sedimentation, with algal cellulose recording the isotopic signature of relatively warm, evaporatively-enriched waters of the epilimnion, while sediment porewaters preserve the composition of hypolimnion waters fed primarily by cold, dense, non-evaporated glacial meltwater issuing from the ice front.