THE OXYGEN-ISOTOPE EVOLUTION OF THE NORTH AMERICAN GREAT LAKES

The Late Pleistocene and Holocene oxygen-isotope compositions of the Great Lakes, as inferred from ostracodes, diatoms and other biogenic materials, are a complex proxy of climatic change in this region. Influencing factors include the varying pathways, abundances and compositions of glacial meltwater input during the retreat of the Laurentide Ice Sheet, variations in upstream inflow through this string of lakes, seasonal and longer term changes in precipitation and runoff within each lake’s catchment, evaporative enrichment, and groundwater input. The modern Great Lakes, as sampled over the last several years, have distinct δD and δ¹⁸O values: Superior, -68‰, -8.7‰; Michigan, -45‰, -5.9‰; Huron, -59‰, -7.4‰; Erie, -51‰, -6.7‰; Ontario, -50‰, -6.6‰. Nearby Lake Simcoe has values of -56‰ and -7.4‰. These compositions reflect inflow from other Great Lakes, varying climatic zones and moisture masses that determine precipitation, surface runoff and evaporation within each lake’s catchment, and water residence times within each lake. Because of evaporation, the lakewater isotopic compositions variably plot to the right of a regional Great Lakes meteoric water line: δD = 7.1 δ¹⁸O +1.0 (defined for monthly precipitation at nine Great Lakes stations from 1996 to 2006). Average annual precipitation δD and δ¹⁸O values vary from -91‰ and -12.1‰ near the northwestern shore of Lake Superior to -50‰ and -7.0‰ in southwestern-most Ontario. Strong seasonal variations in the isotopic composition of precipitation occur at all sites. Present differences between lakewater and precipitation δ¹⁸O values range from ~3‰ for Lake Superior to ~1‰ for Lake Erie. Such differences were likely much larger in the Late Pleistocene to Early Holocene, when long-distance input of low-¹⁸O glacial meltwater drove lakewater compositions to as low as -29 to -20 ‰ at various times in various lakes. By ~8,200-8,100 cal BP, however, lakewater oxygen-isotope compositions again reflected more local climatic conditions (Superior, ~-8.5 ‰; Michigan, -6 ‰; Huron -7 ‰; Simcoe, -7 ‰; Ontario, -7 ‰). Variations since that time, as inferred from lakewater proxies, provide a subtle but integrated indicator of climatic shifting within the Great Lakes Basin that, if appropriately documented, could serve as a bellweather for the future.