GLACIAL LAKES IN NORTHERN MINNESOTA: DRAINAGE RELATIONSHIPS, CHRONOLOGY, DISCHARGE, AND ISOTOPIC COMPOSITION OF MELTWATER

Meltwater along 300 km of the margin of the Laurentide Ice Sheet in northern Minnesota was channeled through a series of glacial lakes. Although the general drainage relationships are well established, new details on the timing of glacial advances in the region have led to significant revisions of the regional chronology. Retreat of the Rainy lobe to the northeast resulted in meltwater ponding in a broad lowland in north-central Minnesota forming a complex of short-lived ice-marginal lakes known as Glacial Lakes Aitkin and Upham I. Before the Rainy Lobe had completely retreated from the basin, the area was overridden by the advance of the St. Louis Sublobe. Stagnation and melting of ice of the St. Louis Sublobe brought into existence Glacial Lakes Aitkin and Upham II and Sucre; first as a series of small interconnected ice marginal lakes and then as an integrated body of water. Retreat of the Rainy Lobe north of the Giants Range formed Glacial Lake Norwood, which drained into Lake Upham. Lake Norwood was succeeded by Glacial Lake Koochiching as a new topographically lower inlet into Lake Aitkin formed. The main outlet for all of these lakes was along the St. Louis River southward along the ice margin of the Superior Lobe into the Mississippi River drainage. Continued retreat of the Superior Lobe opened a meltwater course into Glacial Lake Duluth.

Estimation of discharge and isotopic composition of meltwater requires reconstruction of ice surface profiles, which was accomplished using a specified-mass-balance approach. Discharge of meltwater was estimated using a radiation balance to calculate energy available for melting, which was determined by the incident solar radiation adjusted for albedo and sensible and latent heat fluxes. Oxygen isotopic composition of the meltwater is calculated using analytical and numerical techniques based on the specified mass balance and temperatures determined by elevation of the ice surface.