USING LACUSTRINE SEDIMENT TO TEST THE EVAPORATION HYPOTHESIS FOR THE MOORHEAD LOW WATER PHASE OF LAKE AGASSIZ

Glacial Lake Agassiz was fed by terrestrial runoff and meltwater from the Laurentide Ice Sheet at the end of the last glacial period. The cause of the ~50 m draw down to the Moorhead low-water Phase, ~10,7 00 ¹⁴C yr B.P.( ~12,400 cal yr B.P.), close to the beginning of the Young Dryas chronozone remains elusive. An active outlet routing water eastward or northwestward was recently tested and rejected from available field data. Without evidence for an active outlet, the evaporation hypothesis is being pursued. Here we present initial results for increased aridity during the Younger Dryas from Rabbit Lake, Minnesota.

Rabbit Lake is located just above the Herman strandline, in the southeastern corner of the Lake Agassiz basin. It is expected to record similar hydrological conditions as in Lake Agassiz itself. From a Livingstone core with a radiocarbon age of 11,800 ± 45 ¹⁴C yr B.P. (13,700 ±114 cal yr B.P.), preliminary results indicate increased aridity. The dated unit consists of uniform dark gray mud and is overlain by massive, red-brownish mud with intermixed salts assumed to be of Younger Dryas age. Preliminary results from XRF measurements (ITRAX X-ray Fluorescence Core Scanner) of the S/Ti, Ca/Ti, and Si/Ti profiles are well correlated. High sulfur content together with high calcium content may suggest formation of gypsum within the sediments, a signature of a dry climate. The high Si/Ti ratio may represent a higher quartz content, consistent with gypsum formation during a drier climate.

In addition to the Rabbit Lake cores, sediment cores containing YD-aged sediment and samples for OSL dating were collected from Fargo, ND, within the Lake Agassiz basin. The results of bulk elemental composition (XRF), LOI, and grain size, do not show evidence for evaporative enrichment. However, this does not exclude the closed basin hypothesis due to the planar erosional unconformity at these sites.

To further test the evaporation hypothesis, our future work involves examining a sampling transect north of Grand Forks to an elevation below the lowest point of the Moorhead low.