PAST LAKE-LEVEL AND VEGETATIONAL CHANGES AS INDICATORS OF REGIONAL MOISTURE BALANCE: LAKE MOSOMO, NORTHERN MINNESOTA

To reveal past levels in Lake Mosomo as a regional moisture-balance indicators we used a transect of 5 sediment cores in combination with profiles of ground-penetrating radar and fossil pollen to reconstruct the vegetation respond to moisture-balance fluctuations. The sediment stratigraphy shows that two of the near-shore cores contain a marl unit (with shell fragments) that correlates with a sand layer in the shallowest core, where the GPR data suggest an unconformity. The sediment LOI data show that marls (>35% carbonate) were formed in shallow water between ~ 8 to 5.2 ka cal. yr B.P., when sands accumulated near the shore at about 2.5 m below the modern lake surface and organic silts accumulated off-shore.

The pollen diagram for the central core 3A shows the regional pollen zones for north-central Minnesota -- Picea-Larix at the base, representing widespread late-glacial spruce forest, followed sharply in the early Holocene by Pinus banksiana/resinosa. Halfway through the pine zone prairie pollen types become more abundant (Poaceae, Artemisia, Ambrosia, Chenopodiaceae) as pine savanna succeeds the pine forest. The pine savanna is followed by oak savanna with minor Ulmus Ostrya-type, Corylus, and Tilia, and prairie types persist in the dryer and warmer climate less favorable for pine. In the late Holocene the hardwoods are reduced as Pinus strobus takes over. This change occurs just at the shift from marl to organic silts, found in all cores and dated to 5.2 ka cal. B.P. The pine forest may have developed from an increase in summer rains, resulting in more runoff (and groundwater input) and nutrients, causing increased formation of organic silts and perhaps dissolution of carbonate in anoxic bottom water.

Our results are consistent with the timing of low lake levels throughout Minnesota and with isotopic evidence for low summer precipitation and high evaporation from 8-5.5 ka cal. B.P.