20 Processes and Landforms of Great Lakes Coasts

Friday, 5 May 2023
8:00 AM - 12:00 PM
EC 311

AN EXCEPTIONALLY LOW WATER LEVEL IN LAKE HURON DURING THE 17TH CENTURY

Paper number: 20-2
Program: Theme Sessions
Time: 8:05 AM – 8:25 AM

Authors

Abstract

It is known that extreme lake levels across the Great Lakes basin significantly impact society, but recorded lake level observations are only known back to 1860 CE. Geologists have used a variety of methods to extend water levels further back in time using strandline landforms and sedimentary records. Across the Great Lakes basin, drown forests and individual trees are evidence of fluctuating water levels with the events chronology determined from radiocarbon dating. This presentation documents the dating of a pine tree stump rooted in bedrock in shallow water along the eastern side of Georgian Bay. The stump is in a depth of water below the record low-water of Lake Huron. The stump was disked and examined in the Wooster Tree Ring Lab. Radiocarbon dating of the outer ring (n=3), the pith, and ring 24 (of 42) provide numerous calibrated calendar year solutions extending back to the early 1600’s. Cross-dating of the tree ring-width-series from the disc with local pine tree ring-width series available from the International Tree Ring Database suggests tree growth during the later 17th century. Considering the water depth during sampling, glacioisostatic adjustment, and ~0.5 m freeboard for the pine to grow, the stump records >40 years of water levels at least 2 m below the historic record lowstand.

Additionally, in August of 2022, more recent stumps and standing dead trees were found ~0.5 m above and below lake level, which can be considered a modern analogue for the colonization of trees following receding shorelines in the 1600s. The number of rings present in the recent stumps suggests that the young trees colonized exposed land in a matter of years following the lake level decline in the late 1990’s and were subsequently drowned during the rapid rise in lake level between 2014–2016. Taken together the inundated stumps and rapid colonization of vegetation with dropping lake levels suggest that lake levels lower than recorded in the observational record may have occurred in the mid-late 1600s CE.

Geological Society of America Abstracts with Programs. Vol. 55, No. 3

doi: 10.1130/abs/2023NC-386608

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