| dc.description.abstract | The Laurentian Great Lakes (LGL) are the largest system of surface freshwater on Earth. Three factors, glacial isostatic adjustment (GIA), outlet conveyance, and climate processes contribute to natural rises and falls in LGL lake level over geologic time. Studying the natural history of prehistoric lake levels preserved in coastal landforms helps determine the context of current lake levels and predict potential future lake level changes.
Detailed records of lake level change during the late Holocene are preserved in strandplains of beach ridges. Each beach ridge forms as a result of a lake level rise and fall over many decades and preserves a record of relative lake level elevation at the time of deposition. Multiple beach ridges within a single strandplain contain an account of relative lake level changes over the past 4,500 years.
This study examined beach ridges in the Ipperwash strandplain, southern Lake Huron, that uniquely preserves natural lake level fluctuations at the only unregulated outlet in the LGL, the Port Huron/Sarnia outlet of Lake Michigan-Huron, which is particularly susceptible to natural lake level flucuations. The Ipperwash strandplain is the closest strandplain with the most number of beach ridges to the Port Huron/Sarnia outlet and therefore best records natural lake level fluctuation experienced at the Port Huron/Sarnia outlet of Lake Michigan-Huron.
The study of the Ipperwash strandplain beach ridges used many methods to derive measured elevations and modelled ages of ancient lake levels. Elevation data is combined with age data to create the Ipperwash paleohydrograph. Thirty-six basal foreshore elevations were used to reconstruct the elevation of ancient lake levels. Elevation data shows an oscillatory lake level fall from a maximum elevation of 181.0 m to a minimum elevation of 177.8 m. Ten optically stimulated luminescence ages were used to create a linear age model of the Ipperwash strandplain. The resultant age model shows a maximum age of 3520 years ago and a minimum age of 710 years age.
The multi-millennium trend shows a net linear fall at an average rate of 7 cm/century for the entire Ipperwash paleohydrograph. This trend is interpreted as a record of the rate of GIA at Ipperwash relative to Lake Michigan-Huron’s outlet. The multi-millennium trend suggests the rate of GIA at Ipperwash is 7 cm/century; however, estimates of GIA based on water gauge data suggest the rate of GIA at Ipperwash is 0 cm/century. This discrepancy could result from an underestimation estimated from contoured water level gauge data for the rate of GIA at Ipperwash, erosion at the Port Huron/Sarnia outlet during the deposition of the Ipperwash strandplain and/or the Chicago outlet being dominant during the deposition of the Ipperwash strandplain.
The multi-millennium trend may also be expressed as two millennium trends shown as two vertically offset phases of lake-level lowering from 3520 to 2180 years ago and 2020 to 710 years ago. These age ranges correspond with the Algoma and 1700-high lake level phases in Lake Michigan. Millennium patterns at Ipperwash corresponds to regional climate records and may represent a climate signal. However, the rate of linear lake level lowering for the older lake level phase at Ipperwash corresponds with the difference in rates of GIA, based on water gauge data, between the Chicago outlet and the Ipperwash strandplain. Therefore, the millennium trends may represent either natural climate change or the abandonment of the Chicago outlet of Lake Michigan-Huron. Detailed sedimentologic and lake level records at the Port Huron/Sarnia and Chicago outlets are needed to resolve this controversy.
Centennial lake level fluctuations represent rises and falls in lake levels lasting an average of 208 years ± 114 years with an average amplitude of 0.8 ± 0.4 m about the linear millennium trends. The average timing of the centennial lake level fluctuation at Ipperwash are similar to centennial lake level fluctuations found in Lakes Superior and Michigan-Huron that are interpreted to represent climate driven lake level fluctuations.
Multi-decadal lake level fluctuations cause a single Ipperwash strandplain beach ridge to form average every 73 ± 35 years. The subsurface stratigraphy of Ipperwash beach ridges shows a similarity of other LGL beach ridges which are interpreted to form as a result of a climate driven lake level fluctuations over many decades.
The Ipperwash paleohydrograph provides the context needed to adjust all strandplain data in Lake Michigan-Huron to resolve basin-wide relative lake level changes related to GIA, outlet conveyance, and climate. In addition, the Ipperwash paleohydrograph suggest lake-level may rise and fall on a multi-decadal time scale contributing to erosion and setting the stage to create a new beach ridge, assuming the rate of sediment supply is maintained. | en |
