Dating Deglaciation

VISUALIZING LATITUDE OVER TIME

CALCULATING RETREAT RATES

COSMOGENIC NUCLIDE

RETREAT TIME: 27500ya - 13200ya = 14300ya

RETREAT DISTANCE: 350km

RATE: 350km/14300y = 0.02447km/y = 24.5m/y

BULK SEDIMENT

RETREAT TIME: 22200ya - 10500ya = 11700ya

RETREAT DISTANCE: 428km

RATE: 428km/11700y = 0.03658km/y = 36.6m/y

MACROFOSSILS

RETREAT TIME: 18300ya - 12800ya = 5500ya

RETREAT DISTANCE: 404km

RATE: 404km/5500y = 0.07345km/y = 73.5m/y

I calculated the retreat rate by finding each dataset's total retreat and dividing the rate by total retreat time. I found total retreat distances visualizing recorded coordinates into ArcGIS online and measuring distances in a web app (above). The macrofossil methodology yielded the highest retreat rate, at 73.5m/y, in comparison to values of 36.6m/y for bulk sediment and 24.5m/y for cosmogenic nuclide.

The results for these techniques varied for a few reasons, in part because of the varied number of data points for each. While you might expect either radiocarbon dating method to provide more accurate representations of retreat rate given their greater number of total data points, the fact that both macrofossil and bulk sediment date loose materials may account for their inaccuracy. While fossils or sediment may have been transported independent of this specific glacial retreat and provide a false signal, the cosmogenic nuclide technique seems like a more reliable representation of ice retreat over such a large span of time, as it dates more stationary material.

Visualization of sea level rise; from Lambeck, 1994.

Logically, the retreat of the Laurentide ice sheet led to steep rise in sea level. We can also use the below figure from Dr. Kurt Lambeck's 2014 paper Sea level and global ice volumes from the Last Glacial Maximum to the Holocene to better understand this change. The swift uptick in sea level around 30ka aligns with the Laurentide sheet's retreat, the rate of which aligns with the one I calculated using cosmogenic nuclide data.

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