The Windy Ice Ages

I love a good brisk wind.  Cool windy days seem to put an extra bounce in my step when I do my near daily 3 or 4 mile run up and down the street in front of my house.  And I love hearing the sound of the wind blowing against the house on a winter’s night while I’m snug under the comforter, dreaming of big-breasted women.  Wind is another reason I wish I could jump in a time machine and take a trip back to the Ice Ages of the Pleistocene because when massive glaciers covered most of what today is Canada, the earth heated even more unevenly than it does today, resulting in windier conditions everywhere.

Scientists measure wind in meters per second.  In modern day Georgia and South Carolina, winds average between 1.3-2.2 meters pers second, but during the Last Glacial Maximum winds averaged 4-6 m/s–more than double those of today.  As I’ve written in previous blog entries (See https://markgelbart.wordpress.com/2011/11/07/when-ice-age-georgia-became-desert-like/ and See https://markgelbart.wordpress.com/2013/03/10/when-sand-dunes-buried-herds-of-flat-headed-peccaries/), Ice Age winds often pushed eolian sand dunes across the landscape.  The climate was much drier then because the expanding glaciers locked up so much of earth’s moisture.  The modern precipitation/evapotranspiration ratio in southeastern North America is 1.23.  (Evapotranspiration is the sum of evaporation and plant transpiration.)  But during the LGM it was less than .3 or about 25% that of today.  Precipitation averaged about 33% less than that of today as well.  Moreover, the ocean receded and rivers had to flow longer distances to reach the sea.  As a result, rivers shrank in size, and riverine sand became exposed.  Strong winds pushed these sands into dunes.  Unlike modern day winds which directionally vary according to the seasons, Ice Age winds were consistently west/southwesterly and pushed these dunes to the northeast sides of rivers.  This is the origin of most of the sandhills found in the southeast today.  Modern day winds are not strong enough to  push sands into dunes or as geologists say they could not maintain a “sustained eolian mobilization.” 

The precipitation/evapotranspiration ratio was much lower during the Ice Age than it is today, resulting in a more sparsely vegetated landscape.

Geologists believe these eolian sand dunes were once u or v-shaped in front. The dunes were unvegetated but had partly vegetated parallel arms upwind from the dune fronts.  One such dune is located east of the Savannah River in Jasper County, South Carolina.

Map of South Carolina highlighting Jasper County

Jasper County, South Carolina

Full-size image (84 K)

Exposed Savannah River Dune.  There are many dunes like this located on the northeast side of southeastern rivers.

The Savannah River dune formed between 40,000 BP-19,000 BP.  It alternated between active and inactive stages in concert with variations in precipitation rates.  The surrounding area around the dune was not barren but consisted of grasslands and open woodlands, composed  mostly of pine and oak with less floral species diversity than modern day forests.  Vegetation was less dense than it is today.

At the end of the Ice Age, glaciers began melting, thus releasing moisture into the atmosphere.  Plants took root on the dunes and started holding them down, rendering them inactive.  During the Younger Dryas about 13,000 BP, climate suddenly became as cold and dry as it was during the LGM but just for a few hundred years.  Several smaller dunes formed again, but since then, all of the sand dunes in southeastern North America have become inactive due to the increase in precipitation and the decrease in wind velocities.

Reference:

Sweezey, Christopher; et. al.

“Quaternary Eolian Dunes in the Savannah River Valley, Jasper County, South Carolina”

Quaternary Research (80) 2013

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