The Paleoenvironment of the Ice Free Corridor

The Wisconsinian Ice Age was an epoch when 2 massive Ice Sheets expanded over Canada.  The Cordilleran Ice Sheet expanded over the western Canadian Rocky Mountains south of Alaska, while the Laurentide Glacier covered all of eastern Canada and even extended over New England, Ohio, and other midwestern states.  (Ironically, most of Alaska stayed ice free during this time and consisted of a barren grassy environment known as the mammoth steppe.)  About 30,000 years ago, the Cordilleran Ice Sheet and the Laurentide Ice Sheet conjoined into 1 massive glacial slab that blocked all human and animal migratory routes between Alaska and the rest of America.  Formerly, scientists thought this barrier of ice existed from ~30,000 BP-~11,000 BP, but more recent studies suggest the 2 Ice Sheets began to separate as early as ~15,000 BP, creating an Ice Free Corridor that men and animals could have migrated through.

Map of North America circa 15,000 calender years BP.

A quite interesting environment existed in the Ice Free Corridor as the 2 Ice Sheets began to recede.  Initially, the area between the Ice Sheets was covered by bare soil, rock, and glacial outwash sands.  Wind blew the sands into deep eolian  dunes, some measuring 15 feet high and a mile long.  Scientists used optically stimulated luminescense dating of these sands to determine that deglaciation occurred earlier than previously thought.  Chunks of ice left behind by retreating glaciers and buried by sand became kettle lakes.  Meltwater streams carried chunks of ice into the corridor and these also became kettle lakes.  Glaciers blocked some meltwater streams, and the backflow created massive glacial lakes.  All these bodies of water attracted large flocks of ducks, geese, swans, and cranes.

Black duck (Anas rubripes).  Waterfowl such as ducks, geese, swans, and cranes nested in the abundant kettle lakes that formed in the Ice Free Corridor following the end of the Last Glacial Maximum.  This source of food attracted Paleo-Indians and enticed them to colonize the newly inhabitable region.

Lichens grew on the bare exposed rock, and caribou were the first large mammal able to survive in the corridor because they can subsist on this fungi/algae symbiote.

Caribou likely were the first large mammal species to colonize the Ice Free Corridor.  They can survive on lichens that  grew on rocks centuries before grass returned to the region.

The melting ice and snow made the soils rich in nitrogen, and glacier-pulverized rock added pottasium and phosphorus.  Within centuries, wind blown grass seeds from north and south of the corridor colonized the area, converting it into a grassy mecca that attracted camels, horses, bison, and elk.  The wolf (Canis lupus) was probably the most common large carnivore to colonize the corridor.  Dire wolves (Canis dirus) never made it this far north.  However, this clad of gray wolves is not ancestral to today’s wolves of Canada and Alaska.  Mysteriously, this lineage died out and was replaced by another line of gray wolves.  Lions may have followed prey into the corridor as well, but the lions living to the north were a different species than the lions living to the south, and the 2 species apparently didn’t interbreed.

The extinct yesterday’s camel (Camelops hesternus). It was well adapted for living in the environment of the Ice Free Corridor.  It would have been one of the first large mammals to colonize the arid grasslands that formed on the rich newly deglaciated soils.

The timing of the closure and opening of the Ice Free Corridor is important for archaeologists speculating about when man first entered North America.  Pre-Clovis cultures were present in North America before the Ice Free Corridor became passable, even if the new dates are taken into consideration.  Therefore, many archaeologists believe paleo-Indians took a Pacific coastal route to arrive in North America via boat.  This may be true, but I’m not so quick to dismiss the possibility that man first came to America through an overland route within the corridor before it closed.  The timing of the initial closure is uncertain, and it may not have occurred until ~21,000 BP.  I suspect a population of humans made it to North America before the Ice Free Corridor closed, but they were too few in number to be visible in the archaeological record.

The deglaciation of the Ice Free Corridor attracted humans from north and south.  Archaeologists have discovered a mixture of artifacts here from cultures that originated separately on both sides of the Ice Sheet.  Populations that had been separated for thousands of years came into contact in the corridor while following herds of game and flocks of waterfowls.  It must have been a challenging environment.  Though the mix of grasslands, wetlands, and bare soil provided plenty of protein, it was a melancholy barren world.  The katabatic winds blowing between the 2 Ice Sheets made for some long cold nights.  There were no trees for firewood or boat construction.  Glacial lakes and frigid meltwater streams made the corridor once again impassable during summer, though they did freeze solid for overland travel during winter.  Nevertheless, man did survive in this unique natural conduit for thousands of years.

Reference:

Pinson, Ariane

“The Ice Free Corridor and the Peopling of the Americas”

Mammoth Trumpet 28 (4) October 2013

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Tags: camel, caribou, Cordilleran Ice Sheet, ducks, Eolian Sand Dunes, Ice Free Corridor, kettle lakes, Laurentide Ice Sheet