Paleoclimatology 101 - The Younger Dryas Climactic Event

Figure One - Paleoindians competing with scavenging American Lions for a bison carcass,
sometime around the Younger Dryas Climactic Event. Photo courtesy unknown.    

In my last article, I discussed the "Great Meltdown" when the ice sheets and glaciers of the Wisconsin Ice Age began to melt. The meltdown started around 17,000 years ago, and for a few thousand years, the glaciers and ice sheets slowly retreated northward. Then around 12,900 years ago, the climate in the higher latitudes of the northern Hemisphere reversed course and it became colder. European scientists have known about this cooling event since the mid-twentieth century and dubbed it the Younger Dryas after a flower that grows in Europe in colder climates (figure six). By the 1990s, scientists around the globe were studying the evidence and effects from the Younger Dryas Chronozone in their areas of interest (YDC).      

Not only did ice age conditions return during the YDC from 12,900 to 11,700 years ago, but there were two other key events occurring in North America within the same general time-frame. About the same time that the YDC began, Clovis Paleoindian weaponry disappeared and so did forty or so megafauna species in North America. I am referring to extinct Pleistocene mammals such as mammoths, mastodons, ground sloths, camels, musk ox, horses, short-faced bears, dire wolves, saber-toothed cats, and others. 

Was the occurrence of these three 'Megaevents' - the YDC, the disappearance of Clovis weaponry, and the extinction of megafauna species - a coincidence, or is this the proverbial smoking gun for something else that was going on? 

Scientists have kicked the tires on the YDC at a breakneck pace with an attitude of winner-take-all. So far, the occurrence or coincidence of these three events has led to more questions and speculation than answers from the scientific community. 

Figure Two - My eighth and ninth books. CLICK TO ORDER

Some investigators claimed that the YDC brought back the same icy conditions that were around during the Last Glacial Maximum (LCM). Based on what I have read, that seems unlikely. Archaeologists Meltzer and Bar-Yosef (2012) stated that the biggest impact from the YDC was at higher latitudes in the northern hemisphere. They noted that by the time the YDC showed up, the great North American ice sheets -  Cordilleran and Laurentide - were mere shadows of their awe-inspiring selves. The ice sheets had retreated into Canada and were greatly diminished in both areal extent and thickness. The two archaeologists noted that greenhouse gases - methane and carbon dioxide - had increased to interglacial levels during the YDC which helped moderate the cooling trend of the climate. 

Some scientists wrote that the YDC showed up fast or lickety split, but Meltzer and Bar-Yosef disagreed, reporting that the archaeological evidence indicated that the YDC was a gradual and time-transgressive event, happening at different times in different areas with different overall effects. They also suggested that there were vast areas in the world that were completely unaffected by the YDC. Figure three illustrates the warming and cooling trends of the planet over the past 18,000 years. Around 14,000 years BP, the Earth was on a warming trend when a cooling trend that led to the YDC showed up. This cooling trend lasted for approximately 1200 years.   

Meltzer (2009) reported that during the YDC, glaciers in North America temporarily stopped melting and that a few glaciers actually expanded. He suggested that the climate in the northern part of the United States was cooler and drier. Meltzer's research estimated that average temperatures in the northern part of the United States dropped approximately nine degrees Fahrenheit. The YDC was a time of high winds, blowing sand and silt across the northern plains and the midwest regions of the United States. He pointed out that while the northern United States was colder and drier, the southeastern and midcontinent regions of the United States were warmer and wetter.

Figure Three -  From The Intriguing Problem of the Younger Dryas - What 
Does It Mean and What Caused It? by Anthony Watts.

 

You might be asking yourself what triggered the YDC? In my first paleoclimatology article, I introduced my readers to the Milankovitch Theory of ice age creation. I explained that this orbital theory was based on the gravitational effects of planetary bodies which ultimatly caused variation in the geographic distribution of insolation or solar radiation. I refer you to that article for review. 

There are at least two theories bantered about by scientists for the cause of the Younger Dryas. The more traditional theory proposed that a massive periglacial lake called Lake Agassiz (figure four) sprung a leak in its ice dam when the Laurentide Ice Sheet was in retreat. The breach released a massive amount of freshwater down the St. Lawrence waterway and into the North Atlantic. The deluge of freshwater interrupted the flow of the North Atlantic thermocline and Gulf Stream, the deep, warm current of water flowing north from the tropics. The warm, salty water from the Gulf Stream is responsible for moderating the climate of countries along the North Atlantic, such as Canada, the U.K., Iceland, Greenland, and Norway. 

From our high school science class we remember that fresh water is less dense than seawater (2.5 % less dense). When Lake Agassiz's freshwater flooded the North Atlantic, it floated on top of the warm, salty seawater, much like oil floats on water. The layer of freshwater prevented the warm, salty seawater from mixing and reaching the surface of the ocean. Temperatures in the northern hemisphere were no longer regulated by the Gulf Stream and a portion of the North Atlantic Ocean froze. Once this polar front established its footing, the cold expanded outward from the North Atlantic. 

Detractors of this theory pointed out that the amount of freshwater needed to shut down the circulatory forces of the North Atlantic thermohaline and Gulf Stream for that long of a period was astronomical. The detractors also point out that there is no geologic evidence for a massive flood along the St. Lawrence waterway when the ice dam burst on Lake Agassiz. 

      

Figure Four - Courtesy of Nature Magazine, April 1, 2010. 

The second and more controversial theory about the YDC was based on an extraterrestrial event (Firestone et al 2006). Firestone and his colleagues believed that a comet struck the Laurentide Ice Sheet, melting and breaking off massive blocks of glacial ice which ended up in the North Atlantic Ocean. The freshwater and icebergs weakened North Atlantic thermohaline circulation, triggering  abrupt climate cooling (at this stage this theory is similar to the Lake Agassiz theory). Firestone and his colleagues proclaimed that their theory explained the Younger Dryas, and the extinction of Pleistocene megafauna, and the decline of post-Clovis human populations. The investigators based their theory on the presence of iridium, Helium-3, firestorms, hollow floating spherules, microscopic diamonds, and glasslike carbon in the Younger Dryas aged "carbon rich black layer" (the black mat) found in Clovis sites around North America. 

Detractors to the 'extraterrestrial theory' jumped on it quickly and decisively. They asked Firestone et al where the crater was, and where the remnants of the comet were? Firestone  shot back that the ice sheet buffered the impact from the comet (figure five), therefore no crater. Detractors remained unsold based on a lack of scientific and geologic evidence. Scientific interest in the 'extraterrestrial theory' has waned over the years, but that does not stop its advocates and detractors from debating the pros and cons of the theory.

Is the extraterrestrial theory possible? 

Anything is possible. What about the critics of the extraterrestrial theory, don't they have a case? Of course they have a case, but we must remember that it is inherent in human nature to be skeptical about anything "out of the box". Some of the same critics of the extraterrestrial theory still cling to the "Clovis First" theory even after overwhelming evidence of "Pre-Clovis" has smacked them in the face. 

Figure Five.

    What happened to human populations before and during the YDC? What happened to the Clovis prehistoric culture?

In Hunter-Gatherer Behavior - Human Response during the Younger Dryas, a publication edited by Metin I. Eren (2012), the authors found little evidence of the YDC affecting the lives and behaviors of prehistoric hunter-gatherers around the world. The researchers reported that based on evidence at archaeological sites around the globe, there was more evidence in the YDC for continuity than there was for change. 

In the summary of this same book, Meltzer and Yosef (2012) stated that the efforts to link the YDC to human cultural change was a reasonable endeavor, but that initial enthusiasm often outpaced the empirical evidence found. YDC was mostly a northern hemisphere phenomena, affected the higher latitudes and not much of the rest of the globe. For example in North America, the authors noted that the YDC climate varied from cold and dry to the north, to warm and wet or cold and wet elsewhere on the continent. The authors concluded that the climate changes of the YDC were not globally synchronous or severe, and that there was little archaeological evidence that the YDC impacted human populations enough to motivate cultural change. 

What is my personal opinion on this? 

The Younger Dryas event cooled off the continent. Humans adapted to this change in climate, just like we always have and we always will. Clovis disappeared, but other Paleoindian cultures such as Goshen and Folsom filled the void. Perhaps, they were the same people. The weaponry Clovis used to hunt mammoths evolved into weaponry better suited to a bison-based economy. The now extinct megafauna species met two threats: a changing climate and intense hunting pressure from humans. The extinct mammal species failed to adapt to the threats from the climate and human predators, and went away.

Figure Six - Scientists named the return to near-glacial the Younger Dryas, 

after a flower (Dryas octopetala) that grows in cold conditions 

and that became common in Europe during that time.

Erin, Metin I.  2012   On Younger Dryas Climate Change as a Causal Determinate of Prehistoric Hunter-Gatherer Culture Change in Hunter-Gatherer Behavior - Human Response during the Younger Dryas, edited by Metin I. Eren. Left Coast Press. Walnut Creek.     

Firestone, Richard; West, Allen; Warwick-Smith, Simon   2006  The Cycle of Cosmic Catastrophes. Bear and Company. Rochester.    

Meltzer, David J.  2009  First Peoples in a New World - Colonizing Ice Age America. University of California Press. Berkeley.    

Meltzer, David J., and Ofer Bar-Yosef   2012  Looking for the Younger Dryas in Hunter-Gatherer Behavior - Human Response during the Younger Dryas, edited by Metin I. Eren. Left Coast Press. Walnut Creek.       

Paleoclimatology 101 - The Great Meltdown.

Figure One - South of the Ice. 

In my first article on the ice age, I discussed the orbital theory (Milankovitch Theory) behind the creation of Pleistocene ice ages. In my second article, I explored North America during the height of the Wisconsin Ice Age, at the Last Glacial Maximum (LGM) around 18,000 years ago. In this article, I discuss what happened in North America when the ice sheets began to melt.

The Cordilleran and Laurentide ice sheets altered both land and sea geography and profoundly affected the climate and environment in North America. 

Figure Two - A map depicted the area in North America covered 
by the Cordilleran and Laurentide ice sheets 
and the new coastlines at the LGM.  

With four hundred feet of water from the oceans locked up in ice sheets on land and sea, there was more coastline for humans and animals to explore and inhabit around North America. Unfortunately, there wasn't much interior land in Canada or the northern US that wasn't buried under ice. Figure two depicts North America and where scientists believe the ice sheets were located at the height of the LGM around 18,000 years ago. Note the borderline of the United States now and how far land extended out onto the continental shelf during the LGM.   

In my second article, I explored the Last Glacial Maximum (LGM), or what scientists believe was the height of the Wisconsin Ice Age, around 18,000 years ago. It was not long after the LGM, perhaps around 17,000 years ago, that the Cordilleran and Laurentide ice sheets started to melt. It was a slow melt and scientists believe it took around twelve thousand years to melt most of the two ice sheets. After all, these were not your average, normal-sized, run-of-the-mill glaciers. The two ice sheets covered over a million square miles! Scientists estimate the thickness of the ice sheets from around a half a mile thick along the southern edges to over two miles thick in the middle and northern part of the ice sheets. The ice sheets were so heavy that they compacted the Earth's crust down around one thousand feet! 

Figure Three - My first book and my latest book. The Second Edition 

of my most popular book Shadows on the Trail, the introduction to my 

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According to the Milankovitch or orbital theory, the melting began because of the orbital cycle that allowed higher volumes of summer insolation (radiation from the sun) in the northern hemisphere. In my first article, I explained that for ice sheets to grow, they require less summer insolation which allowed ice from previous winters to survive and not melt during summer heat. By adding more ice year after year, decade after decade, century after century; the ICE SHEETS CAN GREW!    

By 13,000 years ago, the Laurentide ice sheet had decreased in  area by at least twenty-five percent and by volume by at least fifty percent. During this time, most of the meltwater from the Laurentide ice sheet was flowing down the Mississippi River drainage system causing flooding along river banks and eroding sediment as a massive flow of water advanced toward the ocean. Once the sediment-laden water reached the calmer sea, sediment settled out and formed a massive delta along the coastline. At the same time sea levels were rising, flooding river valleys with water and more sediment. Once the weight of the ice sheets was removed from the coastlines of Alaska and British Columbia, the land decompressed and rose in elevation.

One question often asked; how fast did sea levels rise? One estimate stated that sea levels across the globe rose 110 meters (361 feet), or 1.1 meter per century (3.6 feet per century) between 16,000 to 6,000 years ago. This was at a slow enough rate for humans and animals to adapt to the new environment. As a comparison, sea levels today are estimated to be rising 10 to 20 centimeters (.33 to .66 feet) per century. 

During the great meltdown, water was everywhere, including the parched desert basins of the American west. In the Great Basin of Nevada, California, Idaho, Oregon, Utah, and parts of Wyoming, there were over 100 freshwater lakes. Diversion of the jet stream to the south of the ice sheets in Canada, caused heavy rainfall from the Pacific on the deserts. Add in the meltwater from the ice sheets and lakes formed all over the Great Basin. With no outlets and low evaporation rates, some of the lakes grew gigantic enormous. 

One ice age lake example was Lake Bonneville which covered a good portion of Utah. Lake Bonneville was 19,000 square

Figure Four - Lake Bonneville compared to
size of Utah. 

miles and had a depth of over 1000 feet (figure four). Death Valley even carried water during the ice age meltdown. Further east, wind squalls roaring off the ice sheets, sandblasting the land. Since cold air is denser than warm air, the squalls blew down the slopes of the ice sheets creating gusts of wind up to 100 miles per hour. During the winter when the soil near the ice sheets was dry, the winds carried sand and silt in great dust storms across the great plains, creating 35,000 square miles of sand hills in north central Nebraska and South Dakota. Today, it is estimated that silt from the ice ages covers thirty per cent of the United States, now mostly hidden under forests and grasslands.

One of the great floods of all time happened during the meltdown of the ice sheets. A lobe of ice from the Cordilleran ice sheet served as an ice dam for Lake Missoula, a 3000 square mile ice age lake in what is now the Clark Fork Valley in Idaho (figure five). When the Cordilleran ice sheet melted, so did the lobe of ice that held the waters back at Lake Missoula. When the rising level of meltwater breached the weakening ice dam, it failed and a wall of water and ice, as much as 600 feet high, flooded Idaho and eastern Washington on its way down the Columbia River to the Pacific Ocean. The flood lasted for a month and created the scarred Channeled Scablands in eastern Washington. On the other side of Canada, Lake McConnell and Lake Agassiz raised their own havoc.    

Figure Five - a depiction of what the ice dam at Lake Missoula might
have looked like.  

Even after the great meltdown, the ice age had one more gasp. Around 12,900 years ago in a period called the Younger Dryas, a sudden and dramatic drop in temperature occurred in North America, returning us to the climactic conditions of the Last Glacial Maximum. The great plains returned to the cool, wet glacial conditions, reminiscent of the Wisconsin Ice Age. This cold period lasted for over one thousand years! 

The Younger Dryas is important for another reason; it marks the end of what some archaeologists believe to be the Clovis Paleoindian culture in North America. Some investigators believe that the Clovis culture was wiped out by an extraterrestrial event which triggered the Younger Dryas while others believe the Clovis culture ended in a more traditional manner. I will explore this in my next article. 

Figure Six - Author John Bradford Branney. See all of his
books at John Bradford Branney Books

If you missed my first two paleoclimatology articles, here are the links.  
Hey, and check out my books, OK?  
Milankovitch Ice Age Theory
Last Glacial Maximum