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.