Thursday, July 25, 2019

Paleoclimatology 101 - The Younger Dryas


Figure One - Paleoindians competing with scavenging American Lions for a bison carcass,
sometime prior to the Younger Dryas. 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 itself and became colder. European scientists have known about this cooling event since the mid-twentieth century and dubbed it the Younger Dryas, named after a flower that grows in Europe in colder climates (figure one). By the 1990s, scientists around the globe were studying the evidence and effects from the Younger Dryas Chronozone in their areas of concern (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 the YDC began, the Clovis Paleoindian weaponry disappeared from North America. At the same time, forty or so Pleistocene megafauna species in North America went extinct. I am referring to mammals like mammoths, mastodons, ground sloths, camels, musk ox, horses, short-faced bears, dire wolves, saber-toothed cats, and several other large mammal species. 

The occurrence of these three 'megaevents' - the YDC, the disappearance of Clovis, and the extinction of many megafauna species - has prompted scientists to search for the proverbial smoking gun at a winner-take-all pace. So far, the occurrence or coincidence of these three events has led to more questions and conjecture than answers from the scientific community. 



Figure Two - Do you want to go on a prehistoric roller coaster ride? 
Read the SHADOWS on the TRAIL Quadrilogy!  CLICK HERE

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 shadows of their former selves. The ice sheets had retreated into Canada and were greatly diminished in areal extent and thickness. The two archaeologists noted that greenhouse gases - methane and carbon dioxide - had increased to interglacial levels during the YDC, helping to moderate the cooling climate. 

Some scientists write that the YDC showed up lickety split, but Meltzer and Bar-Yosef reported that archaeological evidence indicated that the YDC was a gradual and time-transgressive event, happening at different times in different areas with varying effect. They suggested that there were vast areas in the world that were completely unaffected by the YDC. Figure three shows the warming and cooling trends over the past 18,000 years, as well as the Younger Dryas. The Earth was on a warming trend when the YDC occurred and hung around for approximately 1200 years.   


Meltzer (2009) reported that during the YDC,  North American glaciers stopped melting and some glaciers actually expanded. He reported that the climate in the northern part of the United States was cooler and drier and estimated that average temperatures dropped nine degrees Fahrenheit. It was also a time of high winds, blowing sand and silt across the northern plains and Midwest. 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.

 

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 causing variation in the geographic distribution of insolation or solar radiation. I refer you to that article for further information.

So, what caused a "mini ice age" (YDC) near the end of a "maxi ice age"? There are two main theories bantered about by scientists for the cause of the Younger Dryas. 


The more traditional theory proposed that a massive periglacial lake named 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. This deluge of freshwater interrupted the North Atlantic thermocline and Gulf Stream, that 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 UK, Iceland, Greenland, and Norway. 

F
reshwater is 2.5 per cent less dense than seawater, so when Lake Agassiz's freshwater hit 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. Therefore, temperatures in that part of the northern hemisphere were no longer regulated and a portion of the North Atlantic froze. Once this polar front established its footing, the cold expanded from the North Atlantic area. 

Detractors of this theory pointed out that the amount of freshwater needed to shut down the North Atlantic thermohaline circulation for a long time would be astronomical. And wouldn't there be geological evidence from this type of massive flood along the St. Lawrence waterway?       
Figure Four - Courtesy of Nature Magazine, April 1, 2010. 
The more controversial theory was based on a prehistoric comet striking Earth and triggering the conditions of the Younger Dryas (Firestone et al 2006). Firestone and his colleagues believed that a comet may have struck the Laurentide Ice Sheet, melting and breaking off huge sections of glacial ice which ended up in the North Atlantic. The freshwater and icebergs in the North Atlantic weakened the thermohaline circulation, causing abrupt climate cooling (just like the traditional 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 age "carbon rich black layer" (the black mat) found in Clovis sites around North America. 

Detractors jumped on this theory quickly and passionately. They asked where the crater was, and where the remnant of the comet was? The theorists claimed the ice sheet buffered the impact from the comet (figure five). Critics also pointed out the lack of solid scientific and geologic evidence. Scientific interest in this theory has waned over the years, but advocates on both sides still debate the pros and cons of the theory.  

Is this extraterrestrial theory possible? I reply, "anything is possible". But, what about the critics of the extraterrestrial theory, don't they have a case? Of course, they have a case, but just remember, it is inherent in human nature not to accept things "out of the box". Some of the same critics of the extraterrestrial theory still cling to the "Clovis First" theory even after solid evidence of  "Pre-Clovis" has stared 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 for continuity rather than change. 

In the summary of Hunter-Gatherer Behavior - Human Response during the Younger Dryas, 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 a mostly northern hemisphere phenomena, affected the higher latitudes and not much for the rest of the globe. For example in North America, the authors noted that YDC climate varied from cold and dry in the north to warm and wet and cold and wet elsewhere. The researchers found that no area in North America appeared to be warmer and drier during the YDC. They also 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 take on all of this? Gee, I am glad you asked. The Younger Dryas cooled off the continent, but humans obviously adapted, just like humans always have. Evidence of the Clovis prehistoric culture may have disappeared, but the people behind it showed up in the Goshen and Folsom prehistoric cultures. The weaponry Clovis used to hunt mammoths adapted to better suited weaponry for hunting bison. The megafauna was met with both climate change and intense hunting pressure from humans. The mammals did not adapt.          

Summary...I am unaware of any subject associated with the last ice age that has drawn more attention in the last decade or so than the Younger Dryas. From what I have researched, the YDC was more bark than bite for human populations at the time. There is no question that the YDC occurred, but the big question is what triggered it?   

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.       

Wednesday, July 10, 2019

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! 
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!    

Figure Three - the first book in my
prehistoric book series that took
place after the last ice age.
CLICK this Link!!
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
     

Monday, June 24, 2019

Paleoclimatology 101-Part Two-Last Glacial Maximum




Figure One - North American Paleoindians surviving after the Last Glacial Maximum.  
In my first article on paleoclimatology, I discussed a concept called the Milankovitch Ice Age Theory which explains why ice ages occur and how often their cycle occurs. In this second article, I am writing about what North America looked like during the Last Glacial Maximum (LGM) of the Wisconsin Ice Age. If you missed my first article, here is the link, but be sure to come back after you read my first article;  Paleoclimatology 101 - Milankovitch Ice Age Theory   

According to scientists, the Wisconsin Ice Age reached its Last Glacial Maximum (LGM) sometime between 20,000 and 18,000 years ago. To keep things simple, I am using 18,000 years ago for the LGM in this article. There were probably several advancements and retreats of the ice sheets during the Wisconsin glaciation, but ice sheets and glaciers are very destructive forces. They tend to wipe out everything that gets in their way and previous evidence for advancing and retreating cycles. 

My focus is North America for this article, but readers need to be aware that the last ice age impacted other countries and continents in the northern hemisphere. Figure two is an excellent illustration of the vastness of the last ice age. The map looks down from the North Pole. The areas in blue show the extent of the ice sheets on land and sea. There were places where the ice sheets were as much as three kilometers thick!        
Figure Two - Looking down from the North Pole and showing in blue the 
land mass covered in ice and snow during the last ice age.  
Contrary to social media, fake news, and popular belief, the climate has always been in a state of flux throughout Earth's multi-billion long history, long before humans ever took a step on this planet. Evidence indicates that the Pleistocene, the geologic period of the last ice ages, was a particularly volatile time. Although there have been several events in the geologic past which caused catastrophic climate change, much of the  climatic cycles are related to how the Earth rotates around the sun, and that is the case for the ice ages. 
During the LGM, thick ice sheets covered most of Canada and portions of the northern United States (Figure Three). The massive ice sheets altered geography, climate, and environment on both land and sea. Scientists named the two largest ice sheets covering much of North America, Cordilleran on the west and Laurentide on the east. The Cordilleran and Laurentide ice sheets had a tremendous impact on North America's climate. The water for the these ice sheets was borrowed from the oceans. Scientists believe that to accommodate the estimated size of the ice sheets, global sea levels dropped approximately 120 meters, or 400 feet. The sea level drop exposed the continental shelf around North America and created a land mass northwest of Alaska called Beringia. These new landmasses became available for habitation by animals and humans. There are probably many submerged "Prehistoric Atlantis colonies" on the continental shelf now that sea levels have risen.   
What was it like in North America during the Last Glacial Maximum? It was much colder. 
Figure Three - Key elements of North America during the Last 
Glacial Maximum (LGM). 
Scientists estimate that the presence of the ice sheets may have caused global temperatures to drop nine to twelve degrees Fahrenheit. And though not all scientists agree with the effect on the tropics, some propose that temperatures may have dropped an average of five to nine degrees Fahrenheit. Of course, the closer to the ice sheets, the more the temperature drop. I have seen temperature estimates of eighteen to twenty-two degree Fahrenheit lower than today along the ice sheets and thirty-seven to forty-one degree Fahrenheit drops on top of the ice sheets. The ice sheets were so massive that the jet stream split and went around them. The ice sheets created a high pressure atmospheric zone above them where anticyclonic winds circulating clockwise. These winds were probably fierce and destructive.  
The terrain along the southern margins of the ice sheets was most likely tundra-covered periglacial land resulting from seasonal thawing of snow in areas of permafrost where the runoff, refroze into ice wedges and other structures. Further south from the ice sheets, scientists believe that there were vast spruce forests from the Rocky Mountains to the East Coast of the United States with  interspersed loess and sand hills (Figure Four).  
Figure Four.

 Around 17,000 years ago, the ice sheets began to melt. The northern hemisphere began receiving more summertime insolation from the sun caused an overall reduction in ice sheet thickness and expanse (see Milankovitch Theory). In North America, when the Cordilleran and Laurentide ice sheets melted, it created a mess. Huge lakes formed and oceans received a huge influx of icy freshwater and icebergs, affecting the ocean current circulation patterns. By 15,500 years ago, the ice sheets had melted enough to raise sea levels high enough to create the Bering Strait, but not enough melting to open the ice-free corridor between the Cordilleran and Laurentide ice sheets. It would be hundreds of years later before humans and animals could survive the flooded and boggy passageway. Survival for humans in the ice-free corridor required food, clothing, and firewood availability. 
For decades, scientists believed that the first humans into America migrated through the ice-free corridor between the Cordilleran and Laurentide ice sheets at around 14,500 years ago. However, evidence gathered in the last two decades, indicates there were humans south of the ice sheets, perhaps as early or earlier than the Last Glacial Maximum. So, where did these humans come from? I will cover that story on another day.      
For my next article on Paleoclimatology, I will discuss the Younger Dryas, a period of rapid cooling in the late Pleistocene from 12,800 to 11,500 calendar years ago. It followed closely on the heels of a dramatically abrupt warming that brought the last Ice Age to a close around 17,000 calendar years ago. In the meantime, check out my prehistoric adventures, you will be glad you did! 

                                                          CLICK for INFORMATION

Wednesday, June 5, 2019

Paleoclimatology 101 - Milankovitch Ice Age Theory



Figure One - Ice Age Man. Courtesy of Manhattan's
Museum of Natural History.  
During the Pleistocene, massive sheets of ice flowed southward across Canada into the northern United States about seventeen times. This lasted approximately 1.65 million years. 

Imagine what Canada and other northern hemisphere countries were like with a mile or so of ice on top of the land for thousands of years. The ice sheets were heavy enough to push the Earth's crust down about 1000 feet. Where did all the water for the ice come from. Answer: the oceans! Sea levels dropped substantially. 
Do we know what caused these Ice Ages? Let me present one theory.  
Figure Two - An estimate of the depth of ice in meters
 from the Wisconsin glaciation around 21,000 years ago. 



During the last Ice Age, ice sheets spread and shrank on a 100,000 year-long-cycle. Glaciers dominated the land from 60,000 to 90,000 years during the cold phase of the cycle, and then mostly disappeared for 10,000 to 40,000 years during the warm phase of the cycle (Bonnicksen 2000; p. 5). 
The Wisconsin glaciation began about 100,000 years ago in North America and ice sheets reached maximum thickness around 18,000 years ago. A warming trend started around 17,000 years ago and the ice sheets began to melt. There was a lot of ice to melt. Figure Two represents an estimate of how thick the ice sheets were in meters on four future North American cities. It took another 11,000 years for Canada to completely thaw out, right around 6,000 years ago. Earth has been in an interglacial climate ever since.  
Several things influence climate; the sun’s energy output, carbon dioxide levels, and ocean currents. All of these are important, but the single most important factor in driving our climate is insolation, or the amount of solar energy that reaches the Earth from the Sun. If you don’t believe that the sun has that much influence, try living in Alaska in the middle of winter.

Researchers have found that the sun’s output varies, and that the amount of sunlight that reaches certain parts of the globe is affected by how the Earth orbits the Sun. A Serbian astronomer-
Figure Four - Milutin
Milankovitch 
mathematician by the name of Milutin Milankovitch (1879-1958) hypothesized that past glacial cycles correlated to cyclical changes in insolation, and that the Earth’s circumnavigation around the Sun were the main cause of these cyclical changes in insolation. Milankovitch and others claimed that the Earth’s orbital path had a huge impact on past global cooling and warming cycles. Milankovitch tested his theory against temperature data from the paleoclimate records, and then he proposed a 100,000 year-cycle between ice sheets. He claimed that the ice sheets were not created by dramatic changes in the amount of insolation reaching Earth, but how solar energy was distributed on Earth. He identified three circumnavigation cycles that were responsible for the Ice Ages.
The cycles were eccentricity, axial tilt or obliquity, and wobble or precession.
Eccentricity. Milankovitch defined the first circumnavigational cycle as eccentricity or the shape of the Earth’s orbit around the sun. Due to gravitational pull from other planets, the Earth does not orbit the Sun in a perfect circle. The Earth has an elliptical orbit around the Sun that varies from five percent to zero percent ellipticity over a 100,000 year-long-cycle (Figure Five). An elliptical orbit reduces or increases solar radiation during the various seasons. When the Earth is in its most elliptical orbit, it receives twenty to thirty percent more solar energy at its perihelion (Earth is closest to the Sun) than its aphelion (Earth is farthest from the Sun). Currently, Earth is in an interglacial cycle and its eccentricity is at a minimum.
Figure Five - Eccentricity. The Earth moves in a slightly elliptical
path during its annual revolution around the Sun.  
Axial tilt or Obliquity. The second circumnavigational cycle proposed by Milankovitch was axial tilt or the inclination of the Earth’s spinning axis in relationship to its orbital plane around the Sun. English, please? Earth orbits the Sun at a different angle than the angle the Earth rotates on its own axis (Figure Six). Earth’s rotational axis is currently at an angle of 23.4 degrees from its orbital plane around the Sun. Milankovitch calculated that the Earth’s rotational axis and its orbital plane around the Sun varied from 21.5 to 24.5 degrees over a 41,000 year-long-cycle. Axial tilt or obliquity creates our seasons. When there is less of an axial tilt, the Sun’s solar radiation is better distributed between summer and winter with increased differences in radiation between equatorial and polar regions. His  hypothesis claimed that a smaller axial tilt angle promotes the growth of ice sheets because warmer winters hold more moisture and produce more snowfall while cooler summer temperatures mean less ice melt. Therefore, ice sheets can grow!      
Figure Six - Axial Tilt or Obliquity. The Earth is tilted on its rotational axis
23.4 degrees from a plane perpendicular to the surface over
which it moves during its revolution around the Sun.  
Precession or Wobble. Some years ago, scientists proposed that the Earth’s axis wobbled due to lunisolar forces, changing the orientation of the rotational axis of the Earth. This wobble as Earth spins on its axis is very slow and is on an approximately 23,000 year-long-cycle (Figure Seven). The Earth wobbles enough to change from pointing at the North Star (Polaris) to pointing at another star called Vega.
Figure Seven - Precession or wobble. The effect of the wobble is to
systematically change the timing of the solstices and equinoxes
relative to the extreme positions the Earth occupies on
its elliptical path around the Sun.  
When the Earth’s axis points at Vega, the Northern Hemisphere winter and summer solstices coincide with aphelion (Earth is farthest from the Sun) and perihelion (Earth is nearest to the Sun), respectively. Winter occurs when Earth is farthest from the Sun and summer occurs when Earth is nearest the Sun, leading to the greatest seasonal contrast. In the Northern Hemisphere, winter will end up in July and August, and summer in January and February during the 23,000 year-long-cycle. This happens because axial tilt or obliquity still accounts for the seasons; summer when that hemisphere leans toward the Sun and winter when that hemisphere leans away from the Sun.

Bottom Line. Milutin Milankovitch suggested a combination of conditions that were conducive to glaciation. The first condition is minimal axial tilt or obliquity. Changes in axial tilt has very little effect from solar radiation at lower latitudes, but increases the effect toward the poles. As axial tilt increases, summer radiation increases significantly. Therefore, minimal axial tilt is conducive to ice sheet buildup The second condition is a relatively high eccentricity. Eccentricity variations affect the intensity of the seasons because it alters the distance the Earth is from the Sun. The third condition is the Northern Hemisphere summer should coincide with aphelion (Earth is farthest from the Sun) creating cooler summers which means less melting of the existing ice sheets. When all of these conditions converge, we have what is often referred to as a "cold orbit" and there is a good chance that ice sheets will expand.       
2000    Bonnicksen, Thomas M. America’s Ancient Forests from the Ice Age to the Age of Discovery. John Wiley and Sons. New York.

2015    Bradley, Raymond S.  Paleoclimatology - Reconstructing Climates of the Quaternary. Third Edition. Elsevier Publishing. New York.   

Tuesday, May 7, 2019

Radiocarbon Dating - Part Two - Reporting


Figure One - A handful of Folsom dart points, surface found on private land in the high plains. 
Age is around 10,900 to 10,200 BP. John Branney Collection.    
Welcome to Part Two of my article on Radiocarbon Dating. In Part One, I discussed how the radiocarbon date process worked. In Part Two, I explore how radiocarbon dates are reported in archaeological papers and journals. 

If you missed Part One, click the link to that article; Radiocarbon Dating - Part One - Process . And don't forget to return to Part Two!          

“Radiocarbon dates indicate that Folsom may have had a relatively long residence in the Rocky Mountains and adjacent Plains from about 10,900 to 10,200 BP.

“At the Hell Gap site the investigators defined a Midland level with dates estimated between 10,700 and 10,400 RCYBP (Irwin-Williams et al. 1973).”
                                 -   Marcel Kornfeld, The First Rocky Mountaineers, page 46

When you read the above passage from Dr. Kornfeld’s archaeological book, do you see anything unusual in the way he reports the ages of Folsom and Midland? Do you get an idea of how old the Folsom prehistoric culture was? Do you know what BP means? Do you get an idea of how old the Midland prehistoric culture was? Do you know what RCYBP means? By the time you read this article, you will be able to answer these questions.   
Figure Two CLICK to ORDER the SERIES
In Part One of Radiocarbon Dating, I mentioned certain “gotchas” that archaeologists and scientists must be aware of when using the radiocarbon dating process. One of the bigger problems is that the amount of carbon-14 isotope in the atmosphere was not constant throughout prehistoric time. At different times during the Earth's prehistory, plants absorbed different amounts of the carbon-14 isotope and since the carbon-14 isotope is the basis for measuring age with the radiocarbon dating process, this is a major problem. The radiocarbon model must know how much carbon-14 isotope was available in the atmosphere at a specific time so it can calculate how much radioactive carbon-14 isotope has decayed. 

Scientists recognized this problem early on in the radiocarbon process and went about solving it. They used dendrochronology, or the study of tree rings, to tie tree rings to the level if carbon-14 isotope levels. Using 4,500-year-old bristlecone pines in the Sierra Mountains, scientists determined carbon-14 isotope levels at the time the rings were made by the trees. The scientists then correlated the rings from living bristlecone pines to dead stumps of trees, allowing the scientists to calibrate to around 8,200 years ago. With this information, scientists created a first pass of “calibrating" the raw radiocarbon age to calendar age.   
Figure Three - Paleoindians, waiting their turn
at the cafeteria. 
 
To accommodate the lack of certainty in radiocarbon measurements, scientists added an error factor to the measured radiocarbon age. For example, an archaeological report might read that the age of a site is 10,000 ± 160 B.P. This means that there is a 67 percent chance (one standard deviation) that the real age of the site is 160 years plus and minus on either side of 10,000 years before present (B.P.). Whenever you see Before present or B.P. or BP, it means it is measured from the baseline year of 1950. For the above example, if we want 95 percent accuracy, we must use two standard deviations, so we add two times 160 years, or 320 years to either side of 10,000 years before present (B.P.).         

Archaeologists and scientists oftentimes publish uncalibrated or uncorrected radiocarbon ages, even though uncalibrated radiocarbon ages are misleading and do not tie well to calendar years. For example, 9,000 uncalibrated radiocarbon years corrects to approximately 10,200 calendar years after calibration(see Figure Four). And 11,000 uncalibrated radiocarbon years corrects to approximately 13,000 calendar years; two thousand years of correction! When you see an author quote that Clovis at 11,000 radiocarbon years, it is really around 13,000 calendar years. That is a big difference.  

Most of us think in calendar years, not radiocarbon years. We want to know how old a site or an artifact is in calendar years, not in something as esoteric as radiocarbon years. 
Figure Four - One representation of radiocarbon age conversion 
to calendar age. Enter the vertical axis with an uncalibrated 
radiocarbon age and read the calibrated calendar age 
on the horizontal axis.    
So why do archaeologists and scientists report uncalibrated radiocarbon age instead of calibrated calendar age? I asked that same question to a practicing archaeologist and he told me "because archaeologists write their reports for other archaeologists, not for lay people". I thought his answer was snobbish. I don't think he recognized how condescending his answer was. I mentioned to him that many of us "lowly lay people" are interested in early man and read these reports. I saw this as further evidence of the brick wall that exists between professional archaeologists and avocational archaeologists and collectors.    
Another reason that archaeologists and scientists might report uncalibrated radiocarbon age is that they do not have a high level of confidence in the calibration model or results. After all, models assume a certain amount of carbon-14 isotope in the atmosphere at a certain time. That is a big assumption. 

I became interested in archaeology over fifty years ago and since then I have seen the proposed ages of certain prehistoric cultures change over time. I assume this is because radiocarbon dating models and sampling methodology have improved. I remember when Clovis technology was advertised as around 11,000 years old and now scientists are reporting a better age for Clovis is 13,000 years. I am sure these reporting discrepancies come from improvements in technology, sampling methodology, and computing power.    
When you read archaeological reports, be sure to note what radiocarbon dating nomenclature they are using. Uncalibrated radiocarbon age can be reported in RCYBP (Radiocarbon Years Before Present), C14 ka BP, 14C ka BP, 14C ka BP, radiocarbon years, c14 years before the present, rcbp, carbon-14 years before the present, and CYBP. In all cases, B.P. refers to the baseline year of 1950. Again, if you see this nomenclature behind an age, the age is an uncalibrated radiocarbon age. It is my experience that authors of archaeological site reports don’t explain whether they are using calibrated or uncalibrated radiocarbon age. Readers must figure this out on their own. So, readers be waryUncalibrated to calibrated radiocarbon dates makes a huge difference in ages as you saw above.  

The most common abbreviation for calibrated radiocarbon age is cal B.P., cal yr. BP, B.P. or the one I like the most; XYZ is blankety blank blank years old. 
Figure Five - A handful of Midland dart points surface found on private land in the high plains. According to the 
radiocarbon age nomenclature used by Marcel Kornfeld, Midland is close to two thousand years older than Folsom. 
We know better, Folsom and Midland technologies were contemporaries. John Branney Collection.      


Let me return to the original passage at the top of the article from The First Rocky Mountaineers by archaeologist Marcel Kornfeld. On page 46, Dr. Kornfeld discussed the ages of the Folsom and Midland prehistoric cultures. Most people contend that the Folsom and Midland prehistoric cultures happened pretty much simultaneously. Some people believe the same people made both Folsom and Midland.    
Radiocarbon dates indicate that Folsom may have had a relatively long residence in the Rocky Mountains and adjacent Plains from about 10,900 to 10,200 BP.
At the Hell Gap site the investigators defined a Midland level with dates estimated between 10,700 and 10,400 RCYBP (Irwin-Williams et al. 1973).

It appears from this passage that Dr. Kornfeld mixed uncalibrated radiocarbon age nomenclature, RCYBP in the second paragraph, with calibrated radiocarbon age nomenclature, BP in the first paragraph. By doing this, he is stating that the Midland archaeological level was around 12,620 calendar years old, close to two thousand years older than Folsom. We know better, so I have to assume Dr. Kornfeld meant to use BP for Midland and not RCYBP


Mistake? Probably. Confusing? Definitely, even for the "experts". Be careful when you read radiocarbon ages in archaeological journals and articles! I hoped you learned something. I did when I did the research for this article.   

Figure Six - CLICK for JOHN BRADFORD BRANNEY Books