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 cycles happen. In my second article, I write 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 it;  Paleoclimatology 101 - Milankovitch Ice Age Theory   

According to most 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 advances and retreats of the ice sheets during the Wisconsin glaciation, but since ice sheets and glaciers are very destructive to landscapes, they wipe out a lot of the evidence from previous events. 

As I mentioned in the first paragraph, my focus is on North America, but readers should be aware that the last ice age impacted many other countries and continents in the northern hemisphere. Figure two is an excellent illustration of the vastness of the last ice age. The map is looking down from the North Pole, and the areas in blue are the estimated extent of the ice sheets on both land and sea. There were places in the northern hemisphere 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, Earth's climate has always been in a state of flux throughout our multi-billion-year history. The climate was heating up and cooling off a long time before humans stepped on the planet. Geologic 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 the living 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 these ice sheets came from the oceans. Scientists believe that to accommodate the estimated size of the ice sheets, global sea levels had to drop approximately 120 meters or 400 feet. This sea-level drop exposed the continental shelf around North America and created a landmass northwest of Alaska called Beringia. New landmasses along the continental shelf and in Beringea became available for habitation by both animals and humans. This makes me wonder how many "Prehistoric Atlantis" colonies exist underwater along the continental shelf now that sea levels have risen.   

                                                                                                                                                                             

Figure Three - Key elements of North America 

during the Last 

Glacial Maximum (LGM). 

What was it like in North America during the Last Glacial Maximum? One, it was much colder! That makes sense. 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 scientists propose that temperatures may have dropped an average of five to nine degrees Fahrenheit in the warmer climates of the Earth. Of course, the closer to the ice sheets, the more uncomfortable the temperature drop. I have read temperature estimates of eighteen to twenty-two degrees Fahrenheit lower than today along the front of the ice sheets and thirty-seven to forty-one degrees Fahrenheit lower on top of the ice sheets. We can quibble about whether the temperature was X degrees or Y degrees, but bottom line it was colder. 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 sandhills (figure four).  

Around 17,000 years ago, the ice sheets started to melt. The northern hemisphere received more summertime insolation from the sun causing an overall reduction in ice sheet thickness and expansion (read my article on 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 circulation patterns in the oceans. 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 traverse the flooded and boggy passageway between the two ice sheets. Survival for humans in the ice-free corridor required food, clothing, and firewood availability.

Figure Four - a Mastodon in a spruce forest in a midwestern state in the United States 

during the Last Glacial Maximum.    

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 final 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 dramatic and 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! 

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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. These ice events lasted for 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 approximately 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 plausible 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 roughly 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 started about 100,000 years ago in North America, and ice sheets reached maximum thickness around 18,000 years ago. The warming trend began around 17,000 years ago, and the ice sheets started to melt, and there was a lot of ice to melt! 

Figure Two represents an estimate of how thick the ice sheets were in meters in four future North American cities. It took 11,000 years for Canada to completely thaw out, right around 6,000 years ago. Since then, Earth has had an interglacial climate. 

 

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Several factors influence climate: the sun’s energy output, carbon dioxide levels, and ocean currents. All three factors are important, but the single most important factor in driving our climate is called insolation! Insolation is the amount of solar energy that reaches the Earth from the Sun. If you don’t believe that the sun has much influence on the climate, try living in Alaska in the middle of January. 

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-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 was the main cause of the Earth's 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 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 by how the solar energy was distributed on Earth. He identified three circumnavigation cycles that were responsible for the Ice Ages: 1) eccentricity, 2) axial tilt or obliquity, and 3) wobble or precession.

Eccentricity. Milankovitch defined eccentricity as the shape of the Earth’s orbit around the sun. I always assumed that the Earth rotates around the sun in a circular orbit, but due to the 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). The elliptical orbit of the Earth 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 (when Earth is closest to the sun) than at its aphelion (when Earth is farthest from the sun). Currently, Earth is in its 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 relation to its orbital plane around the sun. 

When I first read that definition, I begged the author to "speak English, please." It is a hard concept to visualize, but it makes sense once it is grasped.  

Earth orbits the sun at a different angle than the angle at which the Earth rotates around 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 vary from 21.5 to 24.5 degrees over a 41,000-year-long cycle. Axial tilt or obliquity is what 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. Milankovitch's 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 cause less ice melt. Under these conditions, ice sheets can grow from year to year, etc.!    

 

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 moves during its revolution around the Sun.  

Precession or Wobble is the third circumnavigational cycle. 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 over time.

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 (when Earth is farthest from the sun) and perihelion (when 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 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. 

Does your head hurt as much as mine did when I first studied this theory? 

You might be asking what the bottom line is. See below.  

Milutin Milankovitch suggested that the right combination at the right time of these three circumnavigational cycles is conducive to glaciation. The first condition is minimal axial tilt or obliquity. Changes in axial tilt have very little effect from solar radiation at lower latitudes but increase 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 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 Northern Hemisphere summer should coincide with an aphelion (when Earth is farthest from the sun), which creates cooler summers, which translates to less melting of existing ice sheets. 

When all three conditions converge, we have what is often referred to as a "cold orbit," and the chances are high that ice sheets will expand! Notice, I never once mentioned "man-made climate change". 

       

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.   

The historical fiction novels written by John Bradford Branney are known for their impeccable research and biting realism. In his latest blockbuster novel BEYOND the CAMPFIRE, Branney catapults his readers back to the Late Pleistocene, where they reunite with some familiar faces from Branney’s best-selling prehistoric adventure series, the SHADOWS on the TRAIL Hexalogy. BEYOND the CAMPFIRE is the eleventh published book by Branney.

Author Branney earned a geology degree from the University of Wyoming and an MBA from the University of Colorado. He lives in the Colorado Mountains with his family. 

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 and read 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 The First Rocky Mountaineers, do you get the impression that both Folsom and Midland are a tad north of 10,000 years old? I did. In fact, the first book I wrote I used 10,500 years ago for the age of the Folsom Complex (figure two). I was wrong. I was using an uncalibrated radiocarbon date. The calibrated radiocarbon date for Folsom is much older. After calibrating the raw radiocarbon date, we find that Folsom occurred around 12,500 years ago during the Younger Dryas climate event! 

Do you know what the BP refers to in the above passage from Dr. Kornfeld? Do you know what RCYBP stands for? By the time you read this article, you will be able to answer these questions.   

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In Part One of my article on Radiocarbon Dating, I mentioned certain “gotchas” that archaeologists and scientists must be aware of when using the radiocarbon dating process. One of the bigger assumptions scientists must account for is that the amount of carbon-14 isotope in the atmosphere has not been constant throughout prehistoric time. At different times during Earth's prehistory, plants absorbed different amounts of the carbon-14 isotope and since the carbon-14 isotope is the basis for measuring in the radiocarbon dating process, this became a major challenge. To determine age, 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 levels of carbon-14 isotope levels. For example, using 4,500-year-old bristlecone pines in the Sierra Mountains, scientists determined carbon-14 isotope levels at the time the tree rings were formed. The scientists then correlated the tree rings from living bristlecone pines to dead stumps,  allowing the scientists to calibrate carbon-14 isotope levels back to around 8,200 years ago. With this information, scientists created the first pass at “calibrating" raw radiocarbon ages to corrected calendar ages.   

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 or minus the 10,000 years before present (B.P.). Whenever you see Before Present or B.P. or BP or bp, it means the age 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 dates instead of calibrated radiocarbon dates. Knowing the difference between calibrated and uncalibrated radiocarbon dates can be misleading and confusing for those who don't know the difference. Raw radiocarbon dates do not tie up well to calendar years. For example, an uncalibrated date of 9,000 radiocarbon years is approximately 10,200 calendar years old. That is a 1,200-year swing! See and play around with the correction curve in figure four. The error between uncorrected and corrected grows substantially as the age of the samples increase! As an example, 11,000 uncalibrated radiocarbon years correct to approximately 13,000 calendar years. That is a 2,000-year difference! So when you read a report where the author quotes the Clovis Complex at 11,000 RCYBP, remember that the author is referring to an uncalibrated date. Most scientists currently believe that Clovis was around 13,000 years old. 

Most of us think in calendar years, not radiocarbon years. We want to know how old a site or an artifact is based on our day-to-day paradigm of calendar years, not 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.   

 

Why do archaeologists and scientists report uncalibrated radiocarbon dates instead of calibrated and/or corrected dates? I asked that same question to a practicing archaeologist and he told me, "Because archaeologists write reports for other scientists and archaeologists, not for laypeople."

Okay...I thought his answer was a tad bit snobbish, but pretty much what I expected. I mentioned to him that most of the sites he and other archaeologists excavate came from "laypeople" and that many of us "laypeople" are just as interested in prehistoric cultures as archaeologists. I saw his response as more evidence for the broad chasm and lack of respect that exists between professional archaeologists and us "laypeople".     

Another reason that archaeologists and scientists might report uncalibrated radiocarbon age is that they might not trust the calibration or correction model. After all, models are only representations, not reality. And in this case, the models assume a certain amount of carbon-14 isotope in the atmosphere at different times in prehistory. That is a big assumption. I remember working on my undergraduate degree in geology and a professor told us that models were only as good as the assumptions behind them, i.e., "garbage in, garbage out."  

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 remember when Clovis technology was reported at around 11,000 years old and now scientists are reporting the more precise age of 13,000 years for Clovis. I am sure these reporting discrepancies come from improvements in technology, modeling, sampling methodology, and computer power.   

When you read archaeological reports, be sure to note the radiocarbon dating nomenclature the authors use. It is not always clear. Uncalibrated radiocarbon dates are reported as bp, RCYBP, 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, BP and bp is referenced from the baseline year of 1950. Again, if you see this nomenclature it means the estimated radiocarbon date is from uncalibrated data. It is my experience that most authors of archaeological site reports don’t explain whether they are using calibrated or uncalibrated radiocarbon dates, they assume the readers will figure it out. Beware! The difference between uncalibrated and calibrated radiocarbon dates can be huge, especially when the samples get older.  

Calibrated radiocarbon dates are reported as cal B.P., cal yr. BP, B.P., BP or the one I like the most; Site XYZ is blankety-blank-blank years old (even though George Frison's fantastic books often used 'years old' but used uncalibrated radiocarbon dates much of the time). 

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 Bradford Branney Collection.      

Let me now return to the original passage at the top of this article from The First Rocky Mountaineers by archaeologist Marcel Kornfeld. 

"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

On page 46, Dr. Kornfeld discussed the ages of the Folsom and Midland prehistoric cultures. Most knowledgeable people contend that the Folsom and Midland prehistoric cultures were contemporaneous, or nearly so. Many people, including me, believe that the people who made Folsom points also made Midland points.    

If we use the appropriate nomenclature I presented earlier in the article, the 10,900 to 10,200 BP in Dr. Kornfeld's upper passage referred to a calibrated radiocarbon date, but this is not the case. Folsom is much older than 10,900 to 10,200 years ago. The author used common calibrated radiocarbon nomenclature for an uncalibrated radiocarbon date!  

The lower passage, 10,700 and 10,400 RCYBP, uses uncalibrated radiocarbon date nomenclature. This is perfectly fine but is in direct conflict with the nomenclature in the upper passage. Dr. Kornfeld mixed uncalibrated radiocarbon age nomenclature with RCYBP with calibrated radiocarbon age nomenclature with BP. By doing this, he is stating that the Midland culture was around 12,620 calendar years old while the Folsom culture was only around 10,500 calendar years old. We know better than that, both cultures were pretty much the same age. Dr. Kornfeld should have used bp instead of BP in the first passage to indicate an uncalibrated radiocarbon date. This just shows that even the experts get confused.   

Be careful when you read radiocarbon dates in archaeological journals and articles, even if written by experts! The dates are not always what they seem. I hoped you learned something from this article. I did when I did the research.   

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Radiocarbon Dating - Part One - The Process

Figure One - Three-inch long Montana Clovis point made from a multi-colored jasper and around 13,000 years old. How do we know who old Clovis points are?  Radiocarbon dating John Bradford Branney Collection.

Radiocarbon dating is a controversial subject. People either believe it works or they don't, and a lot of people including myself have assumptions about the way the process works and how scientists report its results. I am not here to convince you that radiocarbon dating works or does not work. That you must decide on your own after doing your own investigation. The process has its advantages and its limitations, as does any process. I will share what I learned during my investigation on the subject. 

In Part One of my two-part series on radiocarbon dating, I present an overview of the radiocarbon dating process and some of its pitfalls. It is not my intent to cover every nuance and detail of the radiocarbon dating process but to supply readers with enough information for a basic understanding. During my research, I found that the radiocarbon dating process is a lot like making sausage; we all have a general idea of how the sausage-making process works, we just don’t want to know the ingredients that went into making the sausage.  

 

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Over the decades, I have read many books and reports on archaeological sites, especially those related to my stomping grounds, the high plains of North America. I read these books and reports because I have hunted prehistoric artifacts for five decades plus and I have a passion for learning about the people who made my prehistoric artifacts. Reading these archaeological reports also helps me to write good stories for my historical fiction book series titled the SHADOWS on the TRAIL Pentalogy.  

I have always been curious about how and why archaeologists report radiocarbon dates the way they do. There does not appear to be any standard as to how archaeologists and scientists report the dates. Archaeologists use a plethora of confusing terms and phrases when reporting radiocarbon dates. For example, these are radiocarbon dates from two reports: 10,000 RCYBP and 10,000 years BP. Does this mean both sites are 10,000 years old? For someone unfamiliar with the terminology, they might say yes, but the answer is no. The first date reports an uncalibrated radiocarbon date while the second date is a calibrated radiocarbon date. Either the reader or the author must correct the uncalibrated radiocarbon date to get something meaningful like calendar years old.

Radiocarbon dating is indispensable in archaeology. Every archaeologist uses radiocarbon dating in one way or another. If we understand the basics of the radiocarbon dating process, it helps us to better understand archaeological reports and the limitations to the process. As consumers of these reports, we need to have a basic understanding of the process and its strengths and weaknesses. 

Figure Two – Variations in Cody Complex artifacts surface found on private land on the High Plains of North America. From left to right; Eden (Colorado), eccentric Cody Complex knife form (Wyoming), Eden? (Colorado), Scottsbluff knife form (Colorado), Cody knife (Wyoming), and Cody knife (Colorado). Eden on left is two inches long. Based on several radiocarbon dates (Knell and Muñiz 2013:13). Cody Complex's age is 11,600 to 8785 cal BP. John Bradford Branney Collection.  

Radiocarbon dating is one of the most widely used methods for scientists to figure out the relative ages of biological samples, such as wooden artifacts or bones. The process uses a natural phenomenon occurring in the Earth’s atmosphere (figure three). When cosmic rays from the sun bombard nitrogen atoms in our upper atmosphere, it creates an unstable, radioactive carbon isotope called carbon-14. This radioactive carbon-14 isotope oxidizes into a carbon-14 dioxide isotope which settles in the lower atmosphere. Plants and algae take in the carbon-14 dioxide isotope at the same ratio that exists in the atmosphere for that specific time. Other living organisms exchange carbon with the atmosphere through respiration and by eating plants and organisms that have the carbon-14 isotope in their tissue.

 

Figure Three - How the radiocarbon dating process works by Pass My Exams. 

When an organism dies, its carbon intake stops, and the radioactive carbon-14 isotope in its tissue begins to decay into a stable carbon-12 isotope. Radioactive decay is the process by which an unstable atomic nucleus loses energy (in terms of mass) by emitting radiation. The carbon-14 isotope has a known radioactive half-life of approximately 5,730 years. This means that in 5,730 years half of the carbon-14 isotope in the dead organism will decay into the stable carbon-12 isotope. To figure out the age of the dead organism, archaeologists and scientists test the remains for the presence of the radioactive carbon-14 isotope and its ratio to the stable carbon-12 isotope. Wood and charcoal are the best materials to use in the process, but other previously "alive" materials also work.

 

Figure Four - Paleoindians from my book Shadows on the Trail.

Radiocarbon dating is one of the most reliable means in the toolbox for dating archaeological sites, but the process has a few challenges that we should be aware of. The first challenge for the field archaeologist is to find a reliable and uncontaminated sample within the cultural material desired. The archaeologist must ensure that the sample tested is associated with the cultural material in question and that unrelated organic deposits have not contaminated the sample. As an example,  testing a piece of charcoal from a prehistoric fire hearth in a geologic formation where there are clinker coal deposits from an earlier geological episode presents a problem. The radiocarbon date might turn out too old.   

The second challenge is that the site must be less than 50,000 to 60,000 years old. As I previously mentioned, the half-life of the radioactive carbon-14 isotope is 5,730 years. If you cut the amount of carbon-14 isotope in half several times through radioactive decay, there comes a point where there is not enough carbon-14 isotope left to supply a statistically reliable answer. Over the years, advances in measurement technology have improved the ability to measure smaller amounts of carbon-14 isotope , but it still is a challenge.   

The third challenge is the biggest hurdle of all. In my fourth paragraph, I explained how cosmic rays from the sun bombarded nitrogen atoms in our upper atmosphere and created the radioactive carbon-14 isotope which is the key element needed for radiocarbon dating. We know the half-life of carbon-14 and we know the amount of carbon-14 currently produced in the atmosphere. Therefore, we should be able to calculate the amount of carbon-14 isotope left in our sample and therefore figure out its age. Unfortunately, it is not that easy.

The big snafu is that the amount of bombardment of cosmic rays from the sun has not been consistent through time. Therefore, the production of the radioactive carbon-14 isotope has not been consistent through time. The inconsistency in the production of radioactive carbon-14 throws a wrench in the monkey works. Throughout geologic time, there were peaks and valleys in the production of the carbon-14 isotope in the atmosphere. One example of this is between 11,300 to 11,600 years ago; scientists believe the atmosphere produced less carbon-14 isotope. A reduction in the production of carbon-14 isotope in the atmosphere results in a difference between measured radiocarbon years and actual calendar years. The reduction resulted in a flat plateau on the radiocarbon calibration curve and a compression of the true or calendar age.   

To correct for these discrepancies, scientists tied radiocarbon dating to tree rings (dendrochronology) and how much carbon-14 is there. This is challenging at best and probably why so many archaeologists cite uncalibrated radiocarbon years in their reports and not calibrated radiocarbon years. Readers of these reports must be aware of how the author or authors are reporting dates. There is a monstrous difference between uncalibrated and calibrated (corrected) radiocarbon ages, especially in the older Paleoindian sites!  

Figure Five – Generic example. The difference between uncalibrated radiocarbon dates          (vertical axis) and calibrated radiocarbon dates (horizontal axis).

Figure five is a simplified version of a radiocarbon date calibration curve and illustrates my point. The vertical axis (left-hand side) shows years in uncalibrated radiocarbon years. This stands for the raw radiocarbon measurement for my example. The horizontal axis (across the bottom) shows years in calibrated calendar years from the baseline year of 1950. The red curve illustrates a simplified calibration curve for the data. In this example, you enter the uncalibrated radiocarbon date on the vertical axis and where it crosses the red line you read the calendar years on the horizontal axis. For example, a site yielded an age of 10,000 radiocarbon years calibrates to 11,400 calendar years, a correction of 1,400 years! If the author of the data reports the uncalibrated age of 10,000 radiocarbon years, an unsuspecting reader might think this is how old the site is in calendar years. It is not! 

It is easy to see how this can lead to confusion for those not living archaeology every day. While some authors report uncalibrated radiocarbon years others report calibrated radiocarbon years. 

In Part Two of my two-part series on radiocarbon dating, I will cover some of the different ways archaeologists report radiocarbon dates from archaeological sites. 

Reference Cited

Knell, Edward J., and Mark P. Muñiz

2013 Paleoindian Lifeways of the Cody Complex. The University of Utah Press. Salt Lake City.   

 

 

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