ASWT Radiocarbon Update #1

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Editor’s Note:  This blog piece is based on an email newsletter sent out earlier this fall to supporters of the successful 2016 Dating Eagle Cave crowdfunding campaign.  Author Emily McCuistion is a veteran of the 2015 and 2016 ASWT Eagle Nest Canyon Expeditions. She is a graduate student at Texas State University and is studying radiocarbon dating in the Lower Pecos Canyonlands for her thesis under Dr. Steve Black.  Emily can be contacted at:

By Emily McCuistion


Practicing the Texas State Bobcat growl at Sayles Adobe, 2016.

I am a child of Austin, Texas, and I’ve had a lifelong interest in the outdoors and in old things. After graduating from the University of Texas at Austin with a BA in anthropology, I moved west for archaeology work; I called the Great Basin and Mojave Desert home for several years as I worked in Death Valley National Park and in southern Nevada. I also worked in the pine forests of the western Sierra Nevada Mountains, on the Texas Gulf coast on a nautical excavation of a Civil War gunboat, and across that great big Pacific Ocean, in New South Wales, Australia, where I worked on archaeology contract projects in advance of mining developments, mostly. For the past four years, I have spent summers with the National Park Service in Denali, Alaska and winters in Texas. Dr. Black has rightly described me as an itinerant archaeologist, and I describe myself as a generalist—one who is interested in learning about ancient hunter-gatherer people worldwide. This interest meshes well with the thesis research path I have embarked on—learning how the radiocarbon record can be used to inform our understanding of the past, both in broad strokes and fine details.

These updates are part of my education—an exercise in articulating what I am learning, and an opportunity to share what I am learning with the archaeology community and invested public. I hope that the newsletters will provoke you to ask questions about radiocarbon dating and how it shapes our understanding of the prehistory of the Lower Pecos Canyonlands. Some of this material will likely appear in an expanded form in my thesis.

Radiocarbon Basics

roasted hearts

Hearts of sotol and lechuguilla roasted in an experimental earth oven. Were they to preserve for 5730 years, they would have half the 14C they had in 2015 when the ASWT team harvested them. In twice that time, 11,460 years, they would have 1/4th the 14C from 2015, and so on.

Radiocarbon (14C) is an unstable isotope of carbon which makes up only a tiny fraction of the carbon in our atmosphere. Most 14C is created in the earth’s upper atmosphere, when thermal neutrons from cosmic rays react with nitrogen. After that, radiocarbon, and the other naturally occurring carbon isotopes (12C and 13C), react with oxygen to become carbon dioxide (CO2), and become distributed throughout the atmosphere.


Photosynthesis is the primary mechanism by which carbon is incorporated into terrestrial plants. Animals intake carbon through the food chain. Fungi, that often overlooked kingdom, takes in carbon through decomposition of its host. Aquatic organisms are more complex; they take in dissolved carbon in ocean, lakes, and rivers. Therefore, aquatic organisms, and the terrestrial animals that derive a large part of their diet from aquatic resources, often date to older than their contemporaneous terrestrial counterparts. These differences in carbon levels in various environments are called reservoir effects; there are ways to adjust assay results to account for these effects. The important thing to grasp here is that radiocarbon dating rests on the idea that CO2, and therefore 14C, is evenly distributed in the atmosphere; aquatic environments aside, relative quantities of atmospheric carbon should be consistent around the planet at any given time.

Though atmospheric carbon is assumed to be consistent across the planet at any given time, it is known that levels of 14C in the atmosphere vary through time. Amounts of 14C are effected by the earth’s magnetic field, by solar flares, by major volcanic eruptions, and, in more recent centuries, by the burning of fossil-fuels and by nuclear detonations. How are these variations through time accounted for? Dendrochronology (tree ring counting; annual growth in trees reflect environmental conditions through time, and tree ring sequences can be accurately dated by simple counting), and more recently, elemental measurements from corals, are used to establish calibration curves. Calibration curves correlate radiocarbon years with calendar or solar years, which is necessary for relating sample ages to most chronologies. Calibration will be discussed in greater depth in a future update.

Essential Terms and Symbols


Prickly pear cactus incorporating carbon from the atmosphere through photosynthesis.

  • Sample: the organic material which undergoes laboratory processing (e.g., preserved plant material, charcoal, bone, a fragment of a perishable artifact, even residues from artifacts).
  • Assay: the laboratory process performed on the sample which extracts and measures the carbon. “Assay” is a noun and a verb.
  • Date: a term often used loosely; a sample is not technically “dated,” it is assayed. The assay results are reported as a statistical estimate range of possible dates, in radiocarbon years before present (RCYBP, or often simply as BP) .
  • δ (delta): indicates isotopic fractionation differences, and is reported with the conventional age [I’m still learning about this topic; it has to do with the ratio of isotopes and loss of lighter isotopes with time—it will be discussed in the future].
  • σ  (sigma): associated with the statistical age range (the standard deviation from the estimated mean age). Standard deviation is expressed by a “±” followed by a number, which, when added or subtracted from the mean, indicates the upper and lower limits of the estimated age range. The σ will be given as 1σ or 2σ, which indicates the confidence level of an estimated date range: 1σ deviation means that the actual age of the dated material has an approximately 68% probability of dating anywhere in that range. A 2σ deviation means that there is an approximately 95% probability; 2σ will always have a larger range of possible dates than 1σ.

Delta Sigma What?  Reading Radiocarbon Ages

There are several types of radiocarbon ages that archaeologists report:

  • Conventional: normalized for isotopic fractionation (δ 13C) but uncalibrated. Reported as BP (before present, “present” being 1950 AD) which is actually radiocarbon years before present (RCYBP). In these updates I will use RCYBP when discussing conventional dates, for clarity.
  • Reservoir Corrected: adjusted age to account for variation in the carbon reservoir (e.g., aquatic environments).
  • Calibrated: accounts for variation in quantity of 14C through time, and translates radiocarbon years into solar or calendar years. Reported as cal BP, cal AD, or cal BC.

In sum, there are several ways to express an age (e.g., RCYBP, BP, AD, BC, cal BP, etc.). These suffixes are critical to indicating what type radiocarbon data is being presented. The conventional age is generally regarded as the most essential age to report, as it reflects the 14C measurements of the sample, without which reservoir correction and calibration would not be possible. A corrected and calibrated assay, however, is integral to establishing chronologies, and for simply grasping how old something is relative to our own calendar system.

For Example, radiocarbon assay TX-107 (wood charcoal), from excavations at Eagle Cave (Stratum V, Hearth 1) by the University of Texas in 1963, was reported in 1965 by Pearson et al. and by Richard Ross thusly:

8760±150 BP (1σ)
6810 BC
6510-7110 BC (2σ)

This notation indicates that the actual age of the materials has a 68% confidence of dating between 8910-8610 RCYBP (150 added to and subtracted from 8760). The 6810 BC date is the mean conventional age estimate (it has been converted to BC from BP by subtracting 1950 from 8760). Finally, there is a 95% probability the sample age falls in the 2σ range, in this case expressed in BC. The 2σ range of possible ages is several hundred years larger.

Several pieces of information considered key today were not reported in the 1950s and 1960s. Calibration curves were not yet established when this date was published. Additionally, isotopic fractionation was not always reported. The TX-107 assay was neither corrected for isotopic fractionation nor calibrated when reported in 1965. As the sample was run on charcoal, a reservoir corrected age is not applicable. Previously reported Lower Pecos assays such as this one will be recalibrated, or in this case, calibrated and corrected for isotopic fractionation for what is likely the first time, as part of my thesis.

FootprintsCarbon Footprints

In this section I share my everyday experiences of learning about radiocarbon dating so that the reader can walk in my metaphorical radiocarbon footsteps. My journey began last winter, and was propelled forward by a couple of key experiences.

One of these experiences was being invited by Dr. Raymond Mauldin and his colleagues at the University of Texas at San Antonio’s Center for Archaeological Research (CAR) to assist with a poster for the Society for American Archaeology’s (SAA) 2017 annual meeting. The poster presented an investigation of population patterns in Central Texas and the Lower Pecos Canyonlands, using large radiocarbon data sets from each region and comparing the abundance and distribution of dates through time. To this end, I contributed an initial compilation of 490 radiocarbon dates from the Lower Pecos. The bulk of these, 268 assays, had been assembled by Solveig Turpin and published in the 1991 study she edited Papers on Lower Pecos Prehistory. The remaining data, 222 assays, came from project reports and articles from the 1990s and 2000s, and from the Ancient Southwest Texas Project’s 2010-2017 excavations. The data were then vetted to eliminate dates with large standard deviations, because such dates are too imprecise for the requirements of the study. Data were also divided between samples from open sites (upland and terrace) and those from rockshelter sites, because preservation of organic materials differ enormously between these site types. In April, 2017 I attended the SAAs in Vancouver, BC, to help present the poster, and enjoyed my first SAA conference experience very much.

The other milestone in my Spring 2017 semester was writing and defending my thesis proposal. This was my first opportunity to explore the application of the large Lower Pecos dataset to address archaeological questions. Potential research problems include increased use of earth ovens as a response to environmental change, spatial change in earth oven facilities through time, differential preference for sotol and lechuguilla, the uses of plants associated with earth ovens for non-comestible purposes (e.g., sandals, basketry, cordage), bison presence, and population fluctuations and settlement through time. I suspect that there will be insuffcient data to meaningfully address certain of these topics, in which case I will highlight the need for further research. The Lower Pecos radiocarbon data set I assemble will be made available to other researchers through an online database.

Several steps must be taken to prepare the dataset for analyses, including compiling the data needed, and correcting and calibrating the conventional ages. In addition, the archaeological context, laboratory treatments, and sample material will be critically evaluated to understand how the assay can (or can’t) be applied to addressing the aforementioned topics. In addition, I am rolling up my sleeves at CAR this autumn, where I am learning sample preparation methods from Dr. Mauldin. Some of the samples I will be assaying come from Eagle Cave, thanks to the generous contributions of the crowdfunding campaign! After initial processing at CAR, the samples will go to radiocarbon lab DirectAMS, where I hope to follow the samples through their final carbon measurements.

My present focus is on collecting and assessing contextual data at the Texas Archeological Research Lab (TARL), selecting additional samples from the ASWT excavations at Eagle Cave and Kelley Cave for assaying at CAR, and acing my statistics class so that I can do the analysis next spring. I am also working with the Center for Archaeological Studies at Texas State, where I am rehousing and cataloguing the spectacular Skiles family collection— an opportunity indirectly related to my thesis work, but which increases my knowledge of the material culture of the Lower Pecos, in particular the fiber industries. In the coming months I hope to relate to you my experiences in CAR’s radiocarbon lab. Thanks for your interest in my studies!

A Summary of the 2015 ENC Season

Greetings ASWT blog followers!  We realize we have done a poor job of keeping up with our blog for the past few months, but we wanted to share a few highlights from the 2015 season at Eagle Cave.

As we said at the beginning of the spring session, our focus was on the south wall of the Eagle Cave trench (see ENC Act 2).

The look of the trench changed dramatically as the season progressed - as did the crew.

The look of the trench changed dramatically as the season progressed – as did the crew.  Photo at top from February, photo at bottom from late May.

As is often the case in archaeology, our initial goals for finishing our work on the south wall were a little too ambitious, and we had to modify our plan.  While we had hoped to step and profile the entire trench face, what we did instead was focus on the upper “zone” within Eagle Cave, and intensively sample those strats (leaving the lower deposits for the 2016 season).

These are all 2D orthographic images of the trench generated from SfM.  The scale is the same for all 5 images.

These are all 2D orthographic images of the trench generated from SfM during the 2015 season. The scale and alignment is constant.

Rather than creating a tall, vertical profile across the entire shelter we “stepped” the profile as we excavated.  This gives the profile itself a very unique shape, but follows our motto of “Low Impact, High Resolution.” The stepped profiles and excavation units are more stable, and we only excavated small areas to do as little damage to the site as possible.  What was also striking was how far back the edge of the trench had eroded since the UT excavations in 1963.  Where we encountered intact deposits at the top of the trench was nearly 5 meters (~16 feet) south of the original 1963 trench edge.

All of our units were relatively small in size, and were placed to expose the intact deposits on the south side of the trench, but also do as little harm to the site as possible.

All of our units were relatively small in size, and were placed to expose the intact deposits on the south side of the trench, but also do as little harm to the site as possible.

As we were excavating we realized we had strikingly differential preservation of materials between the dripline (front) and the rear wall of the shelter.  We had expected to find more fiber and plant remains preserved towards the rear wall, but in fact we found the best preservation to be towards the dripline.  In this area we had thousands of fragments of lechuguilla, sotol, and yucca leaves along with innumerable seeds, pieces of wood and sticks, and other plant debris, whereas we encountered mainly ashy and reworked deposits toward the rear.

A nearly complete desiccated lechuguilla plant from the fiber zone in Eagle Cave.  The primary component of these fiber zones are thousands of preserved plant parts like this.

A nearly complete desiccated lechuguilla plant from the fiber zone in Eagle Cave. The primary component of these fiber zones are thousands of preserved plant parts like this.

In the fiber zones we also found some really extraordinary artifacts that archaeologists normally do not get to find: fragments of cordage, knotted fibers, sandals, a basketry fragment, and an atlatl dart foreshaft!

Perishable artifacts recovered from Eagle Cave (clockwise from top right): sandal fragment, foreshaft, cordage, and matting fragment.

Perishable artifacts recovered from Eagle Cave (clockwise from top right): sandal fragment, foreshaft, cordage, and matting fragment.

As fascinating as all the perishable artifacts are, we found something even more important: coprolites!  Emily wrote an excellent blog post on coprolites (see From the Bowels of the Lower Pecos) so I won’t go in to much detail, but we recovered hundreds of fragments of coprolites.  These artifacts will become invaluable as we begin to study how the people used Eagle Cave because they provide a direct link to what the people ate.  A pilot study of Eagle Cave coprolites is being undertaken by Texas A&M Ph.D. student Chase Beck.

Over the course of the spring we recovered hundreds of coprolites (in various sizes, shapes, and colors) from the fiber zone in Eagle Cave.

Over the course of the spring we recovered hundreds of coprolites (in various sizes, shapes, and colors) from the fiber zone in Eagle Cave.

Speaking of what people ate, we also found a variety of well-preserved faunal (animal) remains.  Although most of the bones were from small animals (like rabbits, mice, squirrels, and fish), we did find evidence of larger game like deer, bison, and antelope.

This probable antelope skull fragment was recovered during excavations.

This probable antelope skull fragment was recovered during excavations.

Throughout the spring the core crew (Koenig, Heisinger, Larsen, Pagano, and McCuistion) did an outstanding job of managing the thousands of samples, artifacts, 3D models, photographs, and countless other pieces of archaeological data.  We were fortunate to be able to share some of what we had learned in Eagle with our colleagues when we hosted an Eagle Nest Canyon Research Palaver, May 8-9th.  We had nearly 40 people come tour the site and discuss our methods and findings.

The Core Crew discusses our work in Eagle Cave with the palaver participants.

The Core Crew discusses our work in Eagle Cave with the palaver participants.

In honor of our last field day, the ENC crew dressed in 1970's garb.

In honor of our last field day, the ENC crew dressed in 1970’s garb, echoing our dry-shelter ecological predecessors, the Texas A&M excavators of Hinds Cave (see TBH exhibit.)

The January-May 2015 field season was very successful, and we will be learning more about Eagle Cave as we begin analyses.  We will be returning to Eagle Cave next winter to finish the  south wall.  In the meantime, look out for future posts, including one about the 2015 ENC Field School at Horse Trail Shelter!