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: email@example.com
By Emily McCuistion
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 (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
- 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)
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.
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!