EY 693
Project Proposal
Using Bayesian Statistics to Develop Paleoindian Chronologies on the Great Plains,
North America
Lucy Burris
4/06/2007
Project Summary
Accurate and reliable chronologies are important to any study of historic processes.
Accurate chronologies are especially important for answering questions of population
dynamics, resource use, and cultural response to change in the absence of written or oral
records. In this proposal, Bayesian statistical methods will be used to refine the existing
chronologies of the North American Great Plains Paleoindian (13,000 - 8,000 years ago)
period. Within this period at least 16 different cultural complexes have been identified.
Dates of these complexes have been established by ad hoc analysis of the roughly 250
radiocarbon dates available from archaeological sites assigned to this area and time
period. Available site information also includes geographic location, artifact and faunal
assemblage lists, and occasionally paleoenvironmental reconstructions. Bayesian
methods show particular promise for data sets like this since they can be applied to small
data sets, they allow the use of auxiliary information, and they permit reevaluation as new
data is obtained. Application of Bayesian methods is expected to improve the resolution
of the time ranges of the various cultural complexes, to identify where additional dating
information is required, and to potentially, allow reevaluation of site cultural assignment
when available evidence is problematic.
EY693 - Burris Proposal 4/6/2007 Page 1 of 16
Project Description
Many of us are intrigued by our past. We trace our genealogies; we participate in
Civil War re-enactments. We go to museums to find out about King Tut and Lewis and
Clark. We haunt flea markets, thrift stores, and estate auctions to find treasures of the
past. All these activities are reasonably well informed by written records and first hand
accounts. We know with a fair amount of certainty when events occured and when and
where cultures existed and individuals lived. We can build reasonably good chronologies
and spatial maps because we have a lot of information at hand. But as the past becomes
further removed in time few or no written records exist, first hand accounts are lost or
garbled and artifacts become objects without information. One of the goals of
archaeology is to build our knowledge of this lost past, to understand what cultures came
before, how those people lived, and why they vanished. Without written records
chronology building relies upon indirect evidence such as tree-ring and radiocarbon
dating, diagnostic artifacts, and stratigraphy. Using the sparse data available and a
technique called Bayesian statistics, this proposal intends to refine the chronologies of the
ancient past in North America so we can better answer the questions of how early people
lived, where they came from, and why they disappeared.
We know that anatomically modern humans developed in Africa and dispersed
around the globe several hundreds of thousands of years ago. We think that people came
to the Americas only within the last 20,000 years or so. This is subject to intense debate
as just a handful of archaeological sites have been radiocarbon dated to this early date.
Only since 1927 with the discovery of a "Folsom" projectile point embedded in a extinct
bison bone at Folsom, New Mexico, have we known that people have been in the
Americas for more than a few thousand years or so. We now know that a variety of
different cultures - or at least a variety of different tool technologies - occupied the
Americas for several thousand years beginning at the end of the Pleistocene (13,00012,000 years ago). What is less clear is how these cultures (or technologies) relate to each
other through time and space. Were they coincident, were they sequential, did one always
precede another?
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Why do we care about these relationships of the distant past (see for example
Kuzmin and Keates 2005)? Understanding when, how long, and where a culture
developed, fluoresced, and disappeared can tell us how they used the resources around
them and knowing how those resources changed can help us understand how humans
have modified their environments and why a culture may have disappeared (or changed
into a form no longer recognized as the same). If we know cultures developed over a
short period of time over a large spatial area we can suspect that different patterns of
development took place compared to slower development in a smaller area. A good
example is the Clovis culture which produced distinctive projectile points made of high
quality stone with a hafting flute. These points are found across North and northern South
America. At one time, it was thought that a small group of Clovis people arrived from
Asia and through population growth and migration populated the Americas. This seems
reasonable if the culture lasted for one or two thousand years. Recent re-dating of Clovis
sites has suggested that the sites were formed within about a 200 year window (Waters
and Stafford 2007). This sheds doubt on either the small group idea or on the idea of
moderate population growth or on idea of migration. Other explanations for the short
time but wide geographic spread of Clovis include a large initial migration into the
Americas, hyper population growth, or technology transfer to indigenous people rather
than dispersion of Clovis people across the continent. Clearly these alternatives lead to
very different understanding of early cultures.
Having "robust and reliable chronologies" is essential to any investigation of the
past (Buck and Millard 2004). This proposal will revisit the existing chronologies of the
North American Great Plains during the Paleoindian period, roughly 13,000 - 8,000 years
ago (see Appendix A for a full listing of cultures and dates used in this proposal).
Chronology building during this period is important to understanding the peopling of the
Americas both in the variety of cultures initially present and in how they used and
modified the landscapes around them and in understanding why they disappeared.
Building chronologies during this period is difficult due to the scarcity and poor
condition of sites, the scarcity and poor condition of suitable material for radiocarbon
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dating (primarily bone and charcoal but also shell and plant materials), the scarcity of
culturally distinct artifacts (primarily projectile points), and the lack of precise calibration
curves with which to convert radiocarbon dates into calendar dates. Compounding these
problems, many radiocarbon dates that were obtained from sites excavated between 1950
and 1970 are problematic due to field and lab issues (Baillie 1990). In many cases
obtaining new sample material is impossible since the sites were destroyed at the time of
excavation.
The North American Great Plains provides a useful geographic area within which
to test a Bayesian approach to chronology building. In this proposal the Great Plains are
taken to extend from southern Alberta, Canada to the Rio Grande in southern Texas and
from the Rocky Mountains to the tall-grass prairies of Kansas and Nebraska (Figure 1).
Although sites are scarce, relative to other areas of North America they are welldispersed and wide spread, have good preservation, and represent a continuous
occupation from earliest times through the present day. Subsistence patterns during the
Paleoindian period were all nomadic hunting and gathering using stone and bone tool
technologies - a subsistence pattern that leaves a consistent archaeological record
(discounting preservation issues) across time. Sites types generally include short and long
term camps, kill sites, lithic quarries and workshops, burials, and tool caches. Sites in the
Great Plains include the original Folsom site (described below), the late Pleistocene
Clovis site at Blackwater Draw in New Mexico, and the extensive Lindenmeier Folsom
camp site in Colorado. Finally, chronologies have been developed against which the
methods proposed here can be evaluated.
The relatively recent availability of personal computers capable of solving
complex computational problems, the availability of software for Bayesian computation
(for example WinBugs), and recent interest in paleochronologies with the discussion of
"Kennewick Man's" antiquity and connection to modern tribes makes this an opportune
time to apply Bayesian methods to chronology determinations (see Bruning 2006 for a
discussion of the legal complexities of the Kennewick remains).
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Figure 1. Great Plains as defined by Holliday (2001). Numbers indicate major Paleoindian sites in the area.
Background
Since the 1950s archaeologists have used radiocarbon dating to establish ages
(radiocarbon determinations) and associated calendar dates for excavated sites. A more
detailed description of radiocarbon dating is available in Appendix B. Combined with
artifact assemblages, these dates have been used to assign sites to cultural groups. With
enough site assignments cultural chronologies - the earliest start and end dates for a
culture - have been established. Dates have been scarce and a variety of techniques have
been used in the past to distinguish good from bad dates, to average multiple
determinations, and to process dateable material. The scarcity of data has precluded the
definition of precise geographic ranges for cultures and has sometimes caused conflicting
interpretations of cultural assignment, particularly when artifact definitions are not
EY693 - Burris Proposal 4/6/2007 Page 5 of 16
properly applied (see for example Sellet 2001). Cultures which are currently problematic
are the Goshen complex which in some areas seems to be concurrent with Clovis and in
others to post-date Folsom (which in most areas are sequential, see discussion in Frison
1990), the Cody complex which appears across wide areas of the Plains but seems to
have many manifestations, and Midland whose current time frame looks much like
Folsom and whose artifacts look like Folsom projectile points without a diagnostic flute.
Archaeologists are asking if Goshen was really contemporary with Clovis, if Cody is a
single culture or a hybridization, and if Midland is a variant of Folsom. A goal of this
proposal is to attempt to resolve these types of issues in the current chronology.
Radiocarbon dating revolutionized our ability to build absolute chronologies of
cultural groups from material collected in archaeological excavations. Initially, obtaining
dates was an expensive and destructive process, limiting the number of samples
submitted to a minimum. While still destructive and expensive, modern dating techniques
are far more accurate and require only milligrams rather than grams of material. Sample
processing has improved so that issues with marine contamination, bone collagen, and
application of calibration curves have been reduced. Statistical analysis techniques have
also advanced so that the potential exists to use multiple lines of evidence including
stratigraphy, artifact presence, and paleoenvironmental data to refine dating results.
Unfortunately, since archaeology is a destructive process, many sites can not be redated
using modern methods as the sites no longer exist and materials may not have been
preserved. Revising chronologies can require the use of preexisting information, although
it may be problematic.
Several large data sets have been compiled of radiocarbon dates from the North
American Great Plains during the Paleoindian period. Meltzer (2006) has compiled all
the radiocarbon dates for the Folsom site in New Mexico - dates span a collection period
from the 1970s through the 1990s. Waters and Stafford (2007) as already mentioned
have recently evaluated a set of sites that are assigned to the Clovis period and obtained
new radiocarbon dates for questionable sites. Holliday (2000) and Eighmy and LaBelle
(1996) have recently compiled chronologies of the North American Great Plains
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Paleoindian cultures from Clovis to Cody (approximately 8,000 years ago). These
chronologies include uncertainty about culture beginning and end dates, geographic
dispersion, and suggest some inconsistencies where cultures may not always follow the
came sequence in time. All of these compilations rely upon current best practice
(generally ad hoc decisions) to determine the validity of radiocarbon dates to include in
their reporting. Because of the recentness of the Meltzer and Waters and Stafford
evaluations and redating, both offer valuable "truth" data sets due with which to compare
the method proposed here.
The research proposed here intends to refine the work of Holliday (2000) and
Eighmy and LaBelle (1996) by using Bayesian statistics to include/exclude dates based
on geographic location, site stratigraphy, date determination history, and artifact
assemblage composition, to establish a potentially stronger chronology of the area and
period. The data sets of Meltzer (2006) from Folsom and Waters and Stafford (2007)
from the Clovis period will be used as reference data sets to test and develop the method.
The work is also expected to determine the overall validity of the cultural assignments of
both sites to cultures and cultures as meaningful elements in time and space. The work
will also identify specific sites / time frames for which additional radiocarbon
determinations should be obtained. If the Bayesian approach is successful, the
chronology developed by this method can continue to be refined as new dates become
available.
Bayesian statistics has been successfully applied to many problems of radiocarbon
dating, particularly, the calibration of radiocarbon determinations to obtain calendar dates
(Buck et al. 1996). Ox-Cal the primary tool for converting radiocarbon ages to calendar
dates is based on a Bayesian framework and provides a probability range for the calendar
dates of a given radiocarbon age (or set of ranges). Bayesian statistics has been used on a
smaller scale than proposed here to determine an absolute chronology for the JamaCoaque tradition in western Ecuador (Zeidler et al. 1998). The method has also been
successfully applied to the determining the likelihood of a given ship wreck on the Great
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Lakes being a particular vessel based on ship size, propulsion method, cargo, and routing
(O'Shea 2004).
Experimental Design
This effort will be conducted in three phases comprised of two pilot studies and a
final product. The pilot studies will be used to develop and demonstrate the methods in
preparation for the final product. The first pilot study ("Folsom Pilot) will use published
radiocarbon dates from the well-studied Folsom site, New Mexico. In 1927, this site was
the first to explicitly demonstrate the co-existence of humans and Pleistocene fauna in
North America (Meltzer 2006). Primary excavation was conducted in the 1920s with a
revisit to the site in the 1930s to collect material for tree-ring dating which was submitted
for radiocarbon dating to Willard Libby in 1949 (Meltzer 2006:7). These dating attempts
failed and the another visit was conducted in the late 1970s to collect data for
paleoenvironmental reconstruction and new radiocarbon dating. The site was revisited for
a third time in the late 1990s to resolve questions left unanswered by previous work. In
total roughly 50 radiocarbon determinations (both accepted and rejected) from well
documented and ordered stratigraphy have been obtained from the site. This data set will
be used to test for the best form the prior distributions (for example, assuming that dates
are drawn from a uniform distribution on an interval from A to B), to test for the ability
to separate acceptable from reject dates, and to determine the computational tractability
of the problem.
The second pilot study ("Clovis Pilot") will use a recently developed set of
radiocarbon dates from the Clovis period in conjunction with earlier dates for theses sites
(Waters and Stafford 2007; and others). In 2007, Waters and Stafford completed a project
to establish accurate radiocarbon determinations for 30 possible Clovis sites. In addition
to re-evaluating existing determinations, they obtained 16 new determination using stateof-the-art techniques. They were able to exclude some dates from the Clovis period and
to improve the resolution on the Clovis culture's time horizon to roughly 200 years.
Clovis Pilot will endeavor to replicate the Waters and Stafford work and establish both
the date range and the accept/reject determinations for the 150 dates they considered.
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While Folsom Pilot is a "within site and within culture" study, Clovis Pilot is "across
sites and within culture."
The Final Product will use all dates in Holliday (approximately 250 dates [2000])
and Eighmy and LaBelle (1996) plus any new dates (for example those in Meltzer 2006
and Waters and Stafford (2007) that can be obtained for Paleoindian age sites on the
Great Plains including geography, stratigraphy, paleoenvironmental factors and artifact
assemblages. Based on the methods developed during Folsom and Clovis Pilots, this
phase will develop a set of new chronologies based on accept/reject criteria. Finally,
using paleoenvironmental data (if sufficient data is available), the chorology will be
revisited to identify patterns based on landscape use. Compared to Folsom and Clovis
Pilots, the Final Product is "across sites and across culture."
If time permits an additional analysis (Final Product B) will build a culture-free
chronology and then attempt to identify pseudo-cultural boundaries. The data set will be
modified by removing the cultural assignment of all the sites and using the knowledge of
geography, site date, and artifact and faunal assemblage to determine the probability that
a site (or cluster of sites) belong to a given a priori culture or if there are "natural"
groupings of sites which suggests currently unknown cultures. If unknown cultural
groupings emerge, this could suggest that technologies are merely tools and do not carry
as significant a cultural connotation as is currently assumed.
Methods
As described above the proposal will be executed in several stages with the results
of each stage providing a basis for the following stage. A failure to get suitable results in
a given stage will result in a reevaluation of the previous stage results and examination of
the assumptions and theory for the failed stage. Figure 2 provides a sketch of the
proposed process flow. Steps are described in more detail below. Since this is an
investigative proposal, the methods are still being developed and will be refined as new
insight is obtained.
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2.
1.
Explore
Software Tools
Acquire data
Select/learn Tool
3.
Potential
Prior
Distributions
Build Database
Build Models
Increasing
complexity
Final Product / B
Clovis Pilot
Folsom Pilot
Refine/
Validate
4. - 7.
8.
Publish
Results
Figure 2. Conceptual diagram of methodology.
1. Acquire data from sites - location (latitude, longitude, elevation), all
radiocarbon determinations (value, lab date, lab, source material, stratigraphic position,
status), artifact list, paleoenvironmental information, stratigraphic description, faunal
evidence and build database an information database (in Microsoft Access or Excel, GIS
ready).
2. Evaluate potential software tools WinBugs (freeware), Ox-cal (on-line tool),
MSBNx (Microsoft freeware used by O'Shea [2004]), select best, and become proficient
in it. WinBugs is a command line / execution script program which allows many forms of
continuous distributional assumptions, its ability to handle discrete distributions is less
clear. MSBNx, in contrast, handles discrete distributions well (as O'Shea [2004] used it,
he assigned discrete probabilities to the chance of a ship being say wind or steam
powered). The ability of MSBNx to handle continuous distributions is unknown. Ox-Cal
is reported to have the ability to include stratigraphic relationships but whether this
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extends beyond a single site and whether any other type of auxiliary information can be
included is also unknown.
3. Compile a candidate list of potential priors based on literature review, other
examples, and current best practice. Ox-Cal apparently uses a uniform prior and this is
the prior most often suggested in the relevant literature. This may be more due to
convenience than any theoretical basis and this is an area requiring additional research.
4. Build and evaluate Folsom Pilot using uniform prior ("no information"),
modify with other priors from Step 3. Constrain based on stratigraphy relationships.
Goodness of fit test to be determined (Bayesian Information Criterion - BIC, deviance,
etc). Revise model as needed. Compare to results published by Meltzer (2006).
5. Build and evaluate Clovis Pilot using prior model from Folsom Pilot. Constrain
or condition based on diagnostic artifact/faunal presence / absence. Revise model as
needed. Compare results to those published by Waters and Stafford (2007).
6. Build and evaluate Final Product. Constrain based on geography, diagnostic
artifacts, /faunal presence/absence. Use prior from Clovis and Folsom Pilots. Revise
model as needed. Modify model with conditioning on distance to water, elevation,
distance to lithic sources, etc.
7. Build a "culture" free model (Final Product B) to assign a "cultural probability
vector" to sites. (similar to assigning ship names to wrecks on the Great Lakes, O'Shea
[2004])
8. Publish results.
Since this is an investigative proposal, there is no guarantee of success. Potential
pitfalls include the problem that the data and analyses are self-referential and the
computational complexity of the data set. The same data that is used to build the models
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will also be used to test them. Potentially, once a base model is established, the input data
can be partitioned (if there is enough of it) and one part used to build the model and the
remainder used to test. A bootstrap approach may also be valid. The second problem of
computational complexity is, at this point, an unknown. WinBugs for instance uses a
Monte Carlo Markov Chain simulation approach to obtain a solution. The Gibbs sampler
which it uses is fairly efficient and yet in the Final Product, over 250 dates need to be
analyzed. Potential solutions include using a computer with a dual core processor and
possibly using distributed computing methods (running the computation on multiple
machines at once). At least one additional problem may arise and that is during the data
collection phase, the required raw data (radiocarbon determinations rather than calendar
dates) may not be obtainable. Dates discarded prior to publication (those discarded by the
principal investigator) may be impossible to recover. Every effort will be made to acquire
this information, but particularly for older dates, it may simply not be available. In this
case the dates that are available will be used. Potentially a set of "pseudo-dates" can be
constructed to represent lost dates and used to test model robustness.
Expected Results
It is anticipated that the methods described above will provide chronologies
similar to published results for both the Folsom and Clovis Pilots. These results will
provide confidence that the method is sound when applied to a data set without a
supported reference. The Final Product is anticipated to provide a tighter chronological
range for existing cultural partitioning and potentially will suggest that some cultural
partitioning is poorly constructed. If Final Product B is produced, it should be especially
insightful into the validity of current cultural partitioning. The Final Product will clearly
indicate where, if possible, additional radiocarbon determinations should be obtained and
which date ranges have less clarity compared to other date ranges.
Broader Impacts of the Study
The research developed here will be used by archaeologists to establish
appropriate site time ranges and as an integral part of research designs. The chronologies
developed here will become prior information for future refinement of the chronology as
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more dates are obtained (one of the values of a Bayesian approach). Gaps in the
developed chronologies will provide a basis for collecting additional dates to further
refine the timeline where data is scarce. The work will also illustrate benefits to be
obtained by examining old data. Radiocarbon dating is still an expensive process and the
current work will show that old dates that were previously discounted may be
salvageable. At least three publications are anticipated: A Folsom Pilot/Clovis Pilot
demonstration paper, a Final Product - chronology paper, and a use of
paleoenvironmental data to explain cultural disappearance paper. Two additional papers
will depend upon funding and analysis results. If Final Product B demonstrates that there
are a new unknown cultural groups, this will be a publishable result. Depending upon
funding availability, a final paper will use specifically acquired radiocarbon
determinations to fill in gaps in the Final Product chronology. The results and methods
will be made publicly available through the CSU Anthropology Department Paleoindian
web site which is currently under development.
References
Baillie, M. G. L.
1990 Checking Back on Assemblage of Published Radiocarbon Dates. Radiocarbon
32(3):61-366.
Bruning, Susan B.
2006 Complex Legal Legacies: The Native American Graves Protection and
Repatriation Act, Scientific Study, and Kennewick Man. American Antiquity
71(3):501-521.
Buck, Caitlin E., William G. Cavanagh, and Clifford D. Litton
1996 Bayesian Approach to Interpreting Archaeological Data. John Wiley and Sons,
New York.
Buck, Caitlin E., and Andrew R. Millard, editors
2004 Bayesian Chorological Data Interpretation: Where Now? In Tools for
Constructing Chronologies: Crossing Disciplinary Boundaries, edited by Caitlin
E. Buck and Andrew R. Millard, pp. 1-24. Springer, New York.
Eighmy, Jeffrey, and Jason M. LaBelle
1996 Radiocarbon Dating of Twenty-seven Plains Complexes and Phases. Plains
Anthropologist 41:53-69.
Frison, George
EY693 - Burris Proposal 4/6/2007 Page 13 of 16
1990 Clovis-Folsom-Goshen Relationships in the Northern High Plains. In Megafauna
and Man: Discovery of America's Heartland, edited by Larry D. Agenbroad, Jim
I. Mead, and Lisa W. Nelson, pp. 100-108. The Mammoth Hot Springs, South
Dakota, Inc. Scientific Papers, Volume 1, Hot Springs South Dakota.
Holliday, Vance T.
2000 The Evolution of Paleoindian Geochronology and Typology on the Great Plains.
Geoarchaeology 15(3):227-280.
Kuzmin, Yaroslav V., and Susan G. Keates
2005 Dates are not Just Data: Paleolithic Settlement Patterns in Siberia Derived from
Radiocarbon Records. American Antiquity 70(4):773-789.
Meltzer, David J.
2006 Folsom: New Archaeological Investigations of a Classic Paleoindian Bison Kill.
University of California Press, Berkeley.
O'Shea, John M.
2004 The Identification of Shipwreck Sites: A Bayesian Approach. Journal of
Archaeological Science 31:1533-1552.
Renfrew, Colin, and Paul Bahn
1996 Archaeology: Theory, Methods, and Practice, 2nd edition. Thames and Hudson,
New York.
Sellet, Frederic
2001 A Changing Perspective on Paleoindian Chronology and Typology: A View from
the Northwestern Plains. Artic Anthropology 38(2):48-63.
Taylor, R. E., C. Vance Haynes, Jr., and Minze Struiver
1996 Clovis and Folsom Age Estimates: Stratigraphic Context and Radiocarbon
Calibration. Antiquity 70:515-525.
Waters, Michael R., and Thomas W. Stafford, Jr.
2007 Redefining the Age of Clovis: Implications for the Peopling of the Americas.
Science 315:1122-1126.
Zeidler, James A., Caitlin E. Buck, and Clifford D. Litton
1998 Integration of Archaeological Phase Information and Radiocarbon Results from
the Jama River Valley, Ecuador: A Bayesian Approach. Latin American
Antiquity 9(2):160-179.
Budget
No budget is required for the pilot studies as all data and software products are
available in the public domain. It is an expected result of the final product that there will
EY693 - Burris Proposal 4/6/2007 Page 14 of 16
be "holes" or weak spots (time frames with multiple probable cultural assignments or
with few dates) in the chronology. Obtaining and retesting approximately 25 samples is
expected to be able to resolve these gaps. Retesting each sample is expected to cost
~$1500 (including sample procurement, prep, and testing) for a budget need of $37,500.
Appendix A
General Chronology and Culture Groups on the North American Great Plains,
between 13,000- 8,000 years ago (adapted from Holliday 2000).
Cultural Group
Clovis
Folsom
Goshen
Midland
Northern Plains
11,200 - 10,900
10,900 - 10,200
still unclear
still unclear
Plainview
Agate Basin
Dalton
Milnesand / Lubbock Lake
10,000 ± 200
10,500 - 10,000
Southern Plains
11,600 - 11,000
10,900 - 10,100
Eastern Plains
10,000 ± 500
9950
Hell Gap
Alberta /Alberta-Cody
Classic Cody
Firstview
10,000 ± 500
10,200 - 9400
9400 - 8800
Fredrick, Jimmy Allen,
Angostura
Golondrina
St. Mary's Hall
Texas Angostura
9400 - 7800
9400 - 8200
9200 - 8900
9500 - 8500
8800 - 8000
Dates are the start and end times in radiocarbon years before present (where present is
1950 A.D.) If not enough dates are available for a start and end range, a center point with
one standard deviation is given. Because of non-linearities in the radio carbon calibration
curve, a rough rule of thumb for BP dates around 10,000 years is 10,000 BP (radiocarbon
date) ~ 10,000 BC (calendar date) ~ 12,000 calendar years BP. This relationship is less
reliable as dates become younger, note that 0 BP ~ 1950 AD ~ 2000 calendar years BP.
EY693 - Burris Proposal 4/6/2007 Page 15 of 16
Appendix B
Radiocarbon Dating
Radiocarbon dating measures the ratio of carbon 14 (14C) to carbon 12 (12C) in a
sample (Renfrew 1996). When a living organism dies it contains a 14C / 12C ratio that
matches (with some caveats) the environment in which it lived. Over time, the unstable
14
C decays to 12C changing the ratio of these molecules. Knowing the decay rate and
working backward from the current ratio, the time since death as a function of decay rate
(age) can be determined.
While the ratio of molecules is relatively easy to measure using accelerated mass
spectrometry (AMS), determination a calendar date given an age is more difficult. First
the 14C / 12C ratio of the environmental pool is not been constant over time, so instead of
a mathematical relationship between carbon ratio and age, an empirical calibration curve
has been developed based on tree-rings, lake varves, and coral dating. Second, terrestrial
and marine carbon pool ratios appear to be different at given points in time and there may
even be differences in terrestrial pools between North and South America, thus a given
date needs to be conditioned based on the carbon pool or combination of carbon pools
most likely to have contributed to organism stored carbon. Third, prior to AMS dating in
the 1980s, many grams of material were required for dating. To obtain adequate sample
material from some sites, charcoal was collected from several locations with the
possibility that although samples were not contemporary in age they were commingled
for testing resulting in a average date for all the materials. While this problem has been
eliminated with AMS dating which uses only milligrams of material, older dates run this
risk. Forth, extraction of datable carbon from bone material has been problematic and
resulted in incorrect dates (Taylor et al. 1996). Fifth, radiocarbon dating is useful only to
about 40,000 years ago; as samples get older the ability to measure the number of 14C
atoms was problematic prior to AMS dating (which measures the atoms not the decay
rate). Finally, it is possible for cultural materials to be altered by carbon uptake from the
surrounding sediments, say leaching from a nearby coal seam, although this is thought to
be rare and generally detectable.
Calibration curve improvements are particularly important. Although the practice
has been inconsistent, researchers should always publish both the original radiocarbon
determination (giving a BP or rcyBP designation) and the current best calibration dates
(expressed in BC/AD or calibrated yr BP). This way if the calibration curve changes (as
occured in the mid-1990s), new calibration dates can be determined. When only the
calibrated dates are published (which is not uncommon since these are dates that are
comparable in a temporal sense), there is no possibility of reevaluating them in light of
new information. AMS dates for the period from 10,000 - 20,000 years ago, have a
standard deviation of about 100 years.(Kuzmin and Keates 2005).
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