Recent Developments in Calibration for Archaeological and Environmental Samples

Abstract

The curves recommended for calibrating radiocarbon ( ¹⁴ C) dates into absolute dates have been updated. For calibrating atmospheric samples from the Northern Hemisphere, the new curve is called IntCal20. This is accompanied by associated curves SHCal20 for the Southern Hemisphere, and Marine20 for marine samples. In this “companion article” we discuss advances and developments that have led to improvements in the updated curves and highlight some issues of relevance for the general readership. In particular the dendrochronological based part of the curve has seen a significant increase in data, with single-year resolution for certain time ranges, extending back to 13,910 calBP. Beyond the tree rings, the new curve is based upon an updated combination of marine corals, speleothems, macrofossils, and varved sediments and now reaches back to 55,000 calBP. Alongside these data advances, we have developed a new, bespoke statistical curve construction methodology to allow better incorporation of the diverse constituent records and produce a more robust curve with uncertainties. Combined, these data and methodological advances offer the potential for significant new insight into our past. We discuss some implications for the user, such as the dating of the Santorini eruption and also some consequences of the new curve for Paleolithic archaeology.

Full text

Radiocarbon, Vol 00, Nr 00, 2020, p 1–23 DOI:10.1017/RDC.2020.22
© 2020 by the Arizona Board of Regents on behalf of the University of Arizona. This is an Open Access
article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.
org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium,
provided the original work is properly cited.
RECENT DEVELOPMENTS IN CALIBRATION FOR ARCHAEOLOGICAL
AND ENVIRONMENTAL SAMPLES
J van der Plicht1*•C Bronk Ramsey2•T J Heaton3•E M Scott4•S Talamo5
1Center for Isotope Research, Groningen University, Nijenborgh 6, 9747 AG Groningen, The Netherlands
2School of Archaeology, University of Oxford, 1 South Parks Rd., Oxford OX1 3TG, UK
3School of Mathematics and Statistics, University of Sheffield, Sheffield S3 7RH, UK
4School of Mathematics and Statistics, University of Glasgow, Glasgow G12 8QQ, Scotland
5Department of Chemistry, University of Bologna, Via Selmi 2, I-40126 Bologna, Italy
ABSTRACT.The curves recommended for calibrating radiocarbon (14C) dates into absolute dates have been updated.
For calibrating atmospheric samples from the Northern Hemisphere, the new curve is called IntCal20. This is
accompanied by associated curves SHCal20 for the Southern Hemisphere, and Marine20 for marine samples. In
this “companion article”we discuss advances and developments that have led to improvements in the updated curves
and highlight some issues of relevance for the general readership. In particular the dendrochronological based part of
the curve has seen a significant increase in data, with single-year resolution for certain time ranges, extending back to
13,910 calBP. Beyond the tree rings, the new curve is based upon an updated combination of marine corals,
speleothems, macrofossils, and varved sediments and now reaches back to 55,000 calBP. Alongside these data
advances, we have developed a new, bespoke statistical curve construction methodology to allow better
incorporation of the diverse constituent records and produce a more robust curve with uncertainties. Combined,
these data and methodological advances offer the potential for significant new insight into our past. We discuss
some implications for the user, such as the dating of the Santorini eruption and also some consequences of the new
curve for Paleolithic archaeology.
KEYWORDS: calibration, curve construction, IntCal, Paleolithic, Thera.
INTRODUCTION
The main backbone of radiocarbon (14C) calibration has been and will continue to be tree rings
dated by dendrochronology, for the time periods where this is possible. Thanks to the work of a
large community of scientists in extending our dendrochronological record and generating new 14C
measurements, calibration has come a long way since the original curves constructed decades ago
(Ralph et al. 1973; Suess 1980; Pearson and Stuiver 1986; Stuiver and Pearson 1986). In the
early days, the 14C data were mainly obtained from conventional laboratories specializing in
high-precision dating (defined as 2‰at the time). These were Belfast, Heidelberg, Groningen,
Pretoria, Tucson, and Seattle; these laboratories also organized mutual intercomparisons at
that time (Kromer et al. 1996). Today, with IntCal20, the dendro-based calibration curve
extends back to 13,910 calBP1(11,960 calBC), with further extensions to 55,000 calBP from
other records. Furthermore, large sections of the timescale have been redated using AMS.
Providing a calibration curve beyond our knowledge of dendrochonologically dated tree rings
has proved a significant challenge. For the glacial part of the 14C dating range, the first tentative
“calibration curves”were based on very few observations, hence were highly coarse and lacking in
detail. For example, Vogel and Kronfeld (1997) based their curve on only 20 paired 14CandU/Th
speleothem observations covering the past 50,000 years. However, during the past few decades,
considerable progress has been made. Since the advent of AMS, calibration has been
extended gradually to 55,000 years, i.e. the complete dating range, enabled by small
*Corresponding author. Email: J.van.der.Plicht@rug.nl
1While tree-ring measurements extending further back in time to 14,189 calBP are present in IntCal20, they are used
alongside data from other records to estimate the curve beyond 13,910 calBP.
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samples provided by chronological records other than tree rings. The first dataset spanning
the complete 14C dating range was based on varves, i.e. laminated sediments from Lake
Suigetsu (Kitagawa and van der Plicht 1998) and further datasets from other archives
followed soon thereafter (Beck et al. 20012; Hughen et al. 2004; Fairbanks et al. 2005).
However, these different records were clearly inconsistent over the oldest part of the 14C
dating range, 26–50 ka calBP (van der Plicht 2004) and very considerable uncertainties
remained (Mellars 2006). For this reason, IntCal04 (Reimer et al. 2004) terminated at 26
ka, and no internationally agreed, unified recommendation was made for calibration
beyond that age (NotCal04; van der Plicht et al. 2004). Instead, radiocarbon users who did
wish to calibrate beyond this range were required to select between a wide number of different
calibration curves (Weninger and Jöris 2004; Fairbanks et al. 2005; van Andel 2005; Bronk
Ramsey et al. 2006) based on the various individual datasets, and archives, available. This
situation made direct comparability of published “calibrated”ages between studies difficult
since potentially different calibration curves had been used to calibrate the underlying
radiocarbon ages.
Once the reasons for some of the disparities in the datasets became better understood,
the IntCal09 calibration curve (Reimer et al. 2009) was constructed and released to the
community. This curve was derived from atmospheric and marine datasets (in particular
from the Cariaco basin; Hughen et al. 2006) and corrected for estimated reservoir effects.
The IntCal09 curve represented our best estimate of Northern Hemispheric atmospheric
14C at that time. Nevertheless, disparities still remained between this estimate and the only
true atmospheric data available of Lake Suigetsu (Kitagawa and van der Plicht 2000),
suggesting issues remained to be resolved. Between 2009 and 2013, new records became
available, in particular from speleothems (Hulu Cave; Southon et al. 2012) and laminated
sediments (Lake Suigetsu; Bronk Ramsey et al. 2012). The IntCal13 calibration curve
incorporated the information from all of this new data while retaining the information on
which IntCal09 was based (Reimer et al. 2013).
The new IntCal20 (Reimer et al. 2020 in this issue) is the latest update and revision to the
Northern Hemisphere calibration curve. It uses a new statistical methodology (Heaton
et al. 2020a in this issue) throughout which offers more flexibility in modeling and hence
an improved ability to combine the varied, and unique, constituent records. In the older
time period, it is based upon new insights on various chronologies, most significantly for
the Pleistocene (Cheng et al. 2018); and improved modeling of marine reservoir effects. We
therefore hope IntCal20 moves us further towards resolving the challenges in synthesizing
the various archives over this period. Further, the dendrochronological part of the IntCal20
calibration curve is also significantly improved. For example, the dataset for Kauri wood is
extended during the Late Glacial (Hogg et al. 2016,2020 in this issue). It is noteworthy to
mention that, in total, IntCal13, the previous calibration curve, was based on 7019 raw
measurements; for IntCal20, this number is 12,904.
The work has been revolutionized by measurements using the newly developed MICADAS
AMS machines, which are extremely efficient and deliver high-precision dates (Synal et al.
2007). Advances in measurement efficiency and enhanced precision will continue to
improve the calibration curve further in the coming years (e.g. Balter 2006). Annual
resolution, dendro-dated, tree ring 14C determinations are being produced at high speed,
2For completeness, we note that Beck et al. (2001) has been replaced later by Hoffman et al. (2010).
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instigated by the hunt for so-called Miyake events, which are narrow spikes (subannual) of
increased 14C production (Miyake et al. 2012,2013). In IntCal20, for the first time, these
spike events have been specifically incorporated into the curve construction methodology
better enabling their retention in the final calibration product (see below for more details).
Further, the explosion of annual tree ring data has been incorporated into the curve via the
new construction method which also recognizes that other 14C measurements arise from
multiple years, commonly decadal, blocks of tree rings.
The present and upcoming single year series therefore enable fine-tuning of the calibration
curve for the Holocene and Late Glacial time periods. These high temporal resolution data,
improved accuracy in the statistical construction method, and the new records have shown
that adjustments to IntCal13 are needed. These adjustments may be significant for the
interpretation of major events in the past: e.g. the Minoan Santorini/Thera eruption, a
crucial time marker for archaeology in the second millennium BC.
As a historical note, we observe that the 14C dating method has revolutionized several
disciplines, in particular archaeology. As put forward by Renfrew (1999), the first
revolution was that samples could be dated by a scientific method at all; the second
revolution was that calibration turned 14C dates into absolute (calendar) dates. One may
add that Bayesian analysis is often mentioned as a third revolution because it enables
dating to decades, i.e. the scale of a human lifetime (Bayliss 2009). The calibration curve
lies at the foundation of 14C dating. It is ideally based on dendrochronology, which
provides absolute dates with an annual resolution. Present developments, in particular, a
mass of new dates on tree rings, enable further fine-tuning and corrections, as will be
discussed in the examples below. The shape of the curve modeled through the measured
14C data is also becoming more relevant, as we start on the road towards an annual high-
precision calibration curve.
The above applies to dendrochronological records, which provide atmospheric/terrestrial
calibration. Beyond the presently available tree-ring record (i.e. older than 13,910 calBP),
calibration needs to be performed on data produced using other dating methods. There are
several methods available, each with their own “pros and cons”(see the list in van der Plicht
2000). Major calibration records beyond the tree-ring timescale have become available, such as
the varved sediments in Lake Suigetsu (Bronk Ramsey et al. 2012) and, most recently, Hulu
Cave (Cheng et al. 2018). Remarkable progress has been made in recent years, but new data
and insights frequently require changes in the calibration curve. True absolute dating remains
work in progress.
This article is an update to the previous “companion”paper to IntCal13 (Bronk Ramsey
et al. 2013), highlighting new developments and implications for the radiocarbon user
community. We split our discussion into two sections. First, we present a broad
overview of recent developments in our understanding of radiocarbon data and provide
an intuitive explanation of the updated methodology used in the construction of IntCal20.
Secondly, we provide several detailed and specific illustrations of calibration using the updated
IntCal20 curves as opposed to IntCal13. While being of scientific interest in their own
right, these examples have been chosen to illustrate several of the general features
calibration users should expect to observe with their own calibrations, such as increased
frequency of multi-modality in calibrated ages. In particular, we consider the consequences
for the dating of the Minoan Santorini eruption, the Pleistocene/Holocene transition, and
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the replacement of Neanderthals by early Homo sapiens in the Paleolithic but the emphasis in
this paper is to demonstrate some of the important practical changes arising from the IntCal20
developments.
DEVELOPMENTS IN DATA AND THE CALIBRATION CURVE
Data and Understanding Uncertainties
A key component for reliable radiocarbon calibration is the quantification and modeling of
uncertainty, as well as how we approach data from different laboratories, different trees,
different regions, and different environmental compartments. This is critical both for the
construction of a robust IntCal20 curve and later calibration against it. We use the word
uncertainty rather than error since it more correctly captures the natural variations that we are
concerned with. Simply put every 14C measurement comes with a measure of uncertainty
(estimated by the laboratory) which must be incorporated into the curve fitting and calibration
procedures. The better we can understand and represent this uncertainty the more reliable the
calibration process.
Historically, from radiometric days, the quoted error was provided by the laboratory taking
into account the internal measurement processes only. When an assemblage of dates is then
formed, it frequently becomes apparent that the scatter in the results from the individual
laboratory is greater than had been imagined given the quoted uncertainties on the
individual measurements. This can be tackled in a variety of ways; one common approach
that has historically been used is the error multiplier. E.g. in Stuiver (1982): “Analysis of
the Seattle data sets and comparison with those published by the Belfast, La Jolla, and
Heidelberg laboratories show that the total variability in a radiocarbon age determination
is often larger than that predicted from the quoted errors. Upper limits for the error
multiplier (i.e. the factor with which the quoted error has to be multiplied to obtain the
overall laboratory variability) are estimated at 1.5 for Seattle and Belfast, 1.1 to 1.4 for La
Jolla, and 2.0 for Heidelberg.”
When we bring together data from different laboratories (dating the same samples), or for
example, from the same laboratory measuring different trees (perhaps different species or
grown in different regions) but covering the same time period, we will naturally see an
additional variation, i.e. 14C measurements in rings, with the identical calendar date, will not
be identical. This could be characterized as an additional level of uncertainty or variation. We
need to be careful in considering how we deal with this additional variation since some
components may be systematic (sometimes described as an offset), and some will be
stochastic. We can consider modeling this variation in terms of offsets and/or error
multipliers (ISG 1982,1983).
Characterizing these sources of variation is an essential part of the IntCal20 modeling approach
since they impact both on the smoothness and/or wiggliness of the resulting curve and its
uncertainty envelope. They also have the potential to affect how we calibrate against the
IntCal20 curve, as we discuss later on calibration in the case of potential regional offsets.
One contributing element to quantifying variation is the practice that the 14C community
has adopted of organizing and participating in laboratory inter-comparisons. The general
laboratory inter-comparisons have been open to all laboratories and have used a wide variety
of routinely dated materials (including but not limited to tree-rings) (Scott et al. 2018). The
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information gained from these studies informed the choice of both model on the offsets and the
prior on the level of additional variation incorporated in IntCal20 construction (Heaton et al.
2020a this issue). At the same time, more focal intercomparisons have refined estimation of the
additional variation based solely on tree-rings (Wacker et al. 2020 in this issue) and on much
smaller subsets of laboratories.
The improvements in the new generation of calibration curves are based on a number of related
developments. A significant factor has been the newer generation of AMS machines. These provide
improved precision beyond that routinely obtained in many laboratories and have delivered a
wealth of single tree ring data. In combination, this has allowed some of the previously
invisible structures in the historic level of atmospheric 14C to be revealed that was not apparent
when considering decadal or bidecadal samples. At the same time, more records are being
considered in the creation of the calibration curve. This makes even more important the
appropriate quantification of the sources of uncertainty since, without this, we run the risk of
delivering an unrealistically precise calibration curve. Looking to the future, the calibration
process will have to take account of any additional differences that might reasonably be
expected between the calibration curve itself and measurements on samples which are inherently
different from deciduous wood, and by methods that are also different.
Construction of the IntCal20 Curve
In addition to a database of radiocarbon determinations, each with an independently obtained
calendar age and for which all the various uncertainties have been appropriately quantified, we
also require a reliable approach to bring these measurements together to create the IntCal20
curve. We aim to do this in such a way that we retain all the genuine features of the radiocarbon
record but remove any spurious variation in the raw observations. This is made more complex
since some of the 14C determinations have an uncertainty on their associated calendar age e.g.
objects for which calendar ages are estimated by U/Th dating such as corals and speleothems;
by varve counting or paleoclimate tuning as in the case of marine cores; or in the case of some
floating tree-ring sequences uncertainty in the absolute dendrochronological age.
We achieve this via non-parametric regression whereby we place very few underlying assumptions
about the nature of past 14C beyond assuming a certain level of smoothness over time that allows us
to borrow strength/information from the neighboring 14C determinations (i.e. those of a similar
calendar age) in estimating the curve. In essence, this approach is similar in spirit to a moving
average process.
For IntCal20, we use Bayesian splines with errors-in-variables to implement this regression, a
significant change from the random walk approach used in IntCal09 and IntCal13. We believe
this switch to Bayesian splines provides several benefits allowing us to more accurately
represent many of the unique aspects of the radiocarbon data and to test robustness to data
and model assumptions. Maintaining the Bayesian paradigm in construction still allows us
to incorporate any prior knowledge about the data, or curve, we may have and is
complementary to the Bayesian process of calibration itself. We provide only a brief
intuitive overview of the statistical method here, but see Heaton et al. (2020ainthis
issue) for more detail.
Our splines are special smooth piecewise-cubic curves connected together at what are called
“knots”. These knots lie at specific calendar ages generally selected by the user in advance.
Between any pair of adjacent knots, the curve is a separate cubic but constructed in such a
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way to ensure that, at the knot marking the join with the next section, the cubic pieces not only
link up continuously but the overall spline remains smooth. The more knots in a section, the
more the overall spline can vary in that time period (wiggliness).
When fitting a Bayesian spline, we aim to find an overall curve that goes close to the observed
data, taking into account their uncertainties, but which is not so overly wiggly as to overfit the
data and introduce features that are not truly present. Further, we can adapt curve wiggliness
to the data and our calibration needs by making an appropriate choice of the underlying knots
that we place at specific calendar ages. Choice of placing and number of knots is often
application specific, based on judgment concerning the underlying smoothness of the curve
and the data density, as well as computational efficiency. The more knots that we have in a
particular calendar range, the more detail we can provide in the calibration curve. In time
periods where our underlying data is dense and we wish to identify precise detail in the
curve, down even to an annual level, we place our knots similarly densely. In particular,
we can incorporate Miyake-type events by packing knots around their known times.
Conversely, where our underlying data is sparse and it is not possible to resolve fine-scale
details, we place our knots more sparsely. For the predominantly dendrodated portion of
the curve, extending from approximately 0–14 cal kBP, IntCal20 places over 2000 knots.
Such a selection enables close to annual resolution in the detail of the final curve for the
Holocene while still maintaining computational feasibility in curve construction.
The new curve construction approach also contains developments in its incorporation of
uncertainty. Despite, as described above, the best attempts to quantify all 14C uncertainty
there are still some sources of variation which are difficult for a laboratory to capture.
Potential examples include the variation between local region, genera/species, or growing
season. This would manifest itself in 14C observations from the same calendar year that
are more widely spread (i.e. over-dispersed) than would be expected given their initial,
laboratory quoted, error. In order to take account of such potential factors, and hopefully
ensure we do not provide an overly precise calibration curve, we assess the level of over-
dispersion in the IntCal20 data during curve construction. This over-dispersion gives us a
measure of additional variation seen in 14C determinations when compared to a single
hemispheric curve. We propagate this additional variation through to the final curve in the
form of predictive intervals on the basis that it is equally likely to be present in uncalibrated
data. However, users should be aware that our estimate of over-dispersion in 14C observations
is based upon tree-ring determinations only. Caution should be applied when calibrating other
material where further potential sources of additional variation exist.
For the majority of calibration users, their use of the published IntCal20 will remain as for
IntCal13. However, a few general elements are worthy of further description:
Curve Realizations
Being Bayesian, the construction methodology provides not just a single potential 14C history
but rather a large set of possible histories, each of which we call a realization. The published
IntCal20 curve is a summary of all of these individual plausible histories—the posterior mean
and variance of a large collection of realizations at each calendar age independently. These
curve summaries provide the correct calibrated age for a single individual determination in
a fast and efficient manner.
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However, the realizations give further information on curve covariance that is lost in the
pointwise IntCal20 summary. When calibrating multiple 14C determinations jointly, for example
in wiggle matches or to test the duration of an event, this covariance has the potential to be
used (see, for example, Blackwell and Buck 2008). Moving forward, the IntCal group is looking
at the best way to provide and incorporate this covariance information into calibration for such
users seeking ultimate precision in calibrating multiple dates jointly.
Increased Likelihood of Multimodal Calendar Ages
The increase in the annual detail afforded by IntCal20 is likely to lead to a greater chance that a
calibrated age will have a multimodal posterior where annual fluctuations in 14C levels lead to
two or more possible fits to the curve. This is particularly likely when the curve shows plateaus.
Such an occurrence can be seen in our example discussing calibration of the 14C determinations
relating to the Minoan Santorini (Thera) eruption. Multimodality requires care to be taken
when reporting the calibrated age so as not to report calendar ages when there is little
chance of the object arising e.g. the posterior mean may equate to a trough between the
posterior modes. We therefore recommend the use of highest posterior density (HPD)
regions when reporting posterior age intervals which will split the calibrated age range into
disjoint intervals if needed. Multimodality may also increase complexity in practical
interpretation if an object has two or more significantly different calendar dates. To reduce
the likelihood of multimodal posteriors, we recommend the use of wiggle matching/joint
modeling of multiple dates if possible.
Posterior Calibrated Ages of IntCal20 Data
An additional benefit of the Bayesian approach of IntCal20 is that each datum within the
calibration database for which the calendar age is uncertain is itself calibrated during curve
construction. We are able to provide posterior calibrated ages for all such records, for
example, Lake Suigetsu, the various marine datasets, and the speleothem records.
INTCAL20: ILLUSTRATION OF NEW CURVE ASPECTS
The Younger Dryas and Glacial/Holocene Transition
Over the years, every IntCal update showed progress on dendrochronology and 14C dating for
the Glacial/Holocene boundary (e.g. Kromer et al. 2004; Kaiser et al. 2012). This effort has
required both absolute tree-ring dates and wiggle matching of floating chronologies. Since
IntCal13, significant contributions have been made by Hogg et al. (2016), Capano et al.
(2017), and Reinig et al. (2020 in this issue).
The dendrochronological part of the present calibration curve IntCal20 now extends back to
13,910 calBP, well into the Bølling/Allerød (B/A) climatic zone. Relatively small but significant
changes in the curve are made during the end of the B/A and the onset of the Younger Dryas
(YD) cold phase. Compared with IntCal13, the IntCal20 calibration curve is slightly shifted,
generally by about 50 calBP, in the older (calendar years) direction, i.e. a 14C determination
calibrated against IntCal20 will be given an older calendar age estimate than if it had been
calibrated against IntCal13. The dramatic onset of the YD is characterized by a steep slope in
the calibration curve, corresponding to an increase of about 5% in the atmospheric 14C
content (Δ14C≈50‰) and which signifies the shutdown of the “ocean conveyor”
(Broecker 1997).
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During the Pleistocene/Holocene transition period, strong climatic variations took place, as
well as megafauna extinctions. For a review and references, we refer to Fiedel (2011). The
shift in IntCal20 now moves absolute calendar dates older by about 50 years and in the
direction of the Holocene boundary as observed in Greenland ice, dated at 11,653 ±99 cal
BP (maximum counting error (MCE), equivalent to 2-σ, Rasmussen et al. 2014). Note that
the dates in Table 1 in the latter publication are given in b2k, i.e. calendar ages relative to
2000 AD; this has been taken into account.
An important time anchor for the last glacial boundary is the Laacher See eruption in
Germany. The 14C date for this eruption has been established as 11,060 ±10 BP (1-σ,
Kromer et al. 2004). This corresponds to a calibrated range of 12,850–13,050 calBP (95.4%
confidence or 2-σ) using IntCal13. Note that, due to the plateau in the calibration curve
that exists at the start of the YD, the high precision seen in the 14C14C date (in BP) is not
converted into a precise calendar age estimate. However, we can say that using IntCal20
will move this anchor date more into the B/A, and away from the YD boundary with a
calibrated range of 12,920–13,080 calBP (95.4% confidence or 2-σ, see Figure 1). The 14C
date is calibrated using OxCal v.4.3.2 (Bronk Ramsey 2009b) using the new IntCal20
calibration curve. We have rounded the calibrated result to 10 calBP.
The Minoan Santorini (Thera) Eruption
For calibration purposes, chronological anchor points provide crucial tests. A case in point of
major importance is the catastrophic Minoan eruption of the Santorini/Thera volcano in the
second millennium BC, a crucial anchor for Bronze Age prehistory. The precise date of the
eruption has been debated for decades. Using a Greenland ice core chronology, the Thera
eruption was originally thought to date to around 1645 BC based upon volcanic tephra
found in the core. However, a recent and timely analysis shows that these volcanic horizons
are more likely to be the result of eruptions in Alaska rather than Thera (McAneney and
Baillie 2019). 14C dating obviously plays a major role in this discussion. The debate has
Figure 1 Calibrating the Laacher See tephra horizon.
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been and still is that 14C shows older dates than archaeological dating of the eruption, up to
more than a century. For a recent overview of this debate, see Antiquity (2014).
Summarized, the 14C date of the eruption can be taken as 3350 ±10 BP (1-σ), which is an
average of many dates from key sites like Palaikastro and Akrotiri (Bronk Ramsey et al.
2004; Bruins et al. 2008). This number (the 14C date) is confirmed by other analyses
(Höflmayer 2012) and consistent with other records like tsunami deposits on Crete (Bruins
et al. 2008). Calibrating this 14C date with calibration curves prior to the present IntCal20
curve yields a calendar date of the event in the late 17th century BC, most notably by
wiggle matched 14C dates of tree rings from an olive tree killed by the eruption. This
resulted in a date of 1627–1600 BC for the event (Friedrich et al. 2006), between 100–150
years older than previous traditional archaeological assessments.
This difference between archaeology and 14C has spawned debates lasting decades (Kutschera
et al. 2012; Antiquity 2014; Manning 2014; Bruins and van der Plicht 2017). Thus far, the
debate has been largely focused on possible errors on either side, 14C, or archaeology.
However, there is also the option that both are correct(oratleastnotvery wrong), rather that
there is a problem with the connection between the two: that is the calibration, which translates
BP dates into calendar ages.
Very recently, new insights on the matter have been revealed. First, the validity of olive trees for
wiggle match dating was questioned (Ehrlich et al. 2018). This is not further discussed here.
Second, a new single year calibration curve for the Minoan Santorini time range became
available, showing the possibility of an early 16th century BC date for the eruption (Pearson
et al. 2018). Note that a small wiggle in the calibration curve around 1570 BC (3520 calBP) is
just missed by previously released curves. If a 14C age happens to coincide with this wiggle
then it opens up an additional potential calendar age fit at this time and hence increases the
probability of a younger (more recent) calendar date for the eruption.
This development led to major 14C (re)dating efforts of wood dated by dendrochronology for
the relevant time range. Several laboratories measured single year rings with ultimate precision
during the construction time of IntCal20. The datasets are reported in this issue (Friedrich et al.
2020; Kuitems et al. 2020). As a result, the “Thera time-range”presently has the best-dated
calibration curve, with over 800 high-precision measurements on dendrochronologically
dated wood between 1700 and 1500 calBC, obtained by several independent AMS
laboratories.
The result is that indeed between ca. 3600 and 3500 calBP the calibration curve needs a shift of
about 20 BP upwards in 14C age, as can be seen in Figure 2. By itself, this confirms the original
observation by Pearson et al. (2018) and so, after calibration, the calendar dates will, therefore,
become younger by a certain amount. What does this mean in practice for the calendar date of
the Minoan Santorini/Thera eruption? We will illustrate this as follows:
First, we calibrate the well-published averaged date 3350 ±10 BP (1-σ) using both curves; see
Figure 3. Note that the calibrated dates are shown here in BC.
This example highlights that near plateaus in the calibration curve, or during periods
of significant wiggles, it is highly likely that calibrated age estimates arising from single
14C determinations will exhibit significant multimodality. In the presence of such
multimodality, practical interpretation of dating is more complex and it is critical that
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care is taken to provide such interpretation correctly. Reporting single 68.2% (1-σ), or 95.4% (2-σ),
intervals will typically be inappropriate and we instead recommend considering HPD regions.
With IntCal13, the posterior calendar age estimate is approximately unimodal (i.e. shows a
single large peak). In such an instance, it is reasonable to report a single interval—here we
obtain a 68.2% (1-σ) interval extending from 1658–1624 calBC; and a 95.4% (2-σ) interval
of 1683–1617 calBC.
However, with IntCal20 the picture is much more complex as our 14C date of 3350 ±10 BP hits
the plateau in the curve. There are now multiple, disjoint, calendar age ranges consistent with
this 14C date. A single interval is not therefore sufficient to summarize these possibilities.
Instead, we see that there are perhaps five separate calendar age regions each with
significant probabilities attached. In providing interpretation, we would suggest reporting
all these regions with their associated probabilities.
If, for IntCal20, we want to provide the equivalent of a 2-σinterval (i.e. the smallest set of
calendar ages which contains the true age with a probability of 95.4%), then should report
the HPD region. This consists of all the intervals quoted in the 95.4% OxCal summary i.e.
1733–1719, 1688–1651, 1645–1608, 1604–1602, 1583–1559, and 1556–1544 calBC. The
individual probabilities for these ranges are 3.5, 22.1, 53.0, 0.3, 11.4, and 5% respectively.
The latter two, representing calendar dates in the 16th century BC, have a combined
probability of 16.4%. A more recent date is, therefore, a distinct possibility, although we
note that the peak centered around 1625 calBC carries the largest individual probability.
We also note that a further small peak at an older age of ca. 1665 calBC appears, introduced
by the refined revision, and increased wiggliness, of the new curve.
Figure 2 The IntCal20 (red) and IntCal13 (blue) calibration curves for the
time range relevant to the Thera eruption. The thick lines represent the
posterior mean of each curve; the thin lines represent the 1-σcredible/
predictive interval. (Please see electronic version for color figures.)
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We further calculated the probability of a calendar date more recent than 1600 calBC
(equivalently 3550 calBP) by summing the individual posterior probabilities of all calendar
ages in that period. This provided an estimated probability of 19.3% for a calendar date in
the 16th century BC.
Second, we reanalyze the wiggle match date of the olive branch published by Friedrich et al.
(2006). In the original publication, the actual calibration curve at the time was IntCal04.
Subsequent calibration curves (IntCal09, IntCal13) did not significantly change the results.
What will change using IntCal20?
Figure 3 Calibration of the averaged Thera date 3350 ±10 BP, using IntCal20
(top) and IntCal13 (bottom).
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The results of wiggle matching the olive tree dates are shown in Figure 4. For completeness we
give here the four 14C dates of Friedrich et al. (2006), measured by radiometry in Heidelberg:
rings 1–13, 3383 ±11 BP; rings 14–37, 3372 ±12 BP; rings 38–59, 3349 ±12 BP; rings 60–72,
3331 ±10 BP (all at 1-σ).
For IntCal20, we see that through wiggle matching, multimodality in the calendar age
estimate is reduced. This is typically to be expected since, when wiggle matching, we
can use the shape of the curve in addition to its absolute value. The resulting calendar
dates for the last ring are 1612–1592 calBC (68.2% confidence) and 1617–1578 calBC
(95.4% confidence). For IntCal13, this is 1619–1608 calBC (68.2% confidence) and
1625–1604 calBC (95.4% confidence).
However, even here, we need to be somewhat careful with our interpretation as the IntCal20
estimate still retains two distinct peaks suggesting the two most likely periods for the last ring
are either ca. 1605 calBC (3555 calBP) or ca. 1595 calBC (3545 calBP). Both these IntCal20-
based potential calendar dates are more recent than the calendar age estimate obtained using
IntCal13 (or earlier curves) by about 5–15 years showing the effect of the calibration curve
change assuming the validity of the olive wood wiggle match.
Figure 4 Dating the Minoan Thera eruption (Friedrich et al. 2006) by wiggle matching rings from an olive
tree branch, using the calibration curves IntCal20 (top) and IntCal13 (bottom).
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Summarizing, for the Minoan Santorini/Thera eruption, we have with IntCal20 a relatively
small revision to the curve itself which nonetheless has a significant impact not only in the
calibrated ages it provides but also in how those age estimates may need to be reported
and interpreted. Until IntCal20, the calibration curve in this time range was based on
relatively sparse data (conventional dates measured decades ago, with relatively large
temporal resolution). Now we have obtained by way of an unprecedented effort a firm
establishment of the calibration curve during one of the most important prehistoric
chronological anchorpoints.
However, in terms of establishing the exact date of Thera, the new calibration curve interestingly
still leaves questions unanswered due to the presence of the plateau in IntCal20 during the contested
time period. To gain more precise insight into the timing using 14C, modeling of multiple 14Cdates
will likely be needed. The 14C dates are calibrated using OxCal v.4.3.2 (Bronk Ramsey 2009b) with
a 1-year resolution.
The Middle and Upper Paleolithic Period
Chronological studies suggest that Neanderthals disappeared approximately between 39 and
41 ka ago, which implies that they overlapped with Archaic Homo Sapiens for at least 2600
years and up to 5400 years (Higham et al. 2014). Additional evidence for the co-existence
of these two human species is represented by DNA data obtained from the analysis of an
Archaic Homo Sapiens (37–42 ka calBP) from Romania, showing that between 6 and 9%
of the genomic sequence of this individual was derived from Neanderthals (Fu et al. 2015).
Despite the present state of the research, there remains still much to understand on the
chronology of the Middle-to-Upper Paleolithic transition, i.e. on the period during which
Archaic Homo Sapiens replaced Neanderthals in Eurasia, in order to construct a fairly
convincing outline sketch of human prehistory and of the context in which it played out.
Here, very precise and accurate calibration of 14C ages, back to ca. 50,000 years ago, is critical
to limit archaeological speculations, developing solid chronologies of paleoenvironmental
change and a more detailed understanding of the succession of climatic events through the last
glacial period. In this paper, we will show what implications the new IntCal20 has for the
understanding of the relationship between these two fascinating species using some of the most
striking direct dates of human fossils in Eurasia. All the 14C dates are calibrated using OxCal
v.4.3.2 (Bronk Ramsey 2009b), and using both IntCal13 and the new IntCal20 calibration
curve (Figure 5). The Figure 5also shows part of the Greenland ice chronology (Rasmussen
et al. 2014). There are scale differences, but at this time range, the offsets are not significant
beyond the 1-σlevel. See Adolphi et al. (2018) and also Muscheler et al. (2020 in this issue).
All the results are rounded by 10 years.
Oase 1 and Ust-Ishim
AHomo sapiens mandible (Oase 1) found in Romania (Peştera cu Oase) has been directly dated
in the Oxford and Groningen laboratories yielding a mean age of 34,950 ±990 BP (1-σ,
Trinkaus et al. 2003). The IntCal13 calibration places the Oase 1 individual at 40,670–
38,520 calBP at 68.2% confidence, and at 41,770–37,310 calBP at 95.4% confidence.
This calibration provides evidence for an early Homo sapiens in Europe. Moreover, the genetic
study found out that the Neanderthal-like DNA shares more alleles (between 6–9%) with the
Oase 1 individual than it does with any present-day humans in Eurasia. They also estimated
how far in time this introgression happened. They concluded that the Neanderthal contribution
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Figure 5 The calibrated 14C ages using IntCal13 of Oase 1 and Ust’-Ishim are shown in grey, and Arcy-
sur-Cure and Saint Césaire in green on top. The corresponding calendar age intervals using IntCal20 are
shown at the bottom. The results are linked with the (NGRIP) δ18O climate record. The numbers from 12 to
8 represent the warm Dansgaard-Oeschger (DO events 12 to 8), and one cold Heinrich Event (H4).
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to the Oase 1 individual occurred recently in his family tree, four to six generations back, which
means the Neanderthal admixture is dated less than 200 years before the time Oase 1 lived
(Fu et al. 2015).
The recalibration with IntCal20 provides a new time span for this important human fossil,
placing his calendar age back in time, between 41,180–39,190 calBP at 68.2% confidence,
and at 41,900–37,700 calBP at 95.4% confidence (see Figure 5). This shift to older ages of
the presence of Homo sapiens at the site implies a longer overlap with Neanderthals in this
region.
Another directly dated key human fossil was found in Siberia (Ust’-Ishim). In this case, it is the
oldest Homo sapiens so far found in Eurasia (Fu et al. 2014). This individual carries a similar
amount of Neanderthal DNA ancestry as present-day Eurasians. In this case, the Neanderthals
gene flow occurred 7000 to 13,000 years before Ust’-Ishim lived. Here two dates were made in
Oxford using the ultrafiltration method: OxA-25516 with a 14C Age of 41,400 ±1300 BP (1-σ)
and the second OxA-30190 with a 14C Age of 41,400 ±1400 BP (1-σ). Applying R_Combine in
OxCal we obtain an age of 41,400 ±953 (1-σ). Calibrating against IntCal13, this corresponds
to a calendar age of 45,770–44,010 calBP at 68.2% and 46,880–43,210 calBP at 95.4% (see
Figure 5). With this result, the authors concluded that: interestingly the Ust’-Ishim
individual probably lived during a warm Dansgaard-Oeschger period (DO 12) (Fu et al.
2014), which has been proposed to be a time of expansion of Homo sapiens into Europe
(Müller et al. 2011; Hublin 2012).
With the new calibration curve IntCal20, these ranges changes to 44,970–43,340 calBP at
68.2% and 45,950–42,890 calBP at 95.4%. This shift to younger ages puts the Ust'-Ishim
individual closer or even into the stadial following DO 12 (see Figure 5).
The Ch ˆ
atelperronian
Another two, directly dated, important hominins are Neanderthals from France. The first is the
Neanderthal fossil of Saint Césaire (La Roche-à-Pierrot) situated in the Charente-Maritime
department of southwestern France (Lévêque and Vandermeersch 1980). The second one
comes from Grotte du Renne (Arcy-sur-Cure) located on the main road between Auxerre
and Avallon, close to Paris (David et al. 2001). Both of them are crucial to understanding
the replacement processes of Neanderthals by Homo sapiens and the interpretation of so-
called “transitional industries”, here the Chˆatelperronian.
The question of whether Neanderthals manufactured the Chˆatelperronian is the topic of
intense debate since the Chˆatelperronian industry represents a new cultural behavior. The
debated question is if the Chˆatelperronian new behavior demonstrates a cultural influence,
on the last Neanderthals, by contemporaneous Archaic Homo Sapiens populations, already
present further east in Europe, or if it represents a Neanderthal invention. Here, direct 14C
dates of hominins together with a precise calibration curve play a pivotal role.
The Saint Césaire bone was pretreated at the Max Planck Institue, Leipzig, Germany, and
graphitized and dated in Oxford to a 14C age of 36,200 ±750 (OxA-18099, 1-σ) (Hublin et
al. 2012). The Arcy-sur-Cure bone was pretreated at the Max Planck and graphitized and
dated in Mannheim, Germany to a 14C age 36,840 ±660 (MAMS-25149, 1-σ) (Welker
et al. 2016). The respective calibrated ages using IntCal13 are 41,550–40,110 calBP at
68.2% and 42,150–39,340 calBP at 95.4% for Saint Césaire, and 41,980–40,840 calBP
at 68.2% and 42,430–40,180 calBP at 95.4% for Arcy-sur-Cure (Figure 5).
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IntCal20 provides a new calibration for Saint Césaire between 41,860–40,690 calBP at
68.2% and 42,200–39,940 calBP at 95.4%. For Arcy-sur-Cure the ranges are between
42,080–41,270 calBP at 68.2% and 42,370–40,780 calBP (Figure 5).
Conclusion
These examples demonstrate that for some intervals, here 40 ka calBP vs. 45 ka calBP, the new
calibration curve will result in narrower ranges for calibrated ages i.e. more precise dating than
the previous IntCal13. The more detailed IntCal20 calibration curve will, therefore, allow
higher precision for the study of human evolution in terms of chronological overlap
between archaeological sites and different human species and will provide better resolution
in relation to climatic events.
Regional and Seasonal Effects
A full discussion of possible regional effects with associated references is given in the main
IntCal20 paper (Reimer et al. 2020 in this issue). The conclusion drawn there is that there
is as yet insufficient information to quantify such effects or indeed to fully understand the
contribution from different underlying mechanisms. The potential issues are: different
growing seasons for the material dated compared to the calibration datasets, localized
addition of CO
2
from different reservoirs (such as ocean upwelling, anthropogenic sources,
and local volcanic vents), mixture of air masses from both hemispheres in the tropics
(Hogg et al. 2020 in this issue), and overall trends with latitude or altitude. Some of these
effects are potentially related because the known subannual cycle in 14C (Appenzeller et al.
1996; Stohl et al. 2003) will necessarily lead to differences in the 14C signal seen in different
species and different latitudes. Furthermore, even the same species may grow differently
due to circumstances such as water availability and other localized stresses. Fortunately,
with the exception of the major addition of carbon from other reservoirs, most seasonal or
regional effects are likely to be less than 20 14C years or 2.5‰and so comparable to the
measurement precision.
In considering the practical consequences of such regional and seasonal effects, statistically, we
can treat measurements which are systematically offset irrespective of the underlying cause. This
is particularly important where complex Bayesian models or wiggle-matching of tree ring data
is being undertaken to achieve precisions approaching the decadal level. In these cases, it is
prudent to build into the model the possibility that all of the 14C dates might be
systematically offset by some small amount, whether this is due to an offset resulting from
the measurement methodology, or within the sample itself relative to the hemispherical
average atmosphere. Particularly in the case of wiggle-matched tree ring data, the
measurement series itself will often provide sufficient evidence for whether there is such an
offset. In general, it is better to provide a prior which reflects our expectation that such offsets
will be small (for example, N(μ,σ2) ~ N(0,102)). This can be achieved, for example, in OxCal
(Bronk Ramsey 2009a) by using the Delta_R methodology normally applied to marine
samples (as was used as a sensitivity test in Bronk Ramsey et al. 2010). The code for this is:
Delta_R(“offset”,0,10);
or
Delta_R(“offset”,N(0,10));
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Such an approach can also be used for single calibrations by adding in additional error in
quadrature to reflect such uncertainty. However, before doing this, one should know whether
this uncertainty has already been included within the error quoted by the laboratory. The
overall effect on large datasets is likely to be much more significant in that any offsets are
potentially correlated across the dataset as a whole, something which the lab cannot
quantify (unless it is purely a measurement issue). It is important to remember that there is
ultimately an irreducible uncertainty in our chronologies, irrespective of the number of
measurements we make.
If one wishes to use the data to determine such an offset with no bias, a uniform prior (for
example ~U(-20,20)) for the offset can be used, and the marginal posterior for the offset
determined as part of the analysis. The OxCal code for this would be:
Delta_R(“offset”,U(-20,20));
The treatment of samples that come from close to the ITCZ is slightly different. Here we know
the direction of the offset which we expect to see from whichever hemisphere we think is
dominant. In such cases, it would be possible to use a mixed reservoir model, where we
assume that the 14C could be drawn from a mixture of the northern and southern
hemisphere atmospheric reservoirs. An alternative is to use the same approach as above but
with a biased offset. In practice, for example, using the Northern Hemisphere curve with
an offset of ~U(0,40) will give a similar result to using the Southern Hemisphere curve with
an offset of ~U(-40,0). The difficulty in doing this is putting a more constrained prior on
the expected mixture, which will be very location dependent; the above uniform
distributions are very conservative but obviously add in significant extra uncertainty unless
the dataset includes some samples of known age which will constrain the marginal
posterior for the offset. Such situations are potentially very complex in that different samples,
particularly if from different species, might pick up different signals even in the same location
because of growing season changes in the ITCZ.
The Marine Curve
The slow diffusion of CO
2
into and out of the surface ocean combined with the very slow
circulation of the deep ocean results in a 14C age offset or reservoir effect (R(t)) between
samples that lived on land and those that lived at the same time in the ocean. The present day
value of this offset is of the order of approximately 400 14C yr; however, R(t) varies regionally
and is time dependent. For sample 14C calibration of marine-based determinations, the regional
variation has usually been handled by including a regional correction to the marine calibration
curve called ΔR but the time dependence of both R and ΔR has made marine calibration
more uncertain than for terrestrial samples.
The Marine20 curve (Heaton et al. 2020b in this issue) includes a global R(t) correction based
on the BICYCLE box model (Köhler and Fischer 2004,2006; Köhler et al. 2005,2006) forced
by our IntCal20 estimate of atmospheric 14C along with several other paleoclimate records.
While more complex ocean general circulation models (OGCMs) are available (e.g. Butzin
et al. 2017,2020 in this issue) which also provide spatial estimates for the marine 14C
reservoir, most archaeological marine samples derive from near coastal environments which
are not well characterized by the grid size of such OGCMs. Consequently, the use of ΔR
with Marine20 can still be advised for these samples. Pre-bomb ΔR values have been
recalculated with Marine20 and are available at www.calib.org/marine20. Researchers
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calibrating marine 14C ages from the open ocean sites may want to access the LSG OGCM
model results (Butzin et al. 2020 in this issue) within the grid closest to the sample site
available at https://doi.pangaea.de/. This is not discussed further here, for more details we
refer to Heaton et al. (2020b in this issue).
NOTE ON TIMESCALES
For completeness, we summarize here the units and definitions for timescales that are relevant
for 14C dating.
BP
Defined unit for the 14C timescale. Measured activities are to be reported following a
convention. The definition concerns the usage of the halflife value, fractionation correction,
and standard activity. For details, we refer to the literature (Stuiver and Polach 1977;
Mook and van der Plicht 1999).
AD
Anno Domini, calendar date as commonly used in western society.
AD is the same as CE (Common Era) which is mostly used in Near Eastern contexts.
calAD
Calendar date, obtained after calibration of a 14C date (expressed in BP).
BC
Calendar date before (the birth of) Christ, which is traditionally placed in 1 AD. BC is the same
as BCE (Before Common Era).
The year “0”does not exist in the traditional calendar; the year 1 BC precedes 1 AD.
calBC
Calendar date, obtained after calibration of a 14C date (expressed in BP).
calBP
Calibrated 14C date, relative to the standard year 1950 AD.
Thus, calBP = 1950 –calAD = 1949 + calBC.
Note one exception to the above: modeled dates are given as BP. After applying Bayesian
analysis of a series of 14C dates only, this equals calBP. The latter is no longer the case
when also dates resulting from other methods are included.
b2k
Calendar date relative to 2000 AD. This unit is not to be used for 14C dating. We mention it
here because it has recently been introduced by the ice core community to avoid confusion with
“our”calBP (Rasmussen et al. 2014). The units BP and calBP are both used in e.g. Earth and
Environmental sciences when dating methods other than 14C are used. However, generally,
they have a different meaning than the 14C definitions given above. We should be aware
that BP and calBP are not “owned”by the radiocarbon community.
CONCLUSIONS
The calibration curve to be used for calibration of 14C dates has been updated. It is presently
called IntCal20, to be used for terrestrial samples from the Northern Hemisphere. It replaces
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the previous curve IntCal13. Additional calibration curves associated with IntCal20 are
Marine20 for samples from marine reservoirs, and SHCal20 for the Southern Hemisphere.
In this companion article, we have summarized the present status of calibration and discuss
some highlights. The full background of curve construction is briefly summarized in order
to provide users with an understanding of the method used. Details and reference to
datasets are given in the main papers in this issue.
As before, the calibration curve can be considered as based on two kinds of records:
dendrochronology and others. The tree ring part now extends to 13,910 calBP, well into
the Late Glacial period. Here the main improvement since IntCal13 is the temporal
resolution. For significant parts of (pre)history, single-year datasets have become available.
This is spawned by the hunt for “Miyake events”, sudden increases in the natural 14C
content of short duration, observed in tree rings. Equally important, the newest MICADAS
AMS machines are very efficient, enabling the acquisition of an unprecedented amount of
tree-ring dates with high resolution. This has revolutionized the field, including calibration.
A key illustration concerns the dating of the Minoan Santorini/Thera eruption. This is a crucial
anchor point for prehistory in the 2nd millennium BC. For decades, the absolute date of the
eruption has been heavily debated by 14C experts and traditional archaeological thinking.
During the last few years, many high-resolution tree-ring measurements have been
performed by various laboratories, which are now all integrated into IntCal20. The revised
curve in the relevant time period is now annually resolved and shifted by a relatively small
but crucial number of years (ca. 20 BP). As a consequence of this shift, there is a plateau in
the calibration curve with detailed annual fluctuations that provide multiple potential fits
with 14C dates from the eruption. This has an impact not just in shifting calendar age
estimates more recent, but also results in multimodal calendar age estimates that require
significant care in interpretation. A much younger date—ca. 1500 BC, as advertised by
some archaeologists—remains extremely unlikely, based on 14C.
For the Glacial part of the curve, beyond the tree ring record of 13,910 calBP, the main datasets
originate from corals, speleothems, and the laminated sediment of Lake Suigetsu. The new
IntCal20 integrated curve provides calibration back to 50,000 BP, which corresponds to
55,000 calBP. This part of the curve is applied to critical real-life chronological issues in
Paleolithic archaeology. We discuss the absolute dates for the key Paleolithic sites Pestera Cu
Oase (Romania), Ust’-Ishim (Siberia), Arcy-sur-Cure, and Saint Cesaire (France). We show
that the more detailed IntCal20 calibration curve allows better precision for the study of
human evolution in terms of chronological overlap between the presence of Homo sapiens
and Neanderthals. It also provides better resolution in relation to climatic events.
ACKNOWLEDGMENTS
CBR is funded by NERC as part of their 14C facility. TJH is funded by a Leverhulme Trust
Fellowship RF-2019-140\9, “Improving the Measurement of Time Using Radiocarbon”.STis
funded by the European Research Council under the European Union’s Horizon 2020
Research and Innovation Programme (grant agreement No. 803147-RESOLUTION).
Discussions with M.W. Dee have been instrumental.
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REFERENCES
Adolphi F, Bronk Ramsey C, Erhardt T, Lawrence
Edwards R, Cheng H, Turney CSM, Cooper A,
Svensson A, Rasmussen SO, Fischer H, et al.
2018. Connecting the Greenland ice-core and
U/Th timescales via cosmogenic radionuclides:
Testing the synchronicity of Dansgaard-Oeschger
events. Climate of the Past 14:1755–1781.
Antiquity. 2014. Debate feature: Bronze Age
catastrophe and modern controversy: dating the
Santorini eruption. Antiquity 88:267–291.
Appenzeller C, Holton JR, Rosenlof KH. 1996.
Seasonal variation of mass transport across the
tropopause. Journal of Geophysical Research—
Atmospheres 101(D10):15071–15078.
Balter M. 2006. Radiocarbon dating’s final frontier.
Science 313:1560–1563.
Bayliss A. 2009. Rolling out revolution: using Radio-
carbon dating in Archaeology. Radiocarbon
51:123–47.
Beck JW, Richards DA, Edwards RL, Silverman BW,
Smart PL, Donahue DJ, Hererra-Osterheld S,
Burr GS, Calsoyas L, Jull AJT, et al. 2001.
Extremely large variations of atmospheric 14C
concentrations during the last glacial period.
Science 292:2453–2458.
Blackwell PG, Buck CE. 2008. Estimating radiocarbon
calibration curves. Bayesian Analysis 3:225–442.
Broecker WS. 1997. Thermohaline circulation, the
Achilles heel of our climate system: will man-
made CO
2
upset the current balance? Science
278:1582–1588.
Bronk Ramsey C, Manning SW, Galimberti M. 2004.
Dating the volcanic eruption at Thera. Radiocarbon
46:325–344.
Bronk Ramsey C, Buck CE, Manning SW, Reimer P,
van der Plicht J. 2006. Developments in radiocarbon
calibration for archaeology. Antiquity 80:783–798.
Bronk Ramsey C. 2009a. Dealing with outliers and
offsets in radiocarbon dating. Radiocarbon 51:
1023–1045.
Bronk Ramsey C. 2009b. Bayesian analysis of
radiocarbon dates. Radiocarbon 51:337–360.
Bronk Ramsey C, Dee MW, Rowland JM,
Higham TFG, Harris SA, Brock F, Quiles A,
Wild EM, Marcus ES, Shortland AJ. 2010.
Radiocarbon-based chronology for Dynastic
Egypt. Science 328:1554–1557.
Bronk Ramsey C, Staff RA, Bryant CL, Brock F,
Kitagawa H, van der Plicht J, Schlolaut G,
Marshall MH, Brauer A, Lamb HF, et al. 2012.
A complete terrestrial radiocarbon record for 11.2
to 52.8 kyr BP. Science 338:370–374.
Bronk Ramsey C, Scott EM, van der Plicht J. 2013.
Calibration for archaeological and environmental
samples in the timerange 26-50 ka cal BP.
Radiocarbon 55:2021–2027.
Bruins HJ, MacGillivray JA, Synolakis CE,
Benjamini C, Keller J, Kisch HJ, Klugel A, van
der Plicht J. 2008. Geoarchaeological tsunami
deposits at Palaikastro (Crete) and the Late
Minoan IA eruption of Santorini. Journal of
Archaeological Science 35:191–212
Bruins HJ, van der Plicht J. 2017. The Minoan
Santorini explosion and its 14Cpositionin
archaeological strata: preliminary comparison
between Ashkelon and Tell el-Dab’a. Radiocarbon
59:1295–1307.
Butzin M, Köhler P, Lohmann G. 2017. Marine
radiocarbon reservoir age simulations for the
past 50,000 years. Geophysical Research Letters
44:8473–8480.
Butzin M, Heaton TJ, Köhler P, Lohmann G. 2020.
A short note on marine reservoir age simulations
used in IntCal20. Radiocarbon 62. This issue.
Capano M, Miramont C, Guibal F, Kromer B, Tuna T,
Fagault Y, Bard E. 2017. Wood 14C dating with
AixMICADAS: Methods and application to tree-
ring sequences from the Younger Dryas event in
the southern French Alps. Radiocarbon 60:51–74.
Cheng H, Lawrence Edwards R, Southon J,
Matsumoto K, Feinberg JM, Sinha A, Zhou W,
Li H, Li X, Xu Y, et al. 2018. Atmospheric
14C/12C changes during the last glacial period
from Hulu Cave. Science 362:1293–1297.
David F, Connet N, Girard M, Lhomme V,
Miskovsky JC, Roblin-Jouve A. 2001. Le
Chˆatelperronien de la grotte du Renne à Arcy-
sur-Cure (Yonne). Données sédimentologiques
et chronostratigraphiques. Bulletin de la Société
Préhistoire Francaise 98:207–230.
Ehrlich Y, Regev L, Boaretto E. 2018. Radiocarbon
analysis of modern olive wood raises doubts
concerning a crucial piece of evidence in dating
the Santorini eruption. Scientific Reports 8. doi:
10.1038/s41598-018-29392-9.
Fairbanks RG, Mortlock RA, Tzu-Chien C, Cao L,
Kaplan A, Guilderson TP, Fairbanks TW,
Bloom AL, Grootes PM, Nadeau MJ. 2005.
Radiocarbon calibration curve spanning 0 to
50,000 years BP based on paired 230Th/234U/238U
and 14C dates on pristine corals. Quaternary
Science Reviews 24:1781–1796.
Fiedel S. 2011. The mysterious onset of the Younger
Dryas. Quaternary International 242:262–266.
Friedrich WL, Kromer B, Friedrich M, Heinemeier J,
Pfeiffer T, Talamo S. 2006. Santorini eruption
dated to 1627–1600 BC. Science 312:548.
Friedrich R, Kromer B, Wacker L, Olsen J, Remmele S,
Lindauer S, Land A, Pearson C. 2020. A new
annual 14C dataset for calibrating the Thera
eruption. Radiocarbon 62. This issue.
Fu Q, Hajdinjak M, Moldovan OT, Constantin S,
Mallick S, Skoglund P, Patterson N, Rohland N,
Lazaridis I, Nickel B, et al. 2015. An early
modern human from Romania with a recent
Neanderthal ancestor. Nature 524:216–219.
Fu Q, Li H, Moorjani P, Jay F, Slepchenko SM,
Bondarev AA, Johnson PLF, Aximu-Petri A,
20 J van der Plicht et al.
https://www.cambridge.org/core/terms. https://doi.org/10.1017/RDC.2020.22
Downloaded from https://www.cambridge.org/core. IP address: 2.57.71.171, on 22 Apr 2020 at 12:58:29, subject to the Cambridge Core terms of use, available at

Prufer K, de Filippo C, et al. 2014. Genome
sequence of a 45,000-year-old modern human
from western Siberia. Nature 514:445–449.
HeatonTJ,BlaauwM,BlackwellPG,BronkRamseyC,
Reimer PJ, Scott ME. 2020a. The IntCal20
approach to radiocarbon calibration curve
construction: a new implementation using
Bayesian splines and errors-in-variables.
Radiocarbon 62. This issue.
Heaton TJ, Köhler P, Butzin M, Bard E, Reimer RW,
Austin WEN, Ramsey CB, Grootes PM,
Hughen KA, Reimer PJ, et al. 2020b. Marine20—
the marine radiocarbon age calibration curve
(0–55,000 cal BP). Radiocarbon 62. This issue.
Higham T, Douka K, Wood R, Ramsey CB, Brock F,
Basell L, Camps M, Arrizabalaga A, Baena J,
Barroso-Ruiz C, et al. 2014. The timing and
spatiotemporal patterning of Neanderthal
disappearance. Nature 512:306–309.
Hoffman DL, Beck JW, Richards DA, Smart PL,
Singarayer JS, Ketchmak T, Hawkesworth CJ.
2010. Towards radiocarbon calibration beyond
28 ka using speleothems from the Bahamas.
Earth and Planetary Science Letters 289:1–10.
Höflmayer F. 2012. The date of the Minoan Santorini
eruption: quantifying the “offset”. Radiocarbon
54:435–448.
Hogg A, Southon J, Turney C, Palmer J, Bronk
Ramsey C, Fenwick P, Boswijk G, Buntgen U,
Froedrich M, Helle G, et al. 2016. Decadally
resolved Lateglacial radiocarbon evidence from
New Zealand Kauri. Radiocarbon 58:709–733.
Hogg A, Heaton TJ, Hua Q, Bayliss A, Blackwell PG,
Boswijk G, Ramsey CB, Palmer J, Petchey F,
Reimer P, et al. 2020. SHCal20 Southern
Hemisphere calibration, 0–55,000 years cal BP.
Radiocarbon 62. This issue.
Hublin JJ. 2012. The earliest modern human
colonization of Europe. PNAS Commentary.
Hublin JJ, Talamo S, Julien M, David F, Connet N,
Bodu P, Vandermeersch B, Richards MP. 2012.
Radiocarbon dates from the Grotte du Renne
and Saint-Césaire support a Neandertal origin
for the Chˆatelperronian. Proceedings of the
National Academy of Sciences 109:18743–18748.
Hughen KA, Lehman S, Southon J, Overpeck J,
Marchal O, Herring C, Turnbull J. 2004. 14C
activity and global carbon cycle change over the
past 50,000 years. Science 303:202–207.
Hughen KA, Southon JR, Lehman SJ, Bertrand CJH,
Turnbull J. 2006. Marine-derived 14C calibration
and activity record for the past 50,000 years from
thr Cariaco basin. Quaternary Science Reviews
25:3216–3227.
ISG. 1982. An inter-laboratory comparison of
radiocarbon measurements in tree rings Inter-
national Study Group. Nature 298:619–623.
ISG. 1983. An international tree-ring replicate study.
In: Waterbolk HT, Mook WG, editors. Strasbourg.
Pact 8:123–133.
Kaiser KF, Friedrich M, Miramont C, Kromer B,
Sgier M, Schaub M, Boeren I, Remmele S,
Talamo S, Guibal F, et al. 2012. Challenging
process to make the Lateglacial tree-ring
chronologies from Europe absolute—an inventory.
Quaternary Science Reviews 36:78–90.
Kitagawa H, van der Plicht J. 1998. Atmospheric
radiocarbon calibration to 45,000 yr BP: late
glacial fluctuations and cosmogenic isotope
production. Science 279:1187–1190.
Kitagawa H, van der Plicht J. 2000. Atmospheric
Radiocarbon calibration beyond 11,900 cal BP
from Lake Suigetsu laminated sediments.
Radicoarbon 42:369–380.
Köhler P, Fischer H, Munhoven G, Zeebe RE. 2005.
Quantitative interpretation of atmospheric
carbon records over the last glacial termination.
Global Biogeochem. Cycles 19, GB4020. doi:
10.1029/2004GB002345.
Köhler P, Muscheler R, Fischer H. 2006. A model-
based interpretation of low-frequency changes
in the carbon cycle during the last 120,000 years
and its implications for the reconstruction of
atmospheric Δ14C. Geochem. Geophys. Geosyst.
7, Q11N06. doi: 10.1029/2005GC001228.
Köhler P, Fischer H. 2004. Simulating changes in the
terrestrial biosphere during the last glacial/
interglacial transition. Global Planetary Change
43: 33–55. doi: 10.1016/j.gloplacha.2004.02.005.
Köhler P, Fischer H. 2006. Simulating low frequency
changes in atmospheric CO
2
during the last
740 000 years. Climate of the Past 2:57–78. doi:
10.5194/cp-2-57-2006.
Kromer B, Ambers J, Baillie MGL, Damon PE,
Hesshaimer V, Hofmann J, Jöris O, Levin I,
Manning SW, McCormac FG, et al. 1996.
Report: Summary of the workshop “Aspects of
high-precision radiocarbon calibration”.
Radiocarbon 38:607–610.
Kromer B, Friedrich M, Hughen KA, Kaiser KF,
Remmele S, Schaub M, Talamo S. 2004. Late
Glacial 14C ages from a floating, 1382-ring pine
chronology. Radiocarbon 46:1203–1209.
Kuitems M, van der Plicht J, Jansma E. 2020. Wood
from the Netherlands around the time of the
Santorini eruption dated by dendrochronology
and radiocarbon. Radiocarbon 62. This issue.
Kutschera W, Bietak M, Wild EM, Bronk Ramsey C,
Dee M, Golser R, Kopetzky K, Stadler P, Steier P,
Thanheiser U, et al. 2012. The chronology of
Tell el-Daba: a crucial meeting point of 14C
dating, archaeology and Egyptology in the 2nd
millennium BC. Radiocarbon 54:407–422.
Lévêque F, Vandermeersch B. 1980. Découverte des
restes humains dans un niveau castelperronien à
Saint-Césaire (Charente-Maritime). Compte-
Rendus de l’Académie des Sciences 291:187–189.
Manning S. 2014. A test of time and a test of time
revisited: The volcano of Thera and the
chronology and history of the Aegean and East
IntCal20 Companion: Archaeological & Environmental Samples 21
https://www.cambridge.org/core/terms. https://doi.org/10.1017/RDC.2020.22
Downloaded from https://www.cambridge.org/core. IP address: 2.57.71.171, on 22 Apr 2020 at 12:58:29, subject to the Cambridge Core terms of use, available at

Mediterranean in the mid-second millennium BC.
Oxford/Philadelphia: Oxbow. 494 p. ISBN
9781782972198.
McAneney J, Baillie M. 2019. Absolute tree-ring
dates for the Late Bronze Age eruptions of
Aniakchak and Thera in light of a proposed
revision of ice-core chronologies. Antiquity 93:
99–112.
Mellars P. 2006. A new radiocarbon revolution and
the dispersal of modern humans in Eurasia.
Nature 439:931–935.
Miyake F, Nagaya K, Masuda K, Nakamura T. 2012.
A signature of cosmic-ray increase in AD 774–775
from tree rings in Japan. Nature 486:240–242.
Miyake F, Masuda K, Nakamura T. 2013. Another
rapid event in the carbon-14 content of tree
rings. Nature Communications 4. doi: 10.1038/
ncomms2783.
Mook WG, van der Plicht J. 1999. Reporting 14C
activities and concentrations. Radiocarbon 41:
227–239.
Müller UC, Pross J, Tzedakis PC, Gamble C,
Kotthoff U, Schmiedl G, Wulf S, Christanis K.
2011. The role of climate in the spread of
modern humans into Europe. Quaternary
Science Reviews 30:273–279.
Muscheler R, Adolphi F, Heaton TJ, Bronk Ramsey C,
Svensson A, van der Plicht J, Reimer PJ. 2020.
Testing and improving the IntCal20 calibration
curve with independent records. Radiocarbon
62. This issue.
Pearson CL, Brewer PW, Brown D, Heaton TJ,
Hodgins GWL, Jull AJT, Lange T, Salzer MW.
2018. Annual radiocarbon record indicates 16th
century BCE date for the Thera eruption.
Science Advances 4:eaar8241.
Pearson CL, Wacker L, Bayliss A, Brown DM,
Salzer MW, Brewer PW, Bollhalder S, Boswijk G,
Hodgins GWL. 2018. Annual variation in
atmospheric 14C between 1700 BC and 1480
BC. Radiocarbon 62. This issue.
Pearson GW, Stuiver M. 1986. High-precision cali-
bration of the radiocarbon timescale 500–2500 BC.
Radiocarbon 28:839–862.
Ralph EK, Michael HN, Han MC. 1973. Radiocarbon
dates and reality. Masca Newsletter 9:1–20.
Rasmussen SO, Bigler M, Blockley SP, Blunier T,
Buchardt SL, Clausen HB, Cvijanovic I, Dahl-
Jensen D, Johnsen SJ, Fischer H, et al. 2014. A
stratigraphic framework for abrupt climatic
changes during the Last Glacial period based
on three synchronized Greenland ice-core
recor ds: refining and extending the INTIMATE
event stratigraphy. Quaternary Science Reviews
106:14–28.
Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW,
Bertrand CJH, Blackwell PG, Buck CE, Burr GS,
Cutler KB, et al. 2004. INTCAL04 terrestrial
radiocarbon age calibration, 0–26 cal kyr BP.
Radiocarbon 46:1029–1058.
Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW,
Blackwell PG, Bronk Ramsey C, Buck CE,
Burr GS, Edwards RL, et al. 2009. IntCal09
and Marine09 radiocarbon calibration curves,
0–50 cal kBP. Radiocarbon 51:1111–1150.
Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell
PG, Bronk Ramsey C, Buck CE, Cheng H,
Edwards RL, Friedrich M, et al. 2013. IntCal13
and Marine13 radiocarbon age calibration
curves 0–50,000 years cal BP. Radiocarbon
55:1869–1887.
Reimer PJ, Austin WEN, Bard E, Bayliss A,
Blackwell P, Bronk Ramsey C, Butzin M,
Edwards L, Friedrich M, Grootes PM, et al.
2020. The IntCal20 Northern Hemisphere
radiocarbon calibration curve (0–55 kcal BP).
Radiocarbon 62. This issue.
Reinig F, Sookdeo A, Esper J, Friedrich M, Helle G,
Kromer B, Nievergelt D, Pauly M, Tegel W,
Treydte K, et al. 2020. Illuminating IntCal during
the Younger Dryas. Radiocarbon 62. This issue.
Renfrew C. 1999. Before civilization—the radiocarbon
revolution and prehistoric Europe. Pimlico.
Scott EM, Naysmith P, Cook GT. 2018. Why do we
need 14C inter-comparisons?: the Glasgow 14C
inter-comparison series, a reflection over 30 years.
Quaternary Geochronology 43:72–82.
Southon J, Noronha AL, Cheng H, Lawrence
Edwards R, Wang Y. 2012. A high resolution
record of atmospheric 14C based on Hulu Cave
speleothem H82. Quaternary Science Reviews
33:32–41.
Stohl A, Bonasoni P, Cristofanelli P, Collins W,
Feichter J, Frank A, Forster C, Gerasopoulos E,
Gäggeler H, James P, et al. 2003. Stratosphere-
troposphere exchange: A review, and what we
have learned from STACCATO. Journal of
Geophysical Research Atmospheres 108(D12).
doi: 10.1029/2002/JD002490.
Stuiver M, Polach H. 1977. Discussion: Reporting of
14C. Radiocarbon 19:355–363.
Stuiver M. 1982. A high precision calibration of
the AD radiocarbon time scale. Radiocarbon
24:1–26.
Stuiver M, Pearson GW. 1986. High-precision
calibration of the radiocarbon timescale AD
1950–500 BC. Radiocarbon 28:805–838.
Suess HE. 1980. The Radiocarbon record in tree
rings of the last 8000 years. Radiocarbon 22:
200–209.
Synal HA, Stocker M, Suter M. 2007. MICADAS: A
new compact radiocarbon AMS system. Nuclear
Instruments and Methods in Physics Research B
259:7–13.
Trinkaus E, Moldovan O, Milota S, Bîlgar A,
Sarcina L, Athreya S, Bailey SE, Rodrigo R,
Mircea G, Higham T, et al. 2003. An early
modern human from the Pestera cu Oase,
Romania. Proceedings of the National Academy
of Sciences 100:11231–11236.
22 J van der Plicht et al.
https://www.cambridge.org/core/terms. https://doi.org/10.1017/RDC.2020.22
Downloaded from https://www.cambridge.org/core. IP address: 2.57.71.171, on 22 Apr 2020 at 12:58:29, subject to the Cambridge Core terms of use, available at

van Andel TH. 2005. The ownership of time: Approved
14C calibration or freedom of choice? Antiquity
79:944–948.
van der Plicht J. 2000. The 2000 radiocarbon varve/
comparison issue: Introduction. Radiocarbon
42:313–322.
van der Plicht J. 2004. Radiocarbon calibration—
past, present and future. Nuclear Instruments and
Methods in Physics Research B 223–224:353–358.
van der Plicht J, Beck JW, Bard E, Baillie MGL,
Blackwett PG, Buck CE, Friedrich M, Guilderson
TP, Hughen KA, Kromer B, et al. 2004.
NOTCAL04: Comparison/calibration 14C records
26–50 cal kyr BP. Radiocarbon 46:1225–1238.
Vogel JC, Kronfeld J. 1997. Calibration of
Radiocarbon dates for the Late Pleistocene
using U/Th dates on stalagmites. Radiocarbon
39:27–32.
Wacker L, Scott EM, Bayliss A, Brown D, Bard E,
Bollhalder S, Friedrich M, Capano M,
Cherkinsky A, Chivall D, et al. 2020. Findings
from an in-depth annual tree ring radiocarbon
intercomparison. Radiocarbon 62. This issue.
Welker F, Hajdinjak M, Talamo S, Jaouen K,
Dannemann M, David F, Julien M, Meyer M,
Kelso J, Barnes I, et al. 2016. Palaeoproteomic
evidence identifies archaic hominins associated
with the Chˆatelperronian at the Grotte du Renne.
Proceedings of the National Academy of Sciences
113:11162–11167.
Weninger B, Jöris O. 2004. Glacial radiocarbon age
calibration: The CalPal program. In: Higham T,
Bronk Ramsey C, Owen C, editors. Radiocarbon
and Archaeology, Volume Monograph 62.
Oxford: St Catherine’s College, Oxford University
School of Archaeology. p. 9–15.
IntCal20 Companion: Archaeological & Environmental Samples 23
https://www.cambridge.org/core/terms. https://doi.org/10.1017/RDC.2020.22
Downloaded from https://www.cambridge.org/core. IP address: 2.57.71.171, on 22 Apr 2020 at 12:58:29, subject to the Cambridge Core terms of use, available at