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Composition and consequences of the IntCal20 radiocarbon calibration curve

Part of: 50th Anniversary issue

Published online by Cambridge University Press:  15 June 2020

Paula J. Reimer Open the ORCID record for Paula J. Reimer [Opens in a new window]
Paula J. Reimer*
Affiliation:
14CHRONO Centre for Climate, the Environment and Chronology, Queen's University Belfast, BelfastBT7 1NN, UK
*
*Corresponding author email address: p.j.reimer@qub.ac.uk
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Abstract

Radiocarbon calibration is necessary to correct for variations in atmospheric radiocarbon over time. The IntCal working group has developed an updated and extended radiocarbon calibration curve, IntCal20, for Northern Hemisphere terrestrial samples from 0 to 55,000 cal yr BP. This paper summarizes the new datasets, changes to existing datasets, and the statistical method used for constructing the new curve. Examples of the effect of the new calibration curve compared to IntCal13 for hypothetical radiocarbon ages are given. For the recent Holocene the effect is minimal, but for older radiocarbon ages the shift in calibrated ages can be up to several hundred years with the potential for multiple calibrated age ranges in periods with higher-resolution data. In addition, the IntCal20 curve is used to recalibrate the radiocarbon ages for the glaciation of the Puget Lowland and to recalculate the advance rate. The ice may have reached its maximum position a few hundred years earlier using the new calibration curve; the calculated advance rate is virtually unchanged from the prior estimate.

Keywords

RadiocarboncalibrationIntCal20
Type
Review Article
Information
Quaternary Research , Volume 96: 50th Anniversary Issue , July 2020 , pp. 22 - 27
Copyright
Copyright © University of Washington. Published by Cambridge University Press, 2020

INTRODUCTION

Radiocarbon ages require a correction to account for changes in atmospheric concentration of 14C over time. Without this correction, or calibration, radiocarbon ages cannot be directly compared to historical dates or ages measured by other methods nor can rates of change be calculated. Radiocarbon calibrations have been done through the use of a curve based on compilations of 14C measurements of known-age material, such as dendrochronologically dated tree rings, since the 1960s (e.g., Stuiver and Suess, Reference Stuiver and Suess1966; Clark, Reference Clark1975). To prevent confusion from the use of the various calibration curves available, an international working group was established to provide a consensus curve (Klein et al., Reference Klein, Lerman, Damon and Ralph1982). Since that time, updated calibration curves have been ratified by the radiocarbon community. Separate curves for the Northern and Southern Hemisphere terrestrial samples and for marine samples are now available. Numerous researchers have contributed to the calibration effort by providing high-precision 14C measurements from tree rings and other archives.

From 2004, the IntCal Working Group has updated, extended, and refined radiocarbon calibrations semi-regularly. As more independently dated archives have been radiocarbon dated and our understanding of the Earth's systems has increased, calibration curves have been extended and refined. The most recent calibration curves for the Northern and Southern Hemispheres and the ocean surface have recently been published (Heaton et al., Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin and Bronk Ramsey2020a; Hogg et al., Reference Hogg, Heaton, Hua, Bayliss, Blackwell, Boswijk and Ramsey2020; Reimer et al., Reference Reimer, Austin, Bard, Bayliss, Blackwell, Ramsey and Butzin2020) for 0 to 55,000 cal yr BP. The methods and datasets used for the IntCal20 curve are summarized in this paper. Calibrations of some hypothetical radiocarbon ages with both IntCal13 (Reimer et al., Reference Reimer, Bard, Bayliss, Beck, Blackwell, Ramsey and Buck2013a) and IntCal20 are used to highlight some of the similarities and differences between the two curves. An example of the effect of the new calibration curve on previously published radiocarbon ages for the timing and rate of advance of the Cordilleran ice sheet in the Pacific Northwest, USA (Porter and Swanson, Reference Porter and Swanson1998), is given.

NEW STATISTICAL TOOLS

The construction of the IntCal20 calibration curve was underpinned by a new statistical model. Heaton et al. (Reference Heaton, Blaauw, Blackwell, Bronk Ramsey, Reimer and Scott2020b) adapted Bayesian splines to provide a flexible method for curve construction suited to the complexities of the different types of data and error structures. The Bayesian method also allowed for prior knowledge to inform the construction. For instance, reported laboratory errors do not usually include all sources of uncertainty, as evidenced by the Sixth International Radiocarbon Intercomparison exercise (SIRI; Scott et al., Reference Scott, Naysmith and Cook2017). For tree rings, growing season and, potentially, species differences may contribute additional uncertainty (e.g., Kromer et al., Reference Kromer, Manning, Kuniholm, Newton, Spurk and Levin2001; Dee et al., Reference Dee, Brock, Harris, Ramsey, Shortland, Higham and Rowland2010). The Bayesian model used prior information for additional radiocarbon uncertainty based on tree ring data from the SIRI exercise, although the resulting curve uncertainty was dominated by the high-quality IntCal data. In addition, wiggle-matching of floating tree ring series (i.e., not dendrochronologically linked to an absolutely dated chronology) was done internal to the model within estimated uncertainty. The Bayesian spline also allows for rapid changes in 14C to be captured in the curve. Complete details of the Bayesian spline implementation are given in Heaton and colleagues (Reference Heaton, Blaauw, Blackwell, Bronk Ramsey, Reimer and Scott2020b).

CALIBRATION DATASETS

Dendrochronologically dated tree ring archives are still preferred for radiocarbon calibration for terrestrial samples because they are direct recorders of atmospheric 14C and there is little or no uncertainty in the calendar age. The number of calibration-quality radiocarbon measurements on known-age tree rings, many of them single year, has proliferated especially since the discovery of rapid increases in atmospheric 14C at AD 774–775 and AD 993 (Miyake et al., Reference Miyake, Nagaya, Masuda and Nakamura2012, Reference Miyake, Masuda and Nakamura2013). Other time periods have been targeted for single-ring measurements to search for additional 14C events (e.g., Miyake et al., Reference Miyake, Jull, Panyushkina, Wacker, Salzer, Baisan, Lange, Cruz, Masuda and Nakamura2017a, Reference Miyake, Masuda, Nakamura, Kimura, Hakozaki, Jull and Lange2017b; Jull et al., Reference Jull, Panyushkina, Miyake, Masuda, Nakamura, Mitsutani, Lange, Cruz, Baisan and Janovics2018) and to improve calibration around a radiocarbon plateau ca. 2700–2400 cal yr BP (Park et al., Reference Park, Southon, Fahrni, Creasman and Mewaldt2017; Fahrni et al., Reference Fahrni, Southon, Fuller, Park, Friedrich, Muscheler, Wacker and Taylor2020), as well as to attempt to pinpoint the timing of the Minoan eruption of Santorini (Thera) (Pearson et al., Reference Pearson, Brewer, Brown, Heaton, Hodgins, Jull, Lange and Salzer2018, Reference Pearson, Wacker, Bayliss, Brown, Salzer, Brewer, Bollhalder, Boswijk and Hodgins2020; Friedrich et al., Reference Friedrich, Kromer, Wacker, Olsen, Remmele, Lindauer, Land and Pearson2020; Kuitems et al., Reference Kuitems, Plicht and Jansma2020). The oldest dendrochronologically dated tree ring chronology in the Northern Hemisphere is the central European Preboreal Pine Chronology (PPC; Friedrich et al., Reference Friedrich, Remmele, Kromer, Hofmann, Spurk, Kaiser, Orcel and Küppers2004), which has been extended to 12,235 cal yr BP (Reinig et al., Reference Reinig, Sookdeo, Esper, Friedrich, Guidobaldi, Helle and Kromer2020). Older tree rings used in calibration remain floating. Multi-laboratory 14C measurements of the late glacial New Zealand kauri floating chronology (Hogg et al., Reference Hogg, Southon, Turney, Palmer, Ramsey, Fenwick, Boswijk, Büntgen, Friedrich and Helle2016) pointed to an error in the link between the absolutely dated Central European PPC and the floating Swiss Late Glacial Master Chronology (Kaiser et al., Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub and Boeren2012) used in IntCal13. Investigating the previous tree ring links resulted in an improved match with the Swiss floating chronology shifting it 35 ± 8 years older. This was supported by the overlap with new 14C measurements from a floating chronology from the southern French Alps (Capano et al., Reference Capano, Miramont, Guibal, Kromer, Tuna, Fagault and Bard2018, Reference Capano, Miramont, Shindo, Guibal, Marschal, Kromer, Tuna and Bard2020). A major discovery at a construction site in Zurich of hundreds of pine trees buried in situ has provided ample material for chronological replication and increased resolution ca. 13,160–11,950 cal yr BP (Reinig et al., Reference Reinig, Sookdeo, Esper, Friedrich, Guidobaldi, Helle and Kromer2020; Sookdeo et al., Reference Sookdeo, Kromer, Buentgen, Friedrich, Friedrich, Helle and Pauly2020).

Three floating tree ring series from northern Italy, which had previously been fitted to Greenland ice core 10Be to ca. 14,700 to 14,000 cal yr BP (Adolphi et al., Reference Adolphi, Muscheler, Friedrich, Güttler, Wacker, Talamo and Kromer2017), were incorporated by 14C matching to the other IntCal data. These measurements add structure to the otherwise rather smooth calibration curve in this time period. A 2000-year-long floating tree ring series from New Zealand (Turney et al., Reference Turney, Palmer, Ramsey, Adolphi, Muscheler, Hughen, Staff, Jones, Thomas and Fogwill2016) was also incorporated using an interhemispheric offset of 43 ± 23 14C yr (Hogg et al., Reference Hogg, Hua, Blackwell, Niu, Buck, Guilderson and Heaton2013). This series adds structure to the curve during Heinrich Stadial 3. Another glacial New Zealand series with 1300 rings (Turney et al., Reference Turney, Fifield, Hogg, Palmer, Hughen, Baillie and Galbraith2010) was also 14C matched into the curve.

The most influential dataset in the IntCal20 curve older than ca. 14,000 cal yr BP is undoubtedly the U-Th dated Hulu cave 14C record from China (Cheng et al., Reference Cheng, Edwards, Southon, Matsumoto, Feinberg, Sinha, Zhou, Li, Li and Xu2018) which extends to 53,900 cal yr BP. The correction for old carbon (dead carbon fraction) in the 14C ages, estimated from the overlap with tree ring data, can be assumed to be relatively constant within uncertainty because the speleothem formed in a portion of the cave where the limestone had largely been replaced by iron oxides. In addition, a short residence time for the soil carbon above the cave is presumed due to the observation of seasonal δ18O values and lack of high 14C levels from nuclear weapons testing observed in the dripwaters (Cheng et al., Reference Cheng, Edwards, Southon, Matsumoto, Feinberg, Sinha, Zhou, Li, Li and Xu2018). The δ18O fluctuations recorded in the speleothem (Wang et al., Reference Wang, Cheng, Edwards, An, Wu, Shen and Dorale2001; Cheng et al., Reference Cheng, Edwards, Sinha, Spötl, Yi, Chen, Kelly, Kathayat, Wang and Li2016) also serve as tie-points for marine foraminifera records from the Iberian margin, Pakistan margin, and Cariaco basin (Bard et al., Reference Bard, Ménot, Rostek, Licari, Böning, Edwards, Cheng, Wang and Heaton2013; Heaton et al., Reference Heaton, Bard and Hughen2013; Hughen and Heaton, Reference Hughen and Heaton2020). In addition, the Lake Suigetsu varved sediment record, which contains terrestrial macrofossils, has been revised and extended (Schlolaut et al., Reference Schlolaut, Staff, Brauer, Lamb, Marshall, Ramsey and Nakagawa2018), and the calendar age has been modelled with the Hulu cave timescale (Bronk Ramsey et al., Reference Bronk Ramsey, Heaton, Schlolaut, Staff, Bryant, Brauer, Lamb, Marshall and Nakagawa2020). Also included in IntCal20 were two speleothem records from an underwater cave on Grand Bahamas (Beck et al., Reference Beck, Richards, Edwards, Silverman, Smart, Donahue and Hererra-Osterheld2001; Hoffmann et al., Reference Hoffmann, Beck, Richards, Smart, Singarayer, Ketchmark and Hawkesworth2010). The Bahamas speleothem records, despite having large uncertainty on the dead carbon fractions, serve as a check on the Hulu cave data. By incorporating a range of datasets we can assess which features are likely to represent atmospheric signals and which are local features or noise.

Marine 14C measurements of coral and foraminifera from marine sediments have been included with a correction for the marine reservoir age (MRA) of the ocean region where they grew. In the past the MRA was assumed to be constant in time, but there is an abundance of evidence to suggest this is not a valid assumption, especially for the last glacial period. For IntCal20 we used MRAs calculated with the Hamburg Large Scale Geostrophic Ocean General Circulation Model (LSG OGCM) with atmospheric input provided by a curve constructed with a Bayesian spline of the Hulu data (Butzin et al., Reference Butzin, Heaton, Köhler and Lohmann2020). The calculated MRAs were only used as prior information to correct the marine 14C ages in the Bayesian spline, which was then adjusted to best fit the data within uncertainty. The LSG OGCM–modelled MRA for the Cariaco basin did not agree with the data, possibly due to the coarse resolution of the model compared to the size of the basin, so a slowly varying spline was used instead (Heaton et al., Reference Heaton, Blaauw, Blackwell, Bronk Ramsey, Reimer and Scott2020b; Hughen and Heaton, Reference Hughen and Heaton2020).

Data from aragonitic coral that grew close to the surface of the ocean were included in previous IntCal curves if the corals met previously established criteria (Reimer et al., Reference Reimer, Bard, Bayliss, Beck, Blackwell, Ramsey, Brown, Buck, Edwards and Friedrich2013b). However, some of the coral 14C data older than 25,000 cal yr BP is highly variable regardless of meeting the criteria. It is likely there has been some undetected diagenesis due to exposure to freshwater during the Last Glacial Maximum lowstand (21,000 ± 2000 cal yr BP). Therefore, no corals older than 25,000 cal yr BP were used in IntCal20.

RESULTS AND DISCUSSION

The effect of the IntCal20 calibration curve compared to IntCal13 (Reimer et al., Reference Reimer, Bard, Bayliss, Beck, Blackwell, Ramsey and Buck2013a) is shown for hypothetical radiocarbon ages of 5000 ± 20 14C yr BP, 15,000 ± 30 14C yr BP, 30,000 ± 50 14C yr BP, and 40,000 ± 200 14C yr BP (Figure 1). For 5000 14C yr BP there is hardly any noticeable difference between the probability distributions calculated using IntCal13 and IntCal20. However, at 15,000 14C yr BP the distribution for IntCal20 has an additional younger peak compared to that for IntCal13. For 30,000 14C yr BP the distribution calculated with IntCal20 is about 400 years younger than with IntCal13, whereas for 40,000 14C yr BP the IntCal20 distribution is bimodal but the main peak is about 500 years older than with IntCal13.

Figure 1. Calibrated probability distributions are shown for hypothetical radiocarbon ages of 5000 ± 20 14C yr BP, 15,000 ± 30 14C yr BP, 30,000 ± 50 14C yr BP, and 40,000 ± 200 14C yr BP. 1 sigma age ranges are shown as thick lines and 2 sigma as thin lines at the base of each distribution. Calibrations were done using IntCal13 and CALIB 7.04 and using IntCal20 with CALIB 8.1 (Stuiver and Reimer, Reference Stuiver and Reimer1993).

A good example of the consistency of the overall shape of radiocarbon calibration curves over the past three decades, is the case of the advance rate of the Puget Lobe of the Cordilleran ice sheet in Washington State during the last glaciation and the timing of the arrival of the ice in the Issaquah delta calibrated with IntCal93 (Stuiver and Reimer, Reference Stuiver and Reimer1993) and with the new IntCal20 curve. Porter and Swanson (Reference Porter and Swanson1998) presented seven radiocarbon dates on outer wood and branches of pine taken from the top of a pro-glacial delta near Issaquah, Washington (Figure 2). The weighted mean of these radiocarbon dates was 14,546 ± 55 14C yr BP, and the mean intercept with the IntCal93 calibration curve was given as 17,420 ± 90 cal yr BP. Although it is no longer recommended to use the mean intercept (Telford et al., Reference Telford, Heegaard and Birks2004), the calibrated age range from IntCal20 is only slightly older than this at 17,455–18,005 cal yr BP (at 2σ, rounding out to 5 years). The glacier advance rate was calculated from two radiocarbon dates on spruce wood from Allison Pool, southern British Columbia (ca. 200 km from Issaquah) which had a mean radiocarbon age of 16,059 ± 71 14C yr BP (Clague et al., Reference Clague, Saunders and Roberts1988) with a reported IntCal93 calibrated age of 18,925 cal yr BP. The difference in mean calibrated ages between Allison Pool and Issaquah delta gave an advance rate of 135 m/yr. With IntCal20 the mean of the Allison Pool radiocarbon dates calibrates to 19,175–19,550 cal yr BP (at 2 σ, rounding out to 5 years). Calculating the difference in the calibrated probability distributions using OxCal (Bronk Ramsey, Reference Bronk Ramsey2009; Reference Bronk Ramsey2017) gives 1939–1355 years at 95% probability resulting in an advance rate of 103–148 m/yr. The estimated advance rate from Porter and Swanson (Reference Porter and Swanson1998) of 135 m/yr falls well within that range.

Figure 2. Map showing the extent of the Puget Lobe of the Cordilleran ice sheet in Washington State during the last glaciation (shaded region) adapted from Porter and Swanson (Reference Porter and Swanson1998). Dark grey arrows indicate inferred flow direction.

CONCLUSIONS

For much of the Holocene, the IntCal20 curve will not have a large effect on the calibration of radiocarbon ages from single samples with the exception of potentially intercepting younger radiocarbon ages with the sharp radiocarbon declines resulting from the 14C events at AD 774–775 and AD 993. For older periods, calibrated age ranges may shift by several hundred years in either direction compared to IntCal13, and there may be additional calibrated age ranges where the curve comprises higher-resolution data. Despite increased detail in calibration curves over time, the overall shape of the IntCal20 curve back to a least 25,000 cal yr BP does not differ greatly from much older curves as seen by the relatively small change for the advance rate of the Cordilleran ice sheet into the Puget Lowland of Washington State as calibrated with IntCal93.

IntCal20 now extends to 55,000 cal yr BP so that it is now possible to calibrate radiocarbon ages including two standard deviations up to ca. 50,000 14C yr BP. The entire IntCal20 curve is available to download, and access to the database can be found at http://intcal.org. The calibration programs CALIB (http://calib.org ) and OxCal (https://c14.arch.ox.ac.uk/ ) have been updated to use the IntCal20 curve. It should be noted that IntCal20 is intended for the calibration of Northern Hemisphere atmospheric samples. SHCal20 should be used for the calibration of Southern Hemisphere atmospheric samples (Hogg et al., Reference Hogg, Heaton, Hua, Bayliss, Blackwell, Boswijk and Ramsey2020), and Marine20 (with application of a local reservoir adjustment) should be used for the calibration of marine samples (Heaton et al., Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin and Bronk Ramsey2020a).

ACKNOWLEDGMENTS

I would like to thank the IntCal Working Group (http://intcal.org), focus group members, and others who made the IntCal20 curve possible. I would also like to thank Ron Reimer for producing the figures for this paper. I am grateful to Julie Brigham-Grette and an anonymous reviewer for their constructive comments.

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Figure 1. Calibrated probability distributions are shown for hypothetical radiocarbon ages of 5000 ± 20 14C yr BP, 15,000 ± 30 14C yr BP, 30,000 ± 50 14C yr BP, and 40,000 ± 200 14C yr BP. 1 sigma age ranges are shown as thick lines and 2 sigma as thin lines at the base of each distribution. Calibrations were done using IntCal13 and CALIB 7.04 and using IntCal20 with CALIB 8.1 (Stuiver and Reimer, 1993).

Figure 1

Figure 2. Map showing the extent of the Puget Lobe of the Cordilleran ice sheet in Washington State during the last glaciation (shaded region) adapted from Porter and Swanson (1998). Dark grey arrows indicate inferred flow direction.