EVOLUTION OF RADIOCARBON CALIBRATION

ABSTRACT In the late 1950s it was recognized that levels of atmospheric radiocarbon (14C) had not been constant over time. Since then, researchers have sought to document those changes, initially through measurements of known age tree rings and more recently using other archives to create curves to correct or calibrate radiocarbon ages to calendar ages. This paper highlights some, but by no means all, of the efforts to create and extend radiocarbon calibration curves.

Erratum to: Radiocarbon calibration: The next generation

The article Radiocarbon calibration: The next generation, written by Richard A STAFF and Ruiliang LIU, was originally published in Vol. 64 Issue 3 without open access. With the author(s)’ decision to opt for Open Choice the copyright of the article changed in April 2021 to © The Author(s) 2021 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.The original article has been corrected.

Soil Carbon Dynamics and Atmospheric Signals in Stalagmite Radiocarbon at Sofular Cave, Turkey

<p><span>The climatic controls of stalagmite radiocarbon remain one focus of modern paleoclimatology due to recent efforts and achievements in radiocarbon calibration. The Hulu cave radiocarbon record (Cheng et al., 2018) has proven the potential of stalagmites from temperate climate zones for atmospheric radiocarbon reconstruction. However, a constant dead carbon fraction (DCF) in stalagmites over long periods of time is rather exceptional. In our study, a high-resolution radiocarbon record (N>100) of a U-Series dated stalagmite from Sofular Cave, Northern Turkey, with elemental Mg/Ca ratio data is presented. From 14 to 10 kyr BP, the radiocarbon signal reveals changing climatic conditions throughout Termination I with warm periods affiliated with increased soil activity and lower DCF. We observe unstable soil conditions for the period before 14 kyr BP where DCF is strongly variable between a lower threshold of ca. 5% and an upper limit of 25%. The combination of stable isotopes, element ratios, radiocarbon and U-series data allows a multi-proxy analysis of the impact of fast climate changes like D/O events on the incorporation of radiocarbon into stalagmites. Between 15 and 27 kyr BP, hydrological changes and soil carbon cycling have a large impact on limestone dissolution systematics which is reflected in fast changing DCF on sub-centennial time scales. Although the reconstruction of atmospheric radiocarbon variability is not possible for the entire growth period, the stalagmite closely reproduces the increased atmospheric radiocarbon concentration at ca. 40 kyr BP during the Laschamp geomagnetic reversal, which has been implemented into radiocarbon calibration curves with the publication of IntCal20 (Reimer et al. 2020). Our record provides new insights in the climatic influence on stalagmite radiocarbon and as to how precision and accuracy of calibration can benefit from comprehensive multi-proxy stalagmite records.</span></p><p><span>References</span></p><p><span>Cheng, H., Lawrence Edwards, R., Southon, J., et al.: Atmospheric 14C/12C changes during the last glacial period from Hulu cave, Science, 362(6420), 1293–1297, doi:10.1126/science.aau0747, 2018.</span></p><p><span>Reimer, P. J., Austin, W. E. N., Bard, E., Bayliss, A., et al.: The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 cal kBP), Radiocarbon, 62(4), 725-757. doi:10.1017/RDC.2020.41</span></p>

Extended dilation of the radiocarbon time scale between 40,000 and 48,000 y BP and the overlap between Neanderthals andHomo sapiens

The new radiocarbon calibration curve (IntCal20) allows us to calculate the gradient of the relationship between14C age and calendar age over the past 55 millennia before the present (55 ka BP). The new gradient curve exhibits a prolonged and prominent maximum between 48 and 40 ka BP during which the radiocarbon clock runs almost twice as fast as it should. This radiocarbon time dilation is due to the increase in the atmospheric14C/12C ratio caused by the14C production rise linked to the transition into the Laschamp geomagnetic excursion centered around 41 ka BP. The major maximum in the gradient from 48 to 40 ka BP is a new feature of the IntCal20 calibration curve, with far-reaching impacts for scientific communities, such as prehistory and paleoclimatology, relying on accurate ages in this time range. To illustrate, we consider the duration of the overlap between Neanderthals andHomo sapiensin Eurasia.

The IntCal20 Approach to Radiocarbon Calibration Curve Construction: A New Methodology Using Bayesian Splines and Errors-in-Variables

ABSTRACTTo create a reliable radiocarbon calibration curve, one needs not only high-quality data but also a robust statistical methodology. The unique aspects of much of the calibration data provide considerable modeling challenges and require a made-to-measure approach to curve construction that accurately represents and adapts to these individualities, bringing the data together into a single curve. For IntCal20, the statistical methodology has undergone a complete redesign, from the random walk used in IntCal04, IntCal09 and IntCal13, to an approach based upon Bayesian splines with errors-in-variables. The new spline approach is still fitted using Markov Chain Monte Carlo (MCMC) but offers considerable advantages over the previous random walk, including faster and more reliable curve construction together with greatly increased flexibility and detail in modeling choices. This paper describes the new methodology together with the tailored modifications required to integrate the various datasets. For an end-user, the key changes include the recognition and estimation of potential over-dispersion in 14C determinations, and its consequences on calibration which we address through the provision of predictive intervals on the curve; improvements to the modeling of rapid 14C excursions and reservoir ages/dead carbon fractions; and modifications made to, hopefully, ensure better mixing of the MCMC which consequently increase confidence in the estimated curve.

Composition and consequences of the IntCal20 radiocarbon calibration curve

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.

Reanalysis of the Atmospheric Radiocarbon Calibration Record from Lake Suigetsu, Japan

ABSTRACTTerrestrial plant macrofossils from the sedimentary record of Lake Suigetsu, Japan, provide the only quasi-continuous direct atmospheric record of radiocarbon (14C) covering the last 50 ka cal BP (Bronk Ramsey et al. 2012). Since then, new high precision data have become available on U-Th dated speleothems from Hulu Cave China, covering the same time range (Cheng et al. 2018). In addition, an updated varve-based chronology has also been published for the 2006 core from Lake Suigetsu (SG06) based on extended microscopic analysis of the sediments and improved algorithms for interpolation (Schlolaut et al. 2018). Here we reanalyze the radiocarbon dataset from Suigetsu based on the new varve counting information and the constraints imposed by the speleothem data. This enables the new information on the calendar age scale of the Suigetsu dataset to be used in the construction of the consensus IntCal calibration curve. Comparison of the speleothem and plant macrofossil records provides insight into the mechanisms underlying the incorporation of carbon into different types of record and the relative strengths of different types of archive for calibration purposes.