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The IntCal family of radiocarbon (14C) calibration curves is based on research spanning more than three decades. The IntCal group have collated the 14C and calendar age data (mostly derived from primary publications with other types of data and meta-data) and, since 2010, made them available for other sorts of analysis through an open-access database. This has ensured transparency in terms of the data used in the construction of the ratified calibration curves. As the IntCal database expands, work is underway to facilitate best practice for new data submissions, make more of the associated metadata available in a structured form, and help those wishing to process the data with programming languages such as R, Python, and MATLAB. The data and metadata are complex because of the range of different types of archives. A restructured interface, based on the “IntChron” open-access data model, includes tools which allow the data to be plotted and compared without the need for export. The intention is to include complementary information which can be used alongside the main 14C series to provide new insights into the global carbon cycle, as well as facilitating access to the data for other research applications. Overall, this work aims to streamline the generation of new calibration curves.
The European Spallation Source (ESS) is a neutron-based research facility under construction in Lund in southern Sweden. The spallation reactions will generate not only the desired neutrons, but also many radioactive by-products, including 14C. As part of the licensing process, and as recommended by the IAEA, various preoperational studies are being carried out, including mapping the “zero-point” radiation environment around the site. As the city of Lund hosts several facilities using 14C-labeled substances, and since temporary and local 14C contamination have been observed in the past, 14C mapping is an important part of these baseline assessments. We here present a summary of 14C levels in various terrestrial environmental samples in Lund and in southern Sweden during the years 2012 to 2020. These environmental F14C do not display significantly elevated levels compared to sites located remote from Lund. We also describe a local 14C contamination event that was detected at the Lund Radiocarbon Dating Laboratory in 2009. Horse-chestnut leaves collected close to the laboratory exhibited F14C values of up to ∼25% above the clean air background. Elevated values of F14C were also found in a short tree-ring series, especially in 2007. The source of this contamination was identified and successfully removed.
We here present a comparison of methods for the pretreatment of a batch of tree rings for high-precision measurement of radiocarbon at the Aarhus AMS Centre (AARAMS), Aarhus University, Denmark. The aim was to develop an efficient and high-throughput method able to pretreat ca. 50 samples at a time. We tested two methods for extracting α-cellulose from wood to find the most optimal for our use. One method used acetic acid, the other used HCl acid for the delignification. The testing was conducted on background 14C samples, in order to assess the effect of the different pretreatment methods on low-activity samples. Furthermore, the extracted wood and cellulose fractions were analyzed using Fourier transform infrared (FTIR) spectroscopy, which showed a successful extraction of α-cellulose from the samples. Cellulose samples were pretreated at AARAMS, and the graphitization and radiocarbon analysis of these samples were done at both AARAMS and the radiocarbon dating laboratory at Lund University to compare the graphitization and AMS machine performance. No significant offset was found between the two sets of measurements. Based on these tests, the pretreatment of tree rings for high-precision radiocarbon analysis at AARAMS will henceforth use HCI for the delignification.
Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
Connecting calendar ages to radiocarbon (14C) ages, i.e. constructing a calibration curve, requires 14C samples that represent, or are closely connected to, atmospheric 14C values and that can also be independently dated. In addition to these data, there is information that can serve as independent tests of the calibration curve. For example, information from ice core radionuclide data cannot be directly incorporated into the calibration curve construction as it delivers less direct information on the 14C age–calendar age relationship but it can provide tests of the quality of the calibration curve. Furthermore, ice core ages on 14C-dated volcanic eruptions provide key information on the agreement of ice core and radiocarbon time scales. Due to their scarcity such data would have little impact if directly incorporated into the calibration curve. However, these serve as important “anchor points” in time for independently testing the calibration curve and/or ice-core time scales. Here we will show that such information largely supports the new IntCal20 calibration record. Furthermore, we discuss how floating tree-ring sequences on ice-core time scales agree with the new calibration curve. For the period around 40,000 years ago we discuss unresolved differences between ice core 10Be and 14C records that are possibly related to our limited understanding of carbon cycle influences on the atmospheric 14C concentration during the last glacial period. Finally, we review the results on the time scale comparison between the Greenland ice-core time scale (GICC05) and IntCal20 that effectively allow a direct comparison of 14C-dated records with the Greenland ice core data.
As part of the ongoing effort to improve the Northern Hemisphere radiocarbon (14C) calibration curve, this study investigates the period of 856 BC to 626 BC (2805–2575 yr BP) with a total of 403 single-year 14C measurements. In this age range, IntCal13 was constructed largely from German and Irish oak as well as Californian bristlecone pine 14C dates, with most samples measured with a 10-yr resolution. The new data presented here is the first atmospheric 14C single-year record of the older end of the Hallstatt plateau based on an absolutely dated tree-ring chronology. The data helped reveal a major solar proton event (SPE) which caused a spike in the production rate of cosmogenic radionuclides around 2610/2609 BP. This production event is thought to have reached a magnitude similar to the 774/775 AD production event but has remained undetected due to averaging effects in the decadal calibration data. The record leading up to the 2610/2609 BP event reveals a 11-yr solar cycle with varying cyclicity. Features of the new data and the benefits of higher resolution calibration are discussed.
We used the radiocarbon wiggle-match dating technique to date the varved sediments of Lake Gyltigesjön in southern Sweden with the main aim to construct an accurate chronology covering the period between about 3000 and 2000 cal BP. Wiggle-match dating was applied to bulk sediments to evaluate the possibility of constructing accurate chronologies in the absence of terrestrial plant macrofossils and when the amount of old carbon in the sediments is unknown. Facilitated by a floating varve chronology and relatively stable 14C reservoir ages, the results show the possibility to assess the contribution of old carbon solely based on the 14C wiggle-matching of bulk sediments. We confirm the wiggle-matched chronology and the 14C reservoir age of approximately 260 yr by cross-checking the results with 14C dating of macrofossils. The obtained calibrated ages based on bulk sediments have an uncertainty range of about 60–65 yr (95.4% confidence interval). This study confirms that 14C wiggle-match dating of bulk sediments is a viable tool when constructing high-resolution chronologies. The method is especially useful in Sun-climate studies since the timing between solar activity variations (expressed as 14C variations) and climate changes can be accurately determined.
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