Radiocarbon Calibration Research Articles

Combined Techniques to Date the First Turkish Bridge Over the Tisza River, Hungary

Given the fluctuating nature of the radiocarbon calibration curve, the precision of single 14C dates on the calendar timescale is limited. However, 14C dating combined with dendrochronology enabled us to date timbers found in the Tisza River, Hungary, during the dry period of summer 2003. Routine preparation of wood samples gave 14C results spread over 4 centuries. By extracting alpha-cellulose from the samples, 2 distinct and relatively narrow historical time periods were obtained: the first period (AD 1505–1595 and 1612–1673, respectively) coincided with the Turkish occupation period, while the second interval (1733–1813) obtained in the case of 2 samples did not exclude the existence of another bridge constructed later. The dendrochronological data confirmed that the bridge was constructed from oak timbers felled between 1558 and 1565. The 14C and dendrochronological dates correspond with the date of a letter written in 1562 by Antal Verancsics, Bishop of Eger, mentioning the construction of the first bridge. In conclusion, the archaeological excavation revealed proof of the first historically attested wooden bridge over the Tisza River.

Developments in radiocarbon calibration for archaeology

This update on radiocarbon calibration results from the 19th International Radiocarbon Conference at Oxford in April 2006, and is essential reading for all archaeologists. The way radiocarbon dates and absolute dates relate to each other differs in three periods: back to 12400 cal BP, radiocarbon dates can be calibrated with tree rings, and the calibration curve in this form should soon extend back to 18000 cal BP. Between 12400 and 26000 cal BP, the calibration curves are based on marine records, and thus are only a best estimate of atmospheric concentrations. Beyond 26000 cal BP, dates have to be based on comparison (rather than calibration) with a variety of records. Radical variations are thus possible in this period, a highly significant caveat for the dating of middle and lower Paleolithic art, artefacts and animal and human remains.

Dating ancient wood by high-sensitivity liquid scintillation counting and accelerator mass spectrometry—Pushing the boundaries

It is critical that a comprehensive terrestrial radiocarbon ( 14C) calibration curve is developed beyond 26 ka for high-precision calibration and correlation of climatic, environmental and archaeological records. Abundant sub-fossil New Zealand kauri ( Agathis australis) wood, preserved in Oxygen Isotope Stage-2 and 3 peat swamps, provides an important resource for 14C calibration; nowhere else in the world does such an extensive collection of ancient wood exist. Although finite 14C ages beyond 50 ka are becoming routinely reported, few attempts have been made to demonstrate their accuracy or precision. Finite ages beyond 50 ka require optimization of all elements involved in sample preparation and 14C analysis. Here we discuss the methodology employed for optimizing the 14C dating of near-background wood samples by both benzene synthesis for liquid scintillation counting (LSC) or graphite synthesis for accelerator mass spectrometry (AMS). We report the mean background blank activities for both methods and present a statistical model for assigning blank standard errors when blank activity variation is over and above counting statistics. We also present duplicate analyses (using LSC and AMS) of nine successive samples of wood obtained from a sub-fossil kauri log near-background in age to investigate the significance of the measured blank levels and variability.

Bayesian nonparametric estimation of the radiocarbon calibration curve

The process of calibrating radiocarbon determinations onto the calendar scale involves, as a first stage, the estimation of the relationship between calendar and radiocarbon ages (the radiocarbon calibration curve) from a set of available high-precision calibration data. Traditionally the radiocarbon calibration curve has been constructed by forming a weighted average of the data, and then taking the curve as the piece-wise linear function joining the resulting calibration data points. Alternative proposals for creating a calibration curve from the averaged data involve a spline or cubic interpolation, or the use of Fourier transformation and other filtering techniques, in order to obtain a smooth calibration curve. Between the various approaches, there is no consensus as to how to make use of the data in order to solve the problems related to the calibration of radiocarbon determinations. We propose a nonparametric Bayesian solution to the problem of the estimation of the radiocarbon calibration curve, based on a Gaussian process prior structure on the space of possible functions. Our approach is model-based, taking into account specific characteristics of the dating method, and provides a generic solution to the problem of estimating calibration curves for chronology building. We apply our method to the 1998 international high-precision calibration dataset, and demonstrate that our model predictions are well calibrated and have smaller variances than other methods. These data have deficiencies and complications that will only be unravelled with the publication of new data, expected in early 2005, but this analysis suggests that the nonparametric Bayesian model will allow more precise calibration of radiocarbon ages for archaeological specimens.

Redundant 230Th/ 234U/ 238U, 231Pa/ 235U and 14C dating of fossil corals for accurate radiocarbon age calibration

230Th/ 234U/ 238U dating of fossil corals by mass spectrometry is remarkably precise, but some samples exposed to freshwater over thousands of years may gain and/or lose uranium and/or thorium and consequently yield inaccurate ages. Although a δ 234U initial value equivalent to modern seawater and modern corals has been an effective quality control criterion, for samples exposed to freshwater but having δ 234U initial values indistinguishable from modern seawater and modern corals, there remains a need for additional age validation in the most demanding applications such as the 14C calibration (Fairbanks et al., 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(16–17), 1781–1796). In this paper we enhance screening criteria for fossil corals older than 30,000 years BP in the Fairbanks0805 radiocarbon calibration data set (Fairbanks et al., 2005) by measuring redundant 230Th/ 234U/ 238U and 231Pa/ 235U dates via multi-collector magnetic sector inductively coupled plasma mass spectrometry (MC-MS-ICPMS) using techniques described in Mortlock et al. (2005. 230Th/ 234U/ 238U and 231Pa/ 235U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry. Geochimica et Cosmochimica Acta 69(3), 649–657.). In our present study, we regard paired 231Pa/ 235U and 230Th/ 234U/ 238U ages concordant when the 231Pa/ 235U age (±2 σ) overlaps with the associated 230Th/ 234U/ 238U age (±2 σ). Out of a representative set of 11 Fairbanks0805 (Fairbanks et al., 2005) radiocarbon calibration coral samples re-measured in this study, nine passed this rigorous check on the accuracy of their 230Th/ 234U/ 238U ages. The concordancy observed between 230Th/ 234U/ 238U and 231Pa/ 235U dates provides convincing evidence to support closed system behavior of these fossil corals and validation of their 230Th/ 234U/ 238U dates for radiocarbon calibration and Δ 14C reconstruction.

Late Holocene human remains from northwest Queensland, Australia: archaeology and palaeopathology

AbstractA human burial of late Holocene age was recently excavated from inland northwest Queensland and studied prior to reburial by the Indigenous community. Bones from the lower thoracic region to the feet were recovered. The person had been interred in a crouched position, resting on their lower legs (shins) and wrapped in paperbark. Similar burial techniques have been described in the region's ethnographic literature, and this site represents the first known archaeological example.Ascertaining a firm date for the burial is problematic owing to the nature of the radiocarbon calibration curve in recent centuries. A detailed analysis of the bones indicated the individual to be an adult female, most likely of middle age. There are some significant pathological lesions present that are indicative of treponematosis. The geographic and cultural context of the burial leads us to suggest the most likely diagnosis is treponarid.

Extension of the New Zealand kauri (Agathis australis) chronology to 1724 BC

Long tree-ring chronologies have been constructed in the Northern Hemisphere for dendroclimatology and palaeoenvironmental studies, radiocarbon calibration and archaeological dating. Numerous tree-ring chronologies have also been built in the Southern Hemisphere, primarily for dendroclimatology, but multimillennial chronologies are rare. Development of long chronologies from the Southern Hemisphere is therefore important to provide a long-term perspective on environmental change at local, regional and global scales. This paper describes the extension of the New Zealand Agathis australis (kauri) chronology from AD 911 to 1724 BC. Subfossil (swamp) kauri was collected from 17 swamp sites in the upper North Island. Kauri timbers were also obtained from an early twentieth century house on the University of Auckland campus. Twelve site chronologies and 11 independent tree-sequences were constructed and crossmatched to produce a 3631-yr record, which was calendar dated to 1724 BC-AD 1907 against the modern kauri master chronology. A new long chronology, AGAUc04a, was built by combining the modern kauri data with house timbers and subfossil kauri. This new chronology spans 1724 BC-AD 1998. It is of similar length to chronologies from Tasmania and South America and is the longest tree-ring chronology yet built in New Zealand. The greatest significance of the long kauri chronology lies in its potential as a high-quality palaeoclimate proxy, especially with regard to investigation of the El Ninio-Southern Oscillation phenomenon. The chronology also has application to investigation of extreme environmental events, dendroecology, archaeology and radiocarbon calibration.

Important features of the new radiocarbon calibration curves

Calibration from the radiocarbon to the calendar scale is essential for developing chronologies in palaeoenvironmental or archaeological studies. We report here on the new internationally agreed radiocarbon calibration curves, which replace the 1998 curves. We discuss some of the important features of the curves and illustrate how modern Bayesian statistical methods and groups of related radiocarbon determinations can considerably improve chronological resolution.

Testing solar forcing of pervasive Holocene climate cycles

AbstractThe temporal and spatial extent of Holocene climate change is an area of considerable uncertainty, with solar forcing recently proposed to be the origin of cycles identified in the North Atlantic region. To address these issues we have developed an annually resolved record of changes in Irish bog tree populations over the last 7468 years which, together with radiocarbon‐dated bog and lake‐edge populations, extend the dataset back to ∼9000 yr ago. The Irish trees underpin the internationally accepted radiocarbon calibration curve, used to derive a proxy of solar activity, and allow us to test solar forcing of Holocene climate change. Tree populations and age structures provide unambiguous evidence of major shifts in Holocene surface moisture, with a dominant cyclicity of 800 yr, similar to marine cycles in the North Atlantic, indicating significant changes in the latitude and intensity of zonal atmospheric circulation across the region. The cycles, however, are not coherent with changes in solar activity (both being on the same absolute timescale), indicating that Holocene North Atlantic climate variability at the millennial and centennial scale is not driven by a linear response to changes in solar activity. Copyright © 2005 John Wiley & Sons, Ltd.

Extension of the Swiss Lateglacial tree-ring chronologies

Fossil wood finds reveal a high resolution paleoclimatic proxy record of Lateglacial and Early Holocene. Eighty-one buried fossil pine stumps have been excavated on the construction site (Gaenziloo) of the A4-highway tunnel through Uetliberg near Zurich (Switzerland). The trees were buried during their lifetime by loamy alluvia washed down from the upper part of the slopes. The stumps have remained well preserved for more than 13,500 years. The cross sections of the trunks were analyzed dendrochronologically. The sections were dated by 14C, and 14C age vs. ring number were obtained from decadal sample segments. Three floating chronologies were built. They cover a time span of 428 years in the Mid-Allerød (GAEALLCH_A), 561 years in the Late-Allerød (GAEALLCH_D) and 212 years in the Younger Dryas (GAEYD_A). Visual synchronization, t-values, percentages of parallel variation (‘Gleichlaeufigkeit’) and radiocarbon wiggle matching ( 14C age determinations on a decadal scale) as well as a check by the program Cofecha support the validity of the resulting chronologies. The two Allerød-chronologies from Gaenziloo were linked with two chronologies from Daettnau (DAEALCH_1 and DAEALCH_2) published by Kaiser, K.F. (1993. Beiträge zur Klimageschichte vom Hochglazial bis ins frühe Holozän, rekonstruiert mit Jahrringen und Molluskenschalen aus verschiedenen Vereisungsgebieten. Ziegler Druck- und Verlags-AG, Winterthur. 206pp). They extend the existing floating Swiss chronology in the Late-Allerød by a total of 186 years. Unfortunately independent chronology GAEYD_A does not overlap with the actual absolute chronology developed by the tree-ring laboratory of the University of Hohenheim (Friedrich, M. et al., 2004. The 12,460 year Hohenheim oak and pine tree-ring chronology from Central Europe – A unique annual record for radiocarbon calibration and paleo-environment reconstructions. Radiocarbon 46(3), 1111–22.).

Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230Th/ 234U/ 238U and 14C dates on pristine corals

Radiocarbon dating is the most widely used dating technique in the world. Recent advances in Accelerator Mass Spectrometry (AMS) and sample preparation techniques have reduced the sample-size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes. Today, it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates. The production of 14C in the atmosphere varies through time due to changes in the Earth's geomagnetic field intensity and in its concentration, which is regulated by the carbon cycle. As a result of these two variables, a radiocarbon age is not equivalent to a calendar age. Four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12,000 calendar years before present. This paper presents high precision paired 230Th/ 234U/ 238U and 14C age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12,000 to 50,000 years before present. We developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years, taking full account of combined errors in input ages and calibration uncertainties. Our radiocarbon calibration program is publicly accessible at: http://www.radiocarbon.LDEO.columbia.edu/ along with full documentation of the samples, data, and our statistical calibration model.

Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals

Fossil coral is an excellent archive to extend the radiocarbon ( 14C) calibration beyond tree-ring records; however, published coral data older than 26,000 years before present are too disparate for practical calibration. We propose an explanation for these discrepancies: trace amounts of secondary calcite and organic matter in fossil corals shift the 14C ages toward younger ages. In a series of acid-leaching experiments to simulate the standard cleaning procedure prior to 14C analysis, we measure up to a 300% enrichment of diagenetic calcite in corals due to the solubility differences between calcite and aragonite. In model calculations, we show the 14C age offsets produced by acid leaching could be hundreds to thousands of years when typical amounts of calcite are present in samples. We demonstrate the necessity and our ability to detect <0.2% calcite in aragonite by X-ray diffraction and apply “<0.2% calcite” as our a priori criterion for our coral samples. The estimated age offsets of samples with <0.2% calcite fall within the reported analytical uncertainty of 14C dating. In addition, we present 14C results from an extended hydrogen peroxide (H 2O 2) pretreatment for coral samples to remove organic matter. Our data show that treated samples yield consistent or older 14C dates compared to non-treated samples. These rigorous but necessary screening and cleaning techniques provide precise, reproducible and accurate radiocarbon calibration data.

230Th/ 234U/ 238U and 231Pa/ 235U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry

The 230Th/ 234U/ 238U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system 230Th/ 234U/ 238U ages is elevated 234U/ 238U initial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have 234U/ 238U initial values close to seawater, there is a need for age validation. Redundant 230Th/ 234U/ 238U and 231Pa/ 235U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both 230Th/ 234U/ 238U and 231Pa/ 235U ages in fragments as small as 500 mg. We have obtained excellent agreement between 230Th/ 234U/ 238U and 231Pa/ 235U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the 230Th/ 234U/ 238U age accuracy. Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby 238U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of 230Th/ 234U/ 238U and 231Pa/ 235U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for 231Pa and from undetermined sources currently limit the 231Pa/ 235U age uncertainty to about ±2.5%. U isotope data and 230Th/ 234U/ 238U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.

Incorporating δ18O values of past waters in the calibration of radiocarbon dating

All scientists who study the late Quaternary are confronted by the problem of radiocarbon calibration. Since 1969, numerous high-frequency time series have been developed through the use of several stable isotopes (e.g., δ 1 8 O). Such time series are inefficient as dating methods because too many age possibilities are obtained for a given unique value. However, the Bayesian framework permits incorporation of these time series as a prior assumption on ages in order to improve calibrated ages obtained from a more precise absolute dating approach such as that using radiocarbon. The method is tested on data obtained from the Gigny cave (French Jura). Highly discontinuous posterior distributions are obtained with narrow highest posterior density regions. Thus, when the radiocarbon method alone gives large uncertainties, the incorporation of δ 1 8 O values of meteoric waters largely diminishes these uncertainties.

A revised sequence of backed artefact production at Capertee 3, New South Wales

AbstractRe‐analysis of the artefact assemblage from Capertee 3, an Australian rockshelter excavated by F.D. McCarthy in the 1950s and 1960s, yields a revised image of chronological changes in backed artefact production. A technologically‐defined sample of backed retouched flakes gives a new depiction of the vertical distribution of backed artefacts in this site. Analysis of artefact weathering indicates most specimens were probably altered in situ, with minimal large‐scale vertical displacement. Calibration of radiocarbon dates provides refined age‐depth estimates for the site. The result is identification of backed artefacts up to 6000 to 7000 years old, documentation of many backed specimens prior to 3500 cal b.p., and observation of only a relatively brief period, between 1500 and 3500 cal b.p., in which backed artefact production rates were extremely high. Changes in production rates are similar to those previously reported from Upper Mangrove Creek.

Radiocarbon calibration – past, present and future

Calibration of the Radiocarbon timescale is traditionally based on tree-rings dated by dendrochronology. At present, the tree-ring curve dates back to about 9900 BC. Beyond this limit, marine datasets extend the present calibration curve INTCAL98 to about 15600 years ago. Since 1998, a wealth of AMS measurements became available, covering the complete 14C dating range. No calibration curve can presently be recommended for the older part of the dating range until discrepancies are resolved.

Absolute calibration of the Greenland time scale: implications for Antarctic time scales and for Δ 14C

We propose a new age scale for the two ice cores (GRIP and GISP2) that were drilled at Greenland summit, based on accelerator mass spectrometry 14C dating of foraminifera in core MD95-2042 (Paleoceanography 15 (2000) 565), calibrated by means of recently obtained paired 14C and 230Th measurements on pristine corals (Marine radiocarbon calibration curve spanning 10,500 to 50,000 years BP (thousand years before present) Based on paired 230Th/ 234U/ 238U and 14C dates on Pristine Corals Geological Society of America Bulletin, 2003, submitted for publication). The record of core MD95-2042 can be correlated very precisely to the Greenland ice cores. Between 30 and 40 ka BP our scale is 1.4 ka older than the GRIP SS09sea time scale (Journal of Quaternary Science 16 (2001) 299). At the older end of Marine Isotope Stage 3 we use published 230Th dates from speleothems to calibrate the record. Using this scale we show a Δ 14C record that is broadly consistent with the modelled record (Earth Planet. Sci. Lett. 200 (2002) 177) and with the data of Hughen et al. (Science 303 (2004) 202), but not consistent with the high values obtained by Beck et al. (Science 292 (2001) 2453) or by Voelker et al. (Radiocarbon 40 (1998) 517). We show how a set of age scales for the Antarctic ice cores can be derived that are both fully consistent with the Greenland scale, and glaciologically reasonable.

Radiocarbon calibration ? past, present and future

Calibration of the Radiocarbon timescale is traditionally based on tree-rings dated by dendrochronology. At present, the tree-ring curve dates back to about 9900 BC. Beyond this limit, marine datasets extend the present calibration curve INTCAL98 to about 15 600 years ago. Since 1998, a wealth of AMS measurements became available, covering the complete 14 C dating range. No calibration curve can presently be recommended for the older part of the dating range until discrepancies are resolved.

Palaeo-climate reconstruction from stable isotope variations in speleothems: a review

Speleothems are now regarded as valuable archives of climatic conditions on the continents, offering a number of advantages relative to other continental climate proxy recorders such as lake sediments and peat cores. They are ideal materials for precise U-series dating, yielding ages in calendar years, thereby circumventing the radiocarbon calibration problems associated with most other continental records. Stable isotope studies in speleothems have shifted away from attempting to provide palaeo-temperature reconstructions to the attainable goal of providing precise estimates for the timing and duration of major O isotope-defined climatic events characterised by high signal to noise ratios (e.g. glacial/interglacial transitions, Dansgaard–Oeschger oscillations, the ‘8200-year’ event). Unlike the marine records, speleothem data sets are not ‘tuned’, and their independent chronology offers opportunities to critically assess leads and lags in the climate system, that in turn can provide important insights into forcing and feedback mechanisms. Improved procedures for the extraction and measurement of stable isotope ratios in fluid inclusions trapped in speleothems are likely to provide, in the near future, a much enhanced basis for the quantitative interpretation of O isotope ratios in speleothem calcite. The latter developments open up once again the tantalising prospect of palaeo-temperature estimates, but more importantly perhaps, provide a direct test for a new generation of general circulation models whose hydrological cycles will incorporate the ‘water isotopes’. The literature is reviewed briefly to provide for the reader a sense of the current state-of-the-art, and to provide some pointers for future research directions.

Radiocarbon calibration beyond 20,000 14C yr B.P. by means of planktonic foraminifera of the Iberian Margin

We present a new set of 14C ages obtained by accelerator mass spectrometry (AMS) on planktonic foraminifera from a deep-sea core collected off the Iberian Margin (MD952042). This site, at 37°N, is distant from the high-latitude zones where 14C reservoir age is large and variable. Many independent proxies — alkenones, magnetic susceptibility, ice-rafted debris, foraminifera stable isotopes, abundances of foraminifera, pollen, and dinoflagellates — show abrupt changes correlative with Dansgaard-Oeschger and Heinrich events of the last glacial period. The good stratigraphic agreement of all proxies — from the fine to the coarse-size fractions — indicates that the foraminifera 14C ages are representative of the different sediment fractions. To obtain reliable 14C ages of foraminifera beyond 20,000 14C yr B.P., we leached the shells prior to carbonate hydrolysis and subsequent analysis. For a calendar age scale, we matched the Iberian Margin U 37 K′ profile with that of Greenland Summit δ 18O. Both are proxies for temperature, which in models varies synchronously in the two areas. The match creates no spurious jumps in sedimentation rate and requires only a limited number of tie points. Except for ages older than 40,000 14C yr B.P., Greenland's GISP2 and GRIP records yield similar calendars. The 14C and imported calendar ages of the Iberian Margin record are then compared to data — from lacustrine annual varves and from corals and speleothems dated by U–Th — previously used to extend the calibration beyond 20,000 14C yr B.P. The new record follows a smooth pattern between 23,000 and 50,000 cal yr B.P. We find good agreement with the previous data sets between 23,000 and 31,000 cal yr B.P. In the interval between 33,000 and 41,000 cal yr B.P., for which previous records disagree by up to 5000 cal yr, the Iberian Margin record closely follows the polynomial curve that was previously defined by an interpolation of the coral ages and runs between the Lake Suigetsu and the Bahamian speleothem data sets.