Professor Richard G. Fairbanks

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Current Research

Radiocarbon Calibration

 
 

The records of the 14C content of the atmosphere and oceans contain a remarkable array of information about Earth history. Produced by cosmic rays in the upper atmosphere, 14CO2 rapidly mixes throughout the troposphere and exchanges with the reactive carbon reservoirs of the oceans and biosphere, where it decays. For the past 11,000 years, fluctuations in the atmospheric 14C have been largely produced by changes in the solar magnetic field. Many researchers believe that carbon cycle changes, tied to deep ocean circulation changes are a significant cause of atmospheric 14C fluctuations between 11,000 and 15,000 years before present. On longer time scales, changes in the Earth’s magnetic field intensity impact the 14C content of the atmosphere, producing positive 14C anomalies during intervals of weaker geomagnetic field.

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Of practical importance to a wide range of scientific disciplines is the radiocarbon calibration, which is used for converting radiocarbon ages to calendar years; essential for measuring time and rates of change for numerous scientific fields. To access our radiocarbon calibration program, click on the 'Radiocarbon Calibration Program' button above, or here.

Arguably, few research topics engage so many different fields of science and have such a profound impact on our understanding of Earth and Solar science as the history of 14C in the Earth's atmosphere and the surface and deep oceans. Over the past decade we have witnessed a remarkable development and proliferation of accelerator mass spectrometers; these instruments have reduced the counting time by a factor of 100 and reduced the sample size by a factor of 1000 compared to the classic B-counting systems. It is estimated that nearly 90% of all measurements made at the more than 50 active accelerator mass spectrometry laboratories are radiocarbon dates. This dramatic increase in the number of radiocarbon dates is driving the demand for a radiocarbon calibration program that spans the entire radiocarbon timescale from the present to 55,000 years BP. Extension of the 14C record beyond the 0 to 11,900 year long tree ring record is well underway, being measured in many different archives, and undoubtedly an enormous amount of scientific knowledge will stem from these studies. In our laboratory, we have overlapped and extended the tree-ring radiocarbon calibration from 3,000 to 55,000 yrs. BP using coral samples from our offshore coral reef core collections from Barbados (13.10°N; 59.32°W) in the western tropical Atlantic and Kiritimati Atoll (1.99°N, 157.78°W) in the central equatorial Pacific, and from the uplifted reefs of Araki Island (15.63°S; 166.93°E) in the western Pacific. In addition, we have reanalyzed the radiocarbon and 230Th/234U/238U age dates from our earlier radiocarbon calibration work using new pretreatment and analytical techniques and state-of-the-art instrumentation at higher precision; we report these new results in this WEB site and in Fairbanks et al., (2005).

In our radiocarbon calibration paper (Fairbanks et al., 2005), we present paired 230Th/234U/238U (Lamont) and 14C age determinations (Lawrence Livermore National Lab and Leibniz-Labor for Radiometric Dating and Isotope Research Christian-Albrechts University Kiel) that span the entire range of the radiocarbon dating technique and present a radiocarbon calibration curve based on a Bayesian statistical model with rigorous error estimations. Due to the importance of an accurate and precise radiocarbon calibration curve, we have measured many samples in duplicate and validated the quality of the samples by dating the older samples with redundant 231Pa/235U dates. Our online radiocarbon calibration curve presented in this WEB site is a stand alone alternative to existing radiocarbon calibration curves that infer calendar ages based on interpolations and correlations of local climate proxies in deep-sea cores to the chronology of ice core proxies or assumptions about sedimentation rates. Our calibration has the advantage that each data point in the calibration has a measured calendar age (230Th/234U/238U) and radiocarbon age with know errors that are independent from each other. In a series of published papers and manuscripts soon to be published, we present our analytical techniques in detail (Mortlock et al., 2005; Chiu et al., 2005a) and the geochemical (Cao et al., 2005) and geophysical (Chiu et al., 2005b, 2005c) explanations for the departure of radiocarbon dates from the true calendar ages and compare our results to other radiocarbon calibration data. View the entire radiocarbon calibration curve here.

Comparisons of Fairbanks Radiocarbon Calibration data and curve and IntCal04 data and curve

The tree ring atmospheric radiocarbon calibration data set spanning 0 to 12,410 years BP is superior to all other atmospheric radiocarbon calibration data due to the number and quality of the radiocarbon measurements and the accuracy and precision of the tree dendrochronology (Stuiver et al., 1998; Reimer et al., 2004). The Fairbanks curve (Fairbanks et al., 2005; 2007) and IntCal04 curve (Reimer et al., 2004) use the same tree ring data set from 0 to 12,410 cal yr B.P. and the two different radiocarbon calibration programs yield nearly identical results over this interval (See CalPal web site for a comparison). There are presently two choices for calibration curves beyond 12,410 years BP: IntCal04 (Hughen et al., 2004) to 26,000 years or our calibration curve (Fairbanks et al., 2005) from 0 to 55,000 years BP. In our judgment the choice is clear but let us review the differences. The IntCal04 includes predominantly laminated sediments from the Cariaco Basin from 11,900 to 14,000 years BP along with coral data from numerous investigators of varying sample quality based on the reported presence of calcite in samples analyzed by some of the contributors. In contrast, our calibration program (Fairbanks et al., 2005; 2007) uses the 1382-ring floating tree ring data set from Kromer et al., (2004) for the time interval between 12,600 and 14,000 yrs BP anchored by our coral data set. Over this time interval, the Caricao data set used by IntCal04 differs by more than 200 years from the tree ring and our coral calibration data. The photo images of the Cariaco sediment layering (Hughen et al., 2004) shows very weak to indistinct layers in the 13,250 to 14,000 year interval where the offset of Cariaco data with the tree ring and coral data sets is most apparent. In a recent study (*Bondevik et al., 2006), Reimer, the lead author of the IntCal04 paper) and her colleagues retract the Cariaco calibration data in IntCal04 data set and IntCal04 calibration curve and inserted the floating tree ring data set in its place. In their 2006 paper they state that:

 The part of the new calibration curves that relies on tree-ring evidence (IntCal04) dates back to 12,410 calendar (cal) yr B.P. Beyond that and back to 14,700 cal yr B.P., IntCal04 is mainly constructed from 14C dates of foraminiferas from Venezuela.s Cariaco basin that are corrected for a constant reservoir age of 405 years. However, it has been proposed that the latter increased up to 650 years during the Bolling/Allerod. We therefore replaced that part of IntCal04 with a new, floating tree-ring curve composed of 1382 rings between 10,650 and 12,000 14C yr B.P. . Thus our tree-ring record from IntCal04, combined with the new, floating tree-ring curve, represents a true terrestrial curve that extends across most of the studied interval.  

All previous attempts to use layered sediments for radiocarbon calibration purposes have not stood the test of time and the Cariaco basin calibration data used in IntCal04 are apparently no exception (Bondevik et al., 2006).

From 14,000 to 26,000 years BP, 80% of the data in the IntCal04 were provided by our laboratory and we have subsequently doubled the amount of data in the 14,000 to 26,000 year BP time interval (Cao et al., 2006; Fairbanks et al., 2005; 2006).

IntCal04 ends at 26,000 cal yr B.P. representing only 50% of the radiocarbon age dating range. Our calibration extends to the limits of radiocarbon age dating to 50,000 cal yrs B.P. using the same coral sample and data quality control measures and dating techniques as applied to our younger samples and those we provided to IntCal04.

Another difference between the selections of IntCal04 versus our calibration program is philosophical. Seven years elapsed between publication of IntCal98 and IntCal04 although the majority of new data between 14,000 and 26,000 are from our laboratory. It is unclear when the next internationally ratified calibration curve will be approved but it is presumably many years away. Meanwhile, Reimer and colleagues (Bondevik et al., 2006) have already retracted a segment of the Cariaco layered sediment data set used in IntCal04 but the .official. IntCal04 file has not been revised. On the other hand, we are increasing the number of new calibration data pairs by 50% per year over the next 3 years. We appreciate the value of a stable internationally-ratified radiocarbon calibration curve, however researchers want the best available calibration curve in order to make the most accurate and precise conversion of radiocarbon years to calendar years. Considering the dramatic increase in ocean, climate, and archeological research spanning the deglacial, glacial and Marine Isotope Stage 3, there is a high and growing demand for radiocarbon calibration. We recommend that researchers and editors always list their raw radiocarbon ages and laboratory sample identification code and simply identify the calibration version used for their calibrated ages. This permits anyone to revise calibrated ages as our calibration curve is periodically updated over the next three years. In order to ensure that there is not a proliferation of calibration curves using outdated versions of our calibration data, we only offer an on line version. We will update our calibration curve every year and publish the data within 12 months of the update. To better serve the archeological community, we will also release the calibration data and access to our curve to CalPal at the same time we update our curve. The curve will be updated every year on Jan 1 and versions are identified as Fairbanks followed by the month and year such as Fairbanks0107 for the upcoming version. There are already enough calibration data such that our curve is stable and the primary improvements with future versions will be in the reduction in computed calendar year uncertainties. We continue to measure the 231Pa/235U ages of the samples that have been exposed to vados fresh water in order to provide quality assurance. Our goal is to have redundant 231Pa/235U dates on all calibrations ages between 30,000 and 55,000 years BP within one year of publishing new 230Th/234U/238U dates.

*Bondevik, S., J. Mangerud, H.H. Birks, S. Gulliksen and P. Reimer, 2006. Changes in North Atlantic radiocarbon reservoir ages during the Allerod and Younger Dryas. Science, 312, 1514-1517.

Publications:

  • [PDF] Chiu, T-C, R. G. Fairbanks, Li Cao, Richard A. Mortlock, 2007. Analysis of the atmospheric 14C record spanning the past 50,000 years derived from high-precision 230Th/234U/238U and 231Pa/235U and 14C dates on fossil corals. Quaternary Science Reviews, 26, 18-36.
  • [PDF] Chiu, T-C, R.G. Fairbanks, R.A. Mortlock, L. Cao, T.W. Fairbanks, and A.L. Bloom, 2006. Redundant 230Th/234U/238U and 231Pa/235U dating of fossil corals: verification of U-series ages for radiocarbon calibration. Quaternary Science Reviews, in press.
  • [PDF] Fairbanks, R.G., T-C Chiu, Li Cao, Richard A. Mortlock and Alexey Kaplan, 2006. Reply to the comment by Yusuke Yokoyama and Tezer M. Esat. Quaternary Science Reviews Correspondence, 26, 3084-3087.
  • [PDF] Chiu, T-C, R.G. Fairbanks, R.A. Mortlock and A.L. Bloom, 2005. Extending the radiocarbon calibration beyond 26000 years before present using fossil corals. Quaternary Science Reviews, 24, 1797-1808.
  • [PDF] Fairbanks, R.G., R.A. Mortlock, T.-C. Chiu, L. Cao, A. Kaplan, T.P. Guilderson, T.W. Fairbanks and A.L. Bloom, 2005. Marine Radiocarbon Calibration Curve Spanning 10,000 to 50,000 Years B.P. Based on Paired 230Th/234U/238U and 14C Dates on Pristine Corals. Quaternary Science Reviews, 24, 1781-1796.
  • [PDF] Mortlock, R.A., R.G. Fairbanks, T. Chiu, and J. Rubenstone, 2005. 230Th/234U/238U and 231Pa/235U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry. Geochim. et Cosmochim. Acta, 69, 3, 649-657.
  • [PDF] Hughen, KA, Baillie, MGL, Bard, E, Bayliss, A, Beck, JW, Blackwell, PG, Buck, CE, Burr, GS, Cutler, KB, Damon, PE, Edwards, RL, Fairbanks, RG, Friedrich, M, Guilderson, TP, Herring, C, Kromer, B, McCormac, FG, Manning, SW, Ramsey, CB, Reimer, PJ, Reimer, RW, Remmele, S, Southon, JR, Stuiver, M, Talamo, S, Taylor, FW, van der Plicht, J, and Weyhenmeyer, CE. 2004. Marine04 Marine radiocarbon age calibration, 0-26 cal kyr BP. Radiocarbon 46, 3, 1059-1086.
  • [PDF] Reimer, PJ, Baillie, MGL, Bard, E, Bayliss, A, Beck, JW, Blackwell, PG, Buck, CE, Burr, GS, Cutler, KB, Damon, PE, Edwards, RL, Fairbanks, RG, Friedrich, M, Guilderson, TP, Herring, C, Hughen, KA, Kromer, B, McCormac, FG, Manning, SW, Ramsey, CB, Reimer, PJ, Reimer, RW, Remmele, S, Southon, JR, Stuiver, M, Talamo, S, Taylor, FW, van der Plicht, J, and Weyhenmeyer, CE. 2004. IntCal04 Terrestrial radiocarbon age calibration, 0-26 cal kyr BP. Radiocarbon 46, 3, 1029-1058.
  • [PDF] Shackleton, N.J., R.G. Fairbanks, T-C Chiu, and F. Parrenin, 2004. Absolute calibration of the Greenland time scale: implications for Antarctic time scales and for Δ14C. Quaternary Science Reviews, 23, 1513-1522.
  • Bard, E., M. Arnold, R.G. Fairbanks and B. Hamelin, 1993. 230Th/234U and 14C ages obtained by mass spectrometry on corals. In Stuiver, M., A. Long, and R.S. Kra, eds. Calibration 1993. Radiocarbon, 35, 1, 191-200.
  • [PDF] Bard, E., B. Hamelin, R.G. Fairbanks, A. Zindler, 1990. Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals. Nature, 345, 405-410.
  • Bard, E., B. Hamelin, R.G. Fairbanks, A. Zindler, G. Mathieu, M. Arnold, 1990. U/Th and 14C ages of corals from Barbados and their use for calibrating the 14C time scale beyond 9000 years BP. Nuclear Instruments and Methods, B52, 461-468.
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