This article traces the metrological history of radiocarbon, from the initial breakthrough devised by Libby, to minor (evolutionary) and major (revolutionary) advances that have brought (14)C measurement from a crude, bulk [8 g carbon] dating tool, to a refined probe for dating tiny amounts of precious artifacts, and for "molecular dating" at the 10 µg to 100 µg level. The metrological advances led to opportunities and surprises, such as the non-monotonic dendrochronological calibration curve and the "bomb effect," that gave rise to new multidisciplinary areas of application, ranging from archaeology and anthropology to cosmic ray physics to oceanography to apportionment of anthropogenic pollutants to the reconstruction of environmental history. Beyond the specific topic of natural (14)C, it is hoped that this account may serve as a metaphor for young scientists, illustrating that just when a scientific discipline may appear to be approaching maturity, unanticipated metrological advances in their own chosen fields, and unanticipated anthropogenic or natural chemical events in the environment, can spawn new areas of research having exciting theoretical and practical implications.
SRM 1649a; accelerator mass spectrometry; apportionment of fossil and biomass carbon; dual isotopic authentication; metrological history; molecular dating; radiocarbon dating; the Turin Shroud; “bomb” 14C as a global tracer.
Portrait of W. F. Libby, about the time of publication of the first edition of his monograph,
Radiocarbon Dating (1952), and statement of the Nobel Committee (1960) .
Graphical representation of the production, distribution, and decay of natural
14C (courtesy of D. J. Donahue).(Parameter values are approximate.)
Low-level anticoincidence counting apparatus devised by Libby for the original
14C measurements that led to the establishment of the radiocarbon dating technique (Ref. , and Radiocarbon Dating (jacket cover) R. Berger and H. Suess, eds., Univ. California Press, Berkeley (1979).)
Radiocarbon dating validation curve (1952): the “curve of knowns” that first demonstrated that absolute radiocarbon dating “worked.” The validation points represent tree rings and historical artifacts of known age. The exponential function is
not fit to the data, but derived from the independently measured half-life and the 14C content of living matter (, Fig. 1).
Radiocarbon Variations, discovered by comparison of high precision radiocarbon “dates” with high (annual) accuracy tree ring dates. The plot, which covers the period from about 5000 BC to the present, represents an early version of the radiocarbon dating calibration curve (, p. 110). The photo shows the Bristlecone pine, the major source of dendrodates extending back many millennia (Photo is courtesy of D. J. Donahue).
Radiocarbon Variations and Climate: the influence of solar activity (sunspot record) (top) on
14C concentrations (cosmic ray production rates) and climate (Maunder Minimum temperature record) (bottom) [15, 16].
Input function of excess (“bomb”)
14C: a global tracer for carbon cycle dynamics in the atmosphere, biosphere, and oceans .
14C and ocean circulation (GEOSECS). Model (left) and experimental (right) vertical transects of bomb 14C in the North Atlantic .
Polypropylene Terephthalate: biomass and fossil feedstocks. The 1,3, propanediol monomer is derived from a renewable (biomass) feedstock via laboratory biotechnology: conversion of glucose or cornstarch using a single microorganism. The copolymer has potential large volume demand, and is useful as a fiber, film, particle, and a molded article .
Unique Isotopic Signatures: the
14C- 13C plane . The main panel shows dual isotopic signatures f:or (1) NIST (S1, S3) and IAEA (S2) traceability standards, and (2) glucose from biomass (a), the new bio-sourced monomer 3G (b) (from cornstarch), the resulting copolymer 3GT (d), and pre-existing products 3G′, 3G″ (c, e). Expanded views of the authentication regions (red rectangles) for the copolymer (left) and monomer (center) are given in the bottom panels, plus ≈10-fold expansion (right) of the isotopic data for independent batches (A, B) of a biomass feedstock (glucose from corn). The blue “x” represents a blind (3G) validation sample.
14C “decay” curve, representing geochemical relaxation of excess atmospheric 14C from nuclear testing [Levin et al., in (Ref. ; Ref. , Chap. 31). Information critical for the discussion in Sec. 7.2.1 is indicated by the arrow—namely, the sampling date and corresponding biomass 14C enrichment for SRM 1649a (urban dust).
14C variations: fossil-biomass carbon apportionment of particulate air pollution in Albuquerque, New Mexico. (Photos showing visibility reduction in early morning (top) and mid-afternoon (bottom) are courtesy of R.K. Stevens .). 14C measurements quantified atmospheric soot from motor vehicles and residential wood-burning, and helped apportion concomitant data on particulate mutagenicity.
AMS: tandem accelerator at ETH, Zürich. Negative carbon ions, produced with a Cs
+ sputter ion source, undergo low energy mass resolution and then are injected into the 4.5 MV accelerator tube. Molecular ions are destroyed by the stripper gas, and emerging 18 MeV C +3 beams of 12C, 13C, and 14C are mass analyzed and measured in current (stable C ions) and event ( 14C ions) detectors .
Conventional (top) vs accelerator (high energy) (bottom) mass spectrometry:
14C/ 12C sensitivity is enhanced by more than eight orders of magnitude through destruction of molecular ions (and unstable N −) (Ref. , Chap. 16).
The Turin Shroud. Shown in the montage are: (15a, upper left), the cover of the issue of
Nature (16 February 1989) reporting the results of the 14C measurements by AMS laboratories in Tucson, Zürich, and Oxford; and three singular features of the artifact: (15b, lower left), the ≈50 mg dating sample received by the Tucson laboratory, showing the distinctive weave (3:1 herringbone twill), with dimensions about 1 cm × 0.5 cm; (15c, upper right), the characteristic negative image, considered by some as a remarkable piece of mediaeval art; and (15d, lower right), a microphotograph by Max Frei showing individual fibers supporting pollen grains of presumed unique origin [38, 39].
14C dating results (“blind”) for the Turin Shroud (sample-1) and three control samples of known age (samples-2,3,4), from the three AMS laboratories: Z (Zürich), O (Oxford), and A (Arizona). Dates are expressed as “Radiocarbon Years” before present (BP); uncertainties represent 95 % confidence intervals .
Transformation of the Radiocarbon Age (BP) to the Calendar Age (AD) of the Shroud. The
14C age (95 % CI) of (691 ± 31) BP corresponds to a two-valued calendar age as a result of the non-monotonic radiocarbon dating calibration curve. As indicated in the figure, the projected calendar age ranges are: (1262–1312) AD and (1353–1384) AD .
14C apportionment of anthropogenic and natural carbonaceous aerosols at remote sites in Europe and Greenland provides knowledge of their impacts on present and paleoclimate [–51].
Massive (6 d, 3000 km) transport of soot from boreal wildfires in Canada to Summit, Greenland. Left inset : AVHRR satellite image of wildfire region ; Center : 6 d backtrajectories ; Right inset : seven-fold,1 d increase in biomass-C (upper, red curve) at Summit [S] on 5 Aug 1994; fossil-C (lower, black curve), was little changed [49, 29]).
Direct, time lapse observation of the track of the August 1994 smoke parcel by TOMS (differential UV) satellite imagery . Consistent with the
14C (biomass carbon) data, the cloud of smoke, indicated by the light turquoise circles, is present over central Greenland for 1 day only, 5 August 1994.
First evidence of a seasonal pattern in biomass carbon aerosol in surface snow in central Greenland [55, 56]. Fundamental differences were found between the biomass carbon peaks in summer (sample-4 [WO4]), and spring (sample-8 [WO8]) via “multi-spectroscopic” macro- and micro-chemical analysis.
PCA biplot of laser microprobe mass spectral data; compositional contrast between particles from the summer biomass peak (WO4, red: C
n − cluster ions favored) and the spring biomass peak (WO8, green: oxygenates favored) .
“Molecular Dating” of individual amino acids in ancient bone. Radiocarbon ages of commonly dated (collagen, gelatin) fractions were 2000 to 3000 years too young as a result of environmental degradation; pure molecular fractions (amino acids) were self-consistent and in agreement with the Clovis culture age .
NIST Standard Reference Material 1649a (“urban dust”). Photograph of the bulk reference material and derived “filter samples” for QA of atmospheric elemental carbon (EC).
14C data listed indicate the mass fraction (%) of biomass-C in the several chemical fractions [29, 62].
Gas chromatography/accelerator mass spectrometry (GC/AMS): AMS following automated prep-scale capillary GC yields “dates” (equivalent biomass carbon mass fractions) for micromolar amounts of individual polycyclic aromatic hydrocarbons [–65]. (Results shown for NIST SRM 1649a; “I.S.” denotes an internal standard; abscissa indicates retention time (min).)
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