1 HISTORY OF RADIOCARBON DATING ORIGINS: Willard F. Libby ( ) Libby at UC Berkeley: In the Beginning Libby at UC Berkeley: In the Beginning Libby at Columbia: The War Years Libby at Columbia: The War Years Libby at Chicago: Three Critical Experiments Libby at Chicago: Three Critical ExperimentsDEVELOPMENT First Radiocarbon Revolution First Radiocarbon Revolution Second Radiocarbon Revolution: Calibration Second Radiocarbon Revolution: Calibration Third Radiocarbon Revolution: AMS Third Radiocarbon Revolution: AMS

Libby at Berkeley In the Beginning... In the Beginning... Undergraduate (Chemistry): » Graduate (Physical Chemistry): Early 1930s: 14 C = a minor radioisotope Early 1930s: 14 C = a minor radioisotope assumed short half life (< 1 year) assumed short half life (< 1 year) artificially produced before detected in nature artificially produced before detected in nature recoil tracks used to calibrate cloud chamber experiments recoil tracks used to calibrate cloud chamber experiments favored production mechanism: deuterons [nuclei of deuterium, an isotope of hydrogen] on 13 C favored production mechanism: deuterons [nuclei of deuterium, an isotope of hydrogen] on 13 C

Libby at Berkeley Ernest O. Lawrence Ernest O. Lawrence 1939 Nobel Prize in Physics 1939 Nobel Prize in Physics 14 C: A radioisotope with biomedical promise 14 C: A radioisotope with biomedical promise long half life (>1000 years) long half life (>1000 years) favored production mechanism: neutrons on 14 N favored production mechanism: neutrons on 14 N

Libby at Berkeley Serge A. Korff (New York University) cosmic ray physics experiments Serge A. Korff (New York University) cosmic ray physics experiments increase in neutrons with increasing altitude increase in neutrons with increasing altitude neutrons created by cosmic rays neutrons created by cosmic rays natural atmospheric 14 C production by neutron secondaries natural atmospheric 14 C production by neutron secondaries Korff and Danforth (1939) Physical Review Korff and Danforth (1939) Physical Review

Libby at Berkeley UCB Ph.D. (1933) nuclear chemistry UCB Ph.D. (1933) nuclear chemistry Built first Geiger counter in U.S. Built first Geiger counter » in U.S. Dissertation: Weak beta activity in rare earth elements Dissertation: Weak beta » activity in rare earth elements Known at UCB as "Wild Bill Libby" for pushing "far out" ideas Known at UCB as "Wild Bill Libby" for pushing "far out" ideas

Libby at Berkeley Libby at Berkeley Screen-wall counter design: problem of detecting betas close to background Screen-wall counter design: problem of detecting betas close to background » counter envelope glass counter envelope glass wire grid (screen-wall) cathode: define sensitive volume wire grid (screen-wall) cathode: define sensitive volume sample applied to inside surface of glass sleeve sample applied to inside surface of glass sleeve glass sleeve moved in and out of sensitive volume glass sleeve moved in and out of sensitive volume mechanical mechanism for alteration of background and sample counting mechanical mechanism for alteration of » background and sample counting Basic design of detector used in post World War II C experiments Basic design of detector used in post World War II 14 C experiments

Libby at Berkeley Late 1930s: Late 1930s: 14 C half life >1000 years ( years) 14 C half life >1000 years ( years) Favored production: neutrons on 14 N Favored production: neutrons on 14 N Libby reads Korff and Danforth (1939) Libby reads Korff and Danforth (1939) Libby: Thats Radiocarbon Dating! Libby: Thats » Radiocarbon Dating!

Libby at Berkeley Radiocarbon dating: Basic physical model (1939) Radiocarbon dating: Basic physical model (1939) production in atmosphere as 14 CO 2 production in atmosphere as 14 CO 2 photosynthetic fixing in biosphere photosynthetic fixing in biosphere living biosphere 14 C equilibrium living biosphere 14 C equilibrium death withdrawal from exchange death withdrawal from exchange time since death function of residual 14 C concentration time since death function of residual 14 C concentration No experimental confirmation No experimental confirmation

Libby at Columbia: The War Years esearch on hold during World War II. Research on hold during World War II : Manhattan Project : Manhattan Project With Harold Urey [1934 Nobel Prize in chemistry] at Columbia University With Harold Urey [1934 Nobel Prize in chemistry] at Columbia University developing a thermal diffusion method to separate 235 U from 238 U developing a thermal diffusion method to separate 235 U from 238 U later thought this research as the high point of his scientific career later thought this research as the high point of his scientific career Requested 14 C half-life measurement at Argonne: 26,000± 13,000 and 21,000± 4000 years Requested 14 C half-life measurement at Argonne: 26,000± 13,000 and 21,000± 4000 years

10 14 C: A dating isotope? 14 C: A dating isotope? Testing implications of model Testing implications of model : University of Chicago : University of Chicago With Harold Urey in the Institute of Nuclear Studies With Harold Urey in the Institute of Nuclear Studies Libby (1946) Physical Review: first published description of concept: coined term "radiocarbon" Libby (1946) Physical Review: first published description of concept: coined term "radiocarbon" With first Chicago graduate student Ernest C. Anderson: With first Chicago graduate student Ernest C. Anderson: First Critical Experiment: isotopic enrichment documented difference in 14 C activity between modern biomethane and fossil petromethane First Critical Experiment: isotopic enrichment documented difference in 14 C activity between modern biomethane and fossil petromethane Libby at Chicago: Three Critical Experiments

Libby at Chicago: The Critical Experiments 14 C: A dating isotope? 14 C: A dating isotope? Testing implications of model Testing implications of model James Arnold joins group as Postdoctoral Fellow James Arnold joins group as » Postdoctoral Fellow Decay counting using solid carbon in a modified Geiger counter of Libby's Berkeley design Decay counting using solid » carbon in a modified Geiger counter of Libby's Berkeley design Critical technical development: Electronic anticoincidence shielding for low level counting close to background Critical technical development: Electronic anticoincidence » shielding for low level counting close to background Made decay counting 14 C dating practical Made decay counting 14 C dating practical Second Critical Experiment: Anderson dissertation: 14 C activities in living samples from various latitudes were essentially equal (exception: marine shell) Second Critical Experiment: Anderson dissertation: 14 C activities in living samples from various latitudes were » essentially equal (exception: marine shell)

Libby at Chicago: The Critical Experiments : Third Critical experiment : Third Critical experiment First 14 C date: wood from tomb of Zoser (Djoser), 3rd Dynasty Egyptian king (July 12, 1948). First 14 C date: wood from tomb of Zoser (Djoser), 3rd Dynasty Egyptian king (July 12, 1948). Historic age: 4650±75 BP Historic age: 4650±75 BP Radiocarbon age: (C-1) 3979±350 BP [average of 3 determinations 3699±770, 4234±600, 3991±500] Radiocarbon age: (C-1) 3979±350 BP [average of 3 determinations 3699±770, 4234±600, 3991±500] Second 14 C date: wood from Hellenistic coffin Second 14 C date: wood from Hellenistic coffin Historic age: 2300±200 BP Historic age: 2300±200 BP Radiocarbon age: (C-?) Modern! Fake! Radiocarbon age: (C-?) Modern! Fake! First Curve of Knowns: First Curve of Knowns: 6 data points (using seven samples) spanning AD 600 to 2700 BC. 6 data points (using seven samples) spanning AD 600 to 2700 BC. Half life used: 5720± 47 years Half life used: 5720± 47 years

Libby at Chicago: The Critical Experiments 1948: Adaption of screen-wall counter design using elemental (solid) carbon CO 2 from combustion/acidification reduced by reaction with magnesium at high temperature and separation of MgO ("ash") from elemental carbon ("lamp black") by treating with concentrated HCl. Inside surface of sleeve of screen-wall counter coated with elemental carbon: The Procedure from Hell! Construction of large scale (3 story high) thermal diffusion column in case routine screen-wall counter operation not achieved; column never used Destruction of set of counters when crushed under weight of magnets of new accelerator being installed

Libby at Chicago: The Critical Experiments 1949: Critical Experiments Successful The agreement between prediction and observation is seen to be satisfactory." Average overall errors on ages = ±10% Foundation 14 C paper: Arnold and Libby "Age determinations by radiocarbon content: checks with samples of known age." Science, December 23, Birthday of Radiocarbon Dating

Libby at Chicago Chicago Radiocarbon Laboratory operational: samples measured and published in a series of date lists. First Chicago list (September 1, 1950): C dates. » Published with minor modifications in Science in February 1951 (Arnold and Libby 1951) using new Libby half life = 5568±30 yrs. First Revised "Curve of Knowns" substituted Libby t 1/2

First Generation Labs Libby appointed to Atomic Energy Commission (1952) Libby appointed to Atomic Energy Commission (1952) 1st International Radiocarbon Conferences: Copenhagen and Andover (1954). [No proceedings published until 6th in Pullman 1965] 1st International Radiocarbon Conferences: Copenhagen and Andover (1954). [No proceedings published until 6th in Pullman 1965] Second Revised "Curve of Knowns" had eleven data points included "Bible" [(wrappings from Dead Sea Scrolls] (Libby, Radiocarbon Dating, 2nd edition, 1955) Second Revised "Curve of Knowns" had eleven data points » included "Bible" [(wrappings from Dead Sea Scrolls] (Libby, Radiocarbon Dating, 2nd edition, 1955) First Generation (solid carbon) Labs: Arizona*, Michigan*, Pennsylvania,* Yale, Lamont (Columbia), New Zealand, Copenhagen, Saskatchewan. [* archaeologists instrumental in establishing] First Generation (solid carbon) Labs: Arizona*, Michigan*, Pennsylvania,* Yale, Lamont (Columbia), New Zealand, Copenhagen, Saskatchewan. [* archaeologists instrumental in establishing]

First Generation Labs Obsolescence of solid carbon: nuclear fallout. » Conversion to or development of gas proportional and » liquid scintillation counting labs: e.g., U.S.G.S., Scripps (UC San Diego), Groningen, British Museum, Cambridge, Stockholm, Uppsala 1959: Journal Radiocarbon begins publication as annual Radiocarbon Supplement to American Journal of Science publishing date lists, separate standing as Radiocarbon from From 1989, Radiocarbon: An International Journal of Cosmogenic Isotope Research 1960: Nobel Prize in Chemistry: "for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics, and other sciences." Libby had arrived at UCLA in late 1959, established Isotope Laboratory in Institute of Geophysics and Planetary Physics

Nobel Prize in Chemistry At its meeting of November 3, 1960, the Royal Swedish Academy of Science has decided, in conformity with the terms of the November 27, 1895 will of Alfred Nobel, to award the prize to be given this year for the most important chemical discovery or improvement to Willard F. Libby for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics and other sciences.

Second Radiocarbon Revolution: Calibration Calibration: Violation of assumption of constant biospheric "zero age" 14 C activity First test: 1949: First Curve of Knowns: "satisfactory agreement" to 5000 BP with overall precision of ±10% Increased counting sensitivities: 1960: additional Egyptian Old Kingdom (ca BC) samples "too young" by yrs at 3000 BC. Libby (1963) suggested that conventional reconstruction of ancient Egyptian chronology by contemporary Egyptologists was in error rather than 14 C-inferred ages. First detailed study: 1960: Using tree ring series and » proportional gas counters with ±1% (±80 yrs) counting variances. Existence of systematic offsets of up to about 160 years over last 1300 years.

Second Radiocarbon Revolution: Calibration Calibration of 14 C time scale: Distinguishing real (solar, sidereal) time" and " 14 C time Bristlecone pine / 14 C data: First detailed continuous tree ring- » based data set documenting 14 C offsets over last 7000 yrs. (Sues » 1970). Documents: (1) major or long-term trend (apparent sine- wave function) and (2) complex set of shorter-term/higher- frequency variations. Long-term anomaly: maximum Holocene offset about 10% or » 800 years at about 7000 BP Shorter-term anomalies: De Vries effects multi-millennial » and multi-century oscillations in 14 C time spectrum ("wiggles," "kinks," "warps") = accordion-like characteristics of 14 C time scale. At some time periods in Holocene, 14 C time resolution is inherently temporally "blinded" for various intervals irrespective of counting variance on sample measurement.

Conventional Radiocarbon Age: Definition Stuiver and Polach (1977) Reporting of 14 C Data. Radiocarbon 1. Use Libby half-life (5568 years) 2. Use 0.95 NBS Oxalic Acid I [or standards with known relationship] to define zero age 14 C count rate 3. Use A.D as 0 BP. [BP = Before Physics] 4. Normalize 14 C activity to common δ 13 C value = Uncalibrated Defines radiocarbon time expressed in 14 C years

present Third Radiocarbon Revolution: AMS Pre-AMS Pre-AMS 1970: Late Hans Oeschger (Bern) first noted great increase in sensitivity that would be possible from ion counting using isotopic enrichment and mass spectrometry. 1970: Late Hans Oeschger (Bern) first noted great » increase in sensitivity that would be possible from ion counting using isotopic enrichment and mass spectrometry : Michael Anbar [SUNY Buffalo] (1978) reports an effort undertaken earlier at Stanford Research Institute to use conventional mass spectrometry to measure CN - for natural level 14 C detection : Michael Anbar [SUNY Buffalo] (1978) reports an effort undertaken earlier at Stanford Research Institute to use conventional mass spectrometry to measure CN - for natural level 14 C detection. Backgrounds from mass 29 trace impurities prevented the approach from becoming feasible for routine work. Ran out of funds. Backgrounds from mass 29 trace impurities prevented the approach from becoming feasible for routine work. Ran out of funds.

Third Radiocarbon Revolution: AMS Cyclotron AMS : Richard Muller [UC Berkeley] (1977) examined a cyclotron-based AMS technique, principle of using particle accelerator charge-to-mass filter first demonstrated by Luis Alvarez in 1939 in detecting 3 He. Berkeley "high energy mass spectrometry" (HEMS) » combined cyclotron for mass resolution and range » separation of 14 N from 14 C using a xenon cell. Suggested extension of 14 C time frame to 10 6 years with AMS if contamination free sample could be prepared. First published AMS-based 14 C date. » 1980's: Muller's group unsuccessful attempt to develop a 40 » keV "cyclotrino" for routine 14 C measurements using external ion source. (Shanghai group successful in 1990s)

Third Radiocarbon Revolution: AMS Tandem AMS 1977: Rochester-Toronto-General Ionex (Purser et al.) ° and Simon Frazer-McMaster (Nelson et al.) groups essentially simultaneously employed tandem accelerators using negative ions to detect natural level 14 C. History of science footnote: Harry Gove (Rochester group) argues that "Rochester Tandem experiment produced the first ever carbon-14 date by accelerator mass spectrometry, since we made a background measurement on graphite. The work at Rochester had been carried out two weeks before that at McMaster but they beat us by one day in getting their manuscript to the publisher [Science]. 1978: First Conference on Radiocarbon Dating with Accelerators, University of Rochester. Initial AMS spectrometers previously used for nuclear » structure, refitted for AMS work. First dedicated AMS » instrument: University of Arizona. »

25 Radiocarbon Dating Impact: Archaeology First Radiocarbon Revolution ° First Radiocarbon Revolution ° 14 C dating constitutes the great revolution in 20 th century prehistoric studies Glyn Daniel 14 C dating constitutes the great revolution in 20 th century prehistoric studies Glyn Daniel Development of 14 C dating in 20 th century comparable to discovery of antiquity of the human species in the 19 th century Development of 14 C dating in 20 th century comparable to discovery of antiquity of the human species in the 19 th century Made a world-wide chronometric framework for prehistory possible Made a world-wide chronometric framework for prehistory possible

26 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World Advent of 14 C dating transformed approaches to the dating frameworks for Paleoindian and Paleoamerican studies Advent of 14 C dating transformed approaches to the dating frameworks for Paleoindian and Paleoamerican studies History of the application of 14 C data in Paleoindian / Paleoamerican contexts provide classic case studies of some of the issues and problems encountered in the critical application of the 14 C method History of the application of 14 C data in Paleoindian / Paleoamerican contexts provide classic case studies of some of the issues and problems encountered in the critical application of the 14 C method

27 Chronology of the Peopling of the New World Folsom sites 14 C age BP _____________________________ Hanson (4) 10,290±90 Hanson (4) 10,290±90 Carter/Kerr McGee 10,400±600 Carter/Kerr McGee 10,400±600 Lubbock Lake 10,540±100 Lubbock Lake 10,540±100 Indian Creek/1 10,630±280 Indian Creek/1 10,630±280 Owl Cave 10,640±85 Owl Cave 10,640±85 Lindenmeier (3) 10,660±60 Lindenmeier (3) 10,660±60 Agate Basin (2) 10,700±70 Agate Basin (2) 10,700±70 Folsom (6) 10,890±50 Folsom (6) 10,890±50 Indian Creek/2 10,980±150 Indian Creek/2 10,980±150 [20 14 C values from 8 sites] [20 14 C values from 8 sites]

28 Chronology of the Peopling of the New World Clovis sites 14 C age BP __________________________________ Murray Springs (8)10,890±50 Lehner (12)10,940±40 Anzick 10,940±90 Dent10,980±90 UP Mammoth11,280±350 Lange/Ferguson11,140±140 Colby11,200±220 Domebo11,480±450 Blackwater Draw(3)11,300±240 Aubrey (2)11,570±70 [31 14 C values from 10 sites] [31 14 C values from 10 sites]

29 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World Initial Data Tule Springs, Nevada: >23, C years BP (C-914) on what was characterized as charcoal recovered from what had been identified as a "hearth-like feature" by excavators who associated its occurrence with sediments containing the bones of extinct Pleistocene fauna [1953] Old Crow, Yukon Territory, Canada: 27, / C years BP (GX-1640) on an inorganic fraction ("bone mineral apatite") of a bone artifact [1973]

30 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World Revised Data Tule Springs: "charcoal" determined to be decayed plant remains and "hearths" determined to be water channel or spring deposit structures. Oldest artifact(s): 10,000-11, C years BP based on over C dates from site [1974] Old Crow: an organic fraction of the bone artifact yielded an age of 1350± C years BP (RIDDL- 145) [1986]

31 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World New World human skeletons original age assignment skeleton basis age _____________________________ Sunnyvale AAR * 70,000 Del Mar AAR * 41,000-48,000 Los Angeles 14 C >23,000 AAR * 26,000 AAR * 26,000 Taber geologic 22,000-60,000 Yuha 14 C22,000 AAR * 23,000 AAR * 23,000 Laguna 14 C 17,150 _________ * amino acid racemization

32 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World New World human skeletons: original age assignment skeleton basis age _____________________________ Sunnyvale AAR * 70,000 Del Mar AAR * 41,000-48,000 Los Angeles 14 C >23,000 AAR * 26,000 AAR * 26,000 Taber geologic 22,000-60,000 Yuha 14 C22,000 AAR * 23,000 AAR * 23,000 Laguna 14 C 17,150 _________ * amino acid racemization New World human skeletons: revised age assignment basis age ___________________________ 14 C * C * C * C * C * C * 5100 __________ * AMS-based on organic fraction

33 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World skeleton 14 C values (BP) reference_____ skeleton 14 C values (BP) reference_____ Tlapacoya I, Mexico 10,200±65 Gonzalez et al Arroyo Frias, Argentina 10,300±60 Hicks, per. comm Buhl, Idaho 10,675±95 Green et al Peñon III, Mexico 10,775±75 Gonzalez et al Anzick, Montana 10,940±90 Stafford et al Arlington Springs, California 10,960±80 Johnson et al Naharon I, Mexico 11,670±60 * unpublished ______________________________________________________________ * 14 C measurement on non-collagen-like amino acids; minimum age (?)

34 Radiocarbon Dating Impact: Archaeology Chronology of the Peopling of the New World Kennewick, Washington human skeleton Sample bone bone fraction 14 C age (BP) δ 13 C() preservation preservation_______________________________________________________________________ 5 th left metcarpal 68.8%(C) 1 TAA ± st right metacarpal 0.3% 2 BETAI ± st right metacarpal 14.3%(NC) 1 TAA ± Left tibial crest 2.3%(NC) 1 TAA ± Left tibial crest 0.05% 2 gelatin5750± _______________________________________________________________________ 1 % of modern bone standard amino acid content, C= collagen-like amino acid composition. 2 percent carbon yield. 3 TAA = total amino acid. 4 Base extracted acid insoluble. 5 UC Riverside sample prep/Lawrence Livermore National Laboratory AMS measurement. 6 Beta Analytic. 7 University of Arizona accelerator

35 Radiocarbon Dating Impact: Archaeology Radiocarbon dating continues to be the gold standard for assigning age to Holocene and late Pleistocene archaeological, geological, and paleontological contexts Radiocarbon dating continues to be the gold standard for assigning age to Holocene and late Pleistocene archaeological, geological, and paleontological contexts Late UCB Prehistorian J. Desmond Clark: Without the 14 C time scale, prehistorians would be foundering in a sea of imprecisions sometimes bred of inspired guesswork but more often of imaginative speculation. Late UCB Prehistorian J. Desmond Clark: Without the 14 C time scale, prehistorians would be foundering in a sea of imprecisions sometimes bred of inspired guesswork but more often of imaginative speculation. end end