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Radiocarbon Methods Lecture outline: 1) Radiocarbon dating principles 2) Atmospheric & ocean radiocarbon variability 3) The Calibration Curve 4) Radiocarbon as biogeochemical tracer The Shroud of Turin Early ideas • Radiocarbon dating: Basic physical model (1939) – production in atmosphere as 14CO2 – photosynthetic fixing in biosphere – living biosphere 14C equilibrium – death withdrawal from exchange – time since death function of residual 14C concentration • No experimental confirmation, in 1930s 1945-1952: The Critical Experiments • First 14C date: wood from tomb of Zoser (Djoser), 3rd Dynasty Egyptian king (July 12, 1948). Historic age: 4650±75 BP Radiocarbon age: 3979±350 BP • Second 14C date: wood from Hellenistic coffin Historic age: 2300±200 BP Radiocarbon age: (C-?) Modern! Fake! • First “Curve of Knowns”: 6 data points (using seven samples) spanning AD 600 to 2700 BC. Half life used: 5720± 47 years 1960-1980 “Second Radiocarbon Revolution:” Calibration • Calibration of 14C time scale: Distinguishing “real (solar, sidereal) time" and "14C time” • Bristlecone pine / 14C data: First detailed continuous tree ring- » based data set documenting 14C offsets over last 7000 yrs. • 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 14C time spectrum 1977 Conventional Radiocarbon Age: Definition • Stuiver and Polach (1977) Reporting of 14C 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 14C count rate 3. Use A.D. 1950 as 0 BP 4. Normalize 14C activity to common δ13C value = -25.0 ‰ 5. Uncalibrated - defines “radiocarbon time” expressed in “14C years” Carbon dating Carbon has 3 isotopes: 12C – stable 13C – stable 12C:13C 14C = 98.89 : 1.11 – radioactive 108% Abundance: Radiocarbon Forms: in the upper atmosphere 14 N n C p 14 Decays: 14 C N 14 t ½ = 5730 yr. Calculated Ct C0 e 14 14 Measured ??? Constant ln 2 t1 2 Half life time t Radiocarbon (14C) formation and decay 1 0 n 147 N 146 C 11H formed by interaction of cosmic ray spallation products with stable N gas radiocarbon subsequently decays by C N Q back to 14N with a half-life of 5730 yr 14 6 14 7 - decay Radiocarbon dating was first explored by W.R. Libby (1946), who later won the Nobel Prize. Most published dates still use the “Libby” half-life of 5568y to enable comparison of 14C dates. The activity of radiocarbon in the atmosphere represents a balance of its production, its decay, and its uptake by the biosphere, weathering, etc. Which of these three things might change through time, and why? Radiocarbon Dating 1) As plants uptake C through photosynthesis, they take on the 14C activity of the atmosphere. 2) Anything that derives from this C will also have atmospheric 14C activity (including you and I). 3) If something stops actively exchanging C (it dies, is buried, etc), that 14C begins to decay. A A0et where present-day, pre-bomb, 14C activity = 13.56dpm/g C So all you need to know to calculate an age is A0, which to first order is 13.56dpm/g, BUT *small variations (several percent) in atmospheric 14C in the past lead to dating errors of up to 20%! Sources of variability: 1) Geomagnetic field strength 2) Solar activity 3) Carbon cycle changes Radiocarbon Measurements and Reporting Radiocarbon dates are determined by measuring the ratio of 14C to 12C in a sample, relative to a standard, usually in an accelerator mass spectrometer. standard = oxalic acid that represents activity of 1890 wood 14C ages are reported as “14C years BP”, where BP is 1950 Most living things do not uptake C in atmospheric ratios -- they fractionate carbon, (lighter 12C preferentially used), must correct for this fractionation because it affects the 14C/12C ratio Collect the 13C/12C ratio, use it to correct for “missing” 14C So the less 13C a sample has, the less 14C it has, and so the uncorrected 14C age will be _______ than the calendar age? 13C / 12C 13C / 12C spl std 13 C 1 *1000 13 12 C / C std Samples are “normalized” to a 13CPDB value of -25‰ Acorr 2(25 13CPDB ) Ameas 1 dpm / g 1000 The final step is to obtain a “calibrated 14C age” using the atmospheric radiocarbon content when the sample grew. Atmospheric radiocarbon variability through time Convention: The atmospheric radiocarbon anomaly with respect to a standard is defined as 14C 14C / 12C 14C 14 12 spl 1 *1000 C / C std -solar activity changes Note: the 14C is 0 during 1890, b/c that’s the activity of the oxalic acid standard -addition of isotopically light fossil fuel C to atmosphere time But how did somebody construct this curve? Reconstructing atmospheric radiocarbon variability through time What you need: absolute age & radiocarbon age A A0et What you get: history of 14Catmos tree cut in 1999A.D. 1821A.D. by ring-counting Most of the Holocene 14Catmos variability derives from changes in the geomagnetic field The Radiocarbon Calibration Curve (atmospheric 14C history) Principle: compare radiocarbon dates with independent dates Examples of independent dating: tree-ring counting, coral U-Th dates, varve counting, correlation of climate signals in varves with ice core data from: corals (bright red) lake varves (green) marine varves (blue) speleothems (orange) tree rings (black) Observation: radiocarbon dates are consistently younger than calendar ages time Hughen et al., 2004 But what caused these large changes in atmospheric 14C? Use a carbon cycle model that includes radiocarbon, play with different scenarios, check fit with reality. geomagnetic field from paleomag studies only geomagnetic field + mag. anomaly + reduced sedimentation during glacial stop transferring radiocarbon into deep ocean red=observed 14C black=modelled 14C geomagnetic field from paleomag + magnetic anomaly at 44k geomagnetic field + mag. anomaly + reduced sedimentation during glacial + change in overturning circulation Beck et al., 2001 So what is the average geochemist to do? Trust the experts! INTCAL98 – established one curve to use for 14C calibration: Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, G., van der Plicht, J., & Spurk, M. 1998. NTCAL98 Radiocarbon Age Calibration, 24,000-0 cal BP. Radiocarbon 40(3):1041-1084. Use their calibration program (current version = CALIB 4.4): M. Stuiver, P.J. Reimer, and R. Reimer Also, avoid contamination with post-bomb/tracer carbon at all costs! Examples: diagenesis may replace original C with post-bomb (modern) C or contamination with tracer (super-enriched) 14C used by biologists The timing and structure of the “bomb” spike Bomb-produced radionuclides (in 1018 Bq (1Bq=1dps) * * * * The radiocarbon bomb spike – atmosphere vs. other reservoirs +1000‰ = 14C doubles Trumbore, 2000 Source of bomb 14C: stratosphere, Northern Hemisphere Incorporation of bomb 14C into various C reservoirs depends on the residence time of C in that reservoir Why? Ex: short residence time = quick, high-amplitude response long residence time = delayed, low-amplitude response Source of Error in 14C dating 1. Variations in geomagnetic flux. Geomagnetic field strength partly controls 14C production in the atmosphere because of attenuation affects on the cosmic flux with increasing magnetic field strength. 2. Modulation of the cosmic-ray flux by increased solar activity (e.g., solar flares) leads to attenuation of the cosmic-ray flux. 3. Influence of the ocean reservoir. Any change in exchange rate between ocean reservoir and atmospheric reservoir will affect the level of 14C in the atmosphere. 4. Industrial revolution (ratio of 14C to stable carbon decreased because of burning fossil fuels) and bomb effects (14C to stable carbon increased because of increased neutron production from detonation of nuclear bombs in the atmosphere) have made modern organic samples unsuitable for as reference samples.