Presentation on theme: "Radiocarbon 9/18/12 Lecture outline: radiocarbon dating principles"— Presentation transcript:
1 Radiocarbon 9/18/12 Lecture outline: radiocarbon dating principles Atmospheric & ocean radiocarbon variabilityThe Calibration CurveRadiocarbon as biogeochemical tracerWhat makes 14C the“emperor” of isotopes?The Shroud of Turin
2 Radiocarbon (14C) formation and decay -formed by interaction of cosmic ray spallation products with stable N gas-radiocarbon subsequently decays by β- decay back to 14N with a half-life of 5730yRadiocarbon 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 threethings might changethrough time,and why?
3 Radiocarbon Dating1) 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.where present-day, pre-bomb,14C activity = 13.56dpm/g CSo all you need to know to calculatean age is A0, which to first orderis 13.56dpm/g, BUT*small variations (several percent)in atmospheric 14C in the pastlead to dating errors of up to 20%!Sources of variability:geomagnetic field strengthsolar activitycarbon cycle changes
4 Radiocarbon Measurements and Reporting 1) 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 wood14C ages are reported as “14C years BP”, where BP is 19502) Fact: Most living things do not uptake C in atmospheric ratios – i.e. they fractionate carbon,(lighter 12C preferentially used), must correct for this fractionation because it affects the 14C/12C ratioResearchers collect the 13C/12C ratio, use it tocorrect for “missing” 14CSo the less 13C a sample has, the less 14C it has,and so the uncorrected 14C age will be _______than the calendar age?Samples are “normalized” to a d13CPDB value of -25‰3) The final step is to obtain a “calibrated 14C age” using the atmospheric radiocarbon contentwhen the sample grew.
5 Atmospheric radiocarbon variability through time Convention:The atmosphericradiocarbonanomaly withrespect to a standardis defined as D14Csolar activity changesNote:the D14C is 0during 1890,b/c that’sthe activity of theoxalic acid standardaddition of isotopically lightfossil fuel C to atmospheretimeBut how did somebodyconstruct this curve?
6 Reconstructing atmospheric radiocarbon variability through time What you need:absolute age & radiocarbon ageWhat you get:history of 14Catmosradiocarbonsamplingtransecttree cut in 1999A.D.1821A.D. by ring-countingMost of the Holocene 14Catmosvariability derives from changesin the geomagnetic field
7 Over longer timescales, what’s controlling atmospheric Δ14C? Earth’s Carbon Reservoirs 14C content(in Gigatons C):atmosphere 748 highest (site of production)terrestrial 2,000 high (some old organic matter)oceanic 38,000 high (surface) and low (deep)geologic 4,000 zero (isolated from atmos. for long time)Climate changes perturb the E’s carboncycle (and therefore 14Catmos) by:changing the amount of C stored on land(ex ice sheets of LGM)-changing the ability of the deepocean to sequester old carbon fromatmosphere (ex increased stratification)
8 note non-zero age at surface The ocean “conveyor belt” and radiocarbonSeawater radiocarbon measurementsset a time-scale for whole-oceanmixing (~1500y), and identifiedthe locations where deep-mixing occurThe 14C ages of waters along a N-Stransect in the Atlantic.the youngestwaters areat the surfaceand in the deepNorth Atlanticnote non-zero age at surfacethe “bomb”spikethe oldestwaters arein the mid-depthpacifc (age~1500y)14C measurements from verticalprofiles taken in world’s oceans4) If dating a marine sample, you will need to correct its calibrated 14C age with a reservoir age,which ranges from <100yrs in the stagnant subtropical gyres to >1000yrs in the mid-depth Pacific.
9 Paleo-ventilation ages from 14C in marine carbonates 1. compare 14C agesof benthic and planktonicforams in same core horizon2. compare U-series and 14C agesfor deep-sea coralsAtlanticPacific,LGM=large symbolsGEOSECS=small dots100y of growth (study modern corals)diff. in 14C ages=670y!Adkins et al., 1998Broecker et al., 2004
10 The Radiocarbon Calibration Curve (atmospheric 14C history) Principle: compare radiocarbon dates with independent datesexamples of independent dating: tree-ring counting, coral U-Th dates, varve counting,correlation of climate signals in varves with ice coreObservation:radiocarbon datesare consistentlyyounger than calendaragesdata from:corals (bright red)lake varves (green)marine varves (blue)speleothems (orange)tree rings (black)So was atmospheric14C larger orsmaller at 20k (LGM)than today?equilinetimeHughen et al., 2004
11 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.red=observed 14Cblack=modelled 14Cgeomagneticfield frompaleomagstudiesonlygeomagneticfield frompaleomag+ magneticanomalyat 44kgeomagneticfield +mag. anomaly+ reducedsedimentationduring glacial+ changein overturningcirculationgeomagneticfield +mag. anomaly+ reducedsedimentationduring glacialstop transferringradiocarboninto deep oceanBeck et al., 2001
12 So what is the average geochemist to do? Trust the experts!INTCAL09 – established one curve to use for 14C calibration:Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW, Blackwell PG,Bronk Ramsey C, Buck CE, Burr GS, Edwards RL, Friedrich M,Grootes PM, Guilderson TP, Hajdas I, Heaton TJ, Hogg AG, Hughen KA,Kaiser KF, Kromer B, McCormac FG, Manning SW, Reimer RW,Richards DA, Southon JR, Talamo S, Turney CSM, van der Plicht J,Weyhenmeyer CE IntCal09 and Marine09 radiocarbon agecalibration curves, 0–50,000 years cal BP. Radiocarbon 51(4):1111–50.Use their calibration program (current version = CALIB 6.0):M. Stuiver, P.J. Reimer, and R. ReimerAlso, avoid contamination with post-bomb/tracer carbon at all costs!Ex: diagenesis may replace original C with post-bomb (modern) Corcontamination with tracer (super-enriched) 14C used by biologists (done in IBB)
13 The timing and structure of the “bomb” spike Bomb-produced radionuclides (in 1018 Bq (1Bq=1dps)****
14 The radiocarbon bomb spike – atmosphere vs. other reservoirs +1000‰ = 14C doublesTrumbore, 2000Source of bomb 14C: stratosphere, Northern HemisphereIncorporation of bomb 14C into various C reservoirs depends on the residence timeof C in that reservoirEx: short residence time = quick, high-amplitude responselong residence time = delayed, low-amplitude responseWhy?
15 Seawater radiocarbon observations GEOSECS – Geochemical OceanSection Studytransects through all major oceansWOCE – World Ocean CirculationExperimentextensive coverage of the ocean
16 Ocean models of bomb 14C Can ocean general circulation models accurately capture the spatial and temporalevolution of the bomb radiocarbon spike?*Our understanding of ocean mixing (esp.vertical mixing) is extremely limited, 14C can help.Rodgers et al., 1999
17 Atmosphere-Ocean-Biosphere models of bomb 14C evolution The contributions of ocean, terrestrial biosphere, fossil fuel combustion and bomb testing to atmospheric D14C was investigated with an Atmospheric General Circulation Model (GCM) coupled to a carbon cycle model.AtmosphereTerrestrial biospherew/ three differentcarbon residence timesWhy does the terrestrial biospherelag the atmosphere?The detrended atmos D14C,showing large seasonal cycleRanderson et al., Global Biogeochemical Cycles, 2002Why do Southern and NorthernHemisphere D14C values differ in 1970?
18 Why does the Northern Hemisphere have lower Δ14C? Map of mean annual Δ14C relative to South Pole. Randerson et al., 2002Why does the Northern Hemisphere have lower Δ14C?
19 Model D14C-coral D14C comparisons French FrigateShoalsFijiRodgers et al., 1999Why are some records “smoother”than others (ie Galapagos vs. Fiji)?NauruGalapagos
20 Radiocarbon in the Biosphere Approach:Use bomb 14C as a tracer ofcarbon cycling in complexenvironments.Or use natural level 14C todate carbon in various pools.Gaudinski et al., 2000
21 Radiocarbon measurements of carbon cycling in the ocean dissolvedinorganic carboncontains bomb14C at surface,not at deepdissolvedorganic carbonold – highlyrefractorysinking particulateorganic mattercontains bombcarbonFigure 1. Radiocarbon in different reservoirs of carbon at Station M in the North Eastern Pacific Ocean. Red circles represent values for HMW DOM (Aluwihare, 1999); blue bars at 0 m and 1500 m represent the range of values observed for individual monosaccharides isolated from HMW sugars (Aluwihare, 1999). DIC, total DOC, and POC data are from Druffel et al., 1996 and Bauer et al., 1998.