Presentation is loading. Please wait.

Presentation is loading. Please wait.

Radiocarbon Course Students: These slides are provided for your reference. If you wish to use any of these slides, please make sure to contact the appropriate.

Similar presentations

Presentation on theme: "Radiocarbon Course Students: These slides are provided for your reference. If you wish to use any of these slides, please make sure to contact the appropriate."— Presentation transcript:

1 Radiocarbon Course Students: These slides are provided for your reference. If you wish to use any of these slides, please make sure to contact the appropriate lecturer first for permission and attribution. Thank you.

2 Radiocarbon in Ecology and Earth System Science W.M. Keck Carbon Cycle Accelerator Mass Spectrometry Facility Lab Instructors: Guaciara dos Santos-Winston Xiaomei Xu Lecture Instructors: Sue Trumbore*, Ted Schuur*, John Southon, Ellen Druffel, Jim Randerson, Erv Taylor Logistics: Morgan Sibley * Course coordinators

3 Goals of the class Learn about the Earths carbon cycle from a 14 C perspective Lectures on what is learned from 14 C in Ocean, Atmosphere, Land, Paleo C cycles Introduce you to the details of interpreting radiocarbon data Problem Sets – how does the number you get help answer your question Preparation of samples for radiocarbon dating by accelerator mass spectrometry (AMS) Laboratory methods – taking and analyzing the sample so that you know what you are measuring

4 Outline of Todays Lecture Global cycles of carbon and 14 C Isotope basics – how radiocarbon is made and distributed in the environment Reporting/Interpreting of radiocarbon data (an intro to problems part of the course) Steps involved in making a 14 C measurement (a brief introduction to the lab part of the course) A brief orientation to the building and AMS lab

5 Earth System Science is the study of Earth as a coupled and interacting system of Land Atmosphere Hydrosphere Biosphere People Involves: physics of transport phase transformations chemical interactions biological reactions and human resource use

6 Forms of Carbon in the Earth System Atmosphere (750) Carbon dioxide (gas) CO 2 (7) (7) Methane (gas) CH 4 Ocean (38,000) mostly dissolved ions (HCO 3 - (bicarbonate) and CO 3 2- (carbonate) Land (650) Living organic matter (1500) Dead organic matter (soil) (1500) Dead organic matter (soil) Solid Earth Land, air, water Fossil organic matter (28,000,000) coal, petroleum, natural gas Limestone (~50,000,000) CaCO 3

7 ATMOSPHERIC CO 2 640 X10 15 g C LIVING BIOMASS 830 X10 15 g C DISSOLVED ORGANICS 1500 X10 15 g C ORGANIC CARBON IN SEDIMENTS AND SOILS 3500 X10 15 g C CO 2 DISSOLVED IN OCEANS 38,000 X10 15 g C LIMESTONE AND SEDIMENT CARBONATES 18,000,000 X10 15 g C TRAPPED ORGANIC CARBON: NATURAL GAS, COAL PETROLEUM, BITUMEN, KEROGEN 25,000,000 X10 15 g C Distribution of Carbon; 10 15 grams = 1 Petagram (Pg) Response times are seasons to centuries Response times are centuries to millennia Response times are tens of thousands to millions of years Timescales we can address with 14 C

8 Changes in CO 2 on thousand year timescales – glacial to interglacial change indicates past changes in C cycle linked to climate Methane Temperature Carbon Dioxide http://


10 IPCC high scenario 2100, 975 ppm IPCC low scenario 2100, 540 ppm 2009, 387 ppm 1959, 316 ppm Where we are predicted to end up in the next century is far outside thenorm - what kinds of climate/CO 2 feedbacks will operate in future? Years before present

11 What makes us sure current CO 2 increase is caused by humans? Depletion of Atmospheric 14 C – the Suess Effect Fossil fuel contains zero radiocarbon (millions of years means so many half-lives of 14 C that it is all decayed away – so adding CO 2 derived from fossil fuel burning reduces 14 C over time Tree rings Preindustrial atmosphere Fewer 14 C atoms per 12 C atom in CO 2

12 Deforestation: Clearing of forests (formerly in the northern hemisphere, now in the tropics) Responsible for ~40% of total C emissions since 1850 In 1990s 0.5 to 2 GtC/year (8- 25% of total emissions)

13 Source: Ralph Keeling, Where does the rest go? Also, what happens to CO 2 from deforestation (not counted here)? Carbon dioxide mixing ratio (parts per million) 1 ppm = 1 liter CO 2 in 1,000,000 liters air


15 Fossil C Where it goes Emissions Gigatons of Carbon per year Fossil C Where it goes Emissions Atmo- sphere Ocean Atmo- sphere Ocean Land 1989 – 2003/71960 – 1988 Fossil Fuel Emission Observed atmosphere increase Land and Ocean Sinks = + ? ? Sarmiento et al. 2010

16 Some of the emitted CO 2 is dissolving in the oceans (tomorrows lecture) Surface waters equilibrate quickly; CO 2 reacts with water Falling particles move organic carbon into the deep ocean Sinking waters in polar regions isolate water that has equilibrated at the surface, removing CO 2 for thousands of years

17 Fossil C Where it goes Emissions Gigatons of Carbon per year Fossil C Where it goes Emissions Atmo- sphere Ocean Atmo- sphere Ocean Land 1989 – 2003/71960 – 1988 Fossil Fuel Emission Observed atmosphere increase Land and Ocean Sinks = + Sarmiento et al. 2010

18 Sarmiento et al. 2010 Biogeosciences 7, 2351-2367 Net Land Flux Deforestation Land Uptake = - Year-to-year variation in land uptake Is ± 3 PgC yr - 1

19 Foster, Motzkin and Slater 1998 Forest Cover in Massachusetts 1830 to 1985 Processes on Land that could be taking up the residual CO 2 : -Regrowth of some forests that were previously cut -Thickening of forests because of forest fire suppression? -Fertilization of forests by increased N deposition or CO 2 - Improved agricultural management

20 Some big questions for the future Can we count on ocean and land sinks to continue to take up ~ half of the CO 2 we emit? What process(es) is (are) responsible for the land uptake? What feedback mechanisms could lead to large changes in the future C cycle? What can we learn from past changes in the C cycle? More details in the weeks lectures…..

21 Carbon Isotopes – stable and radioactive The naturally occurring isotope of carbon: C-12 (98.8%) 6 protons, 6 neutrons C-13 (1.1%) 6 protons, 7 neutrons C-14 (< 10 -10 %) 6 protons, 8 neutrons 14 C is the longest lived radioactive isotope of C, and decays to 14 N by emitting a particle (electron):

22 13 C – patterns in the environment reflect mass-dependent fractionation (partitioning among phases at equilibrium and differences in reaction rates) 14 C – Reflects time or mixing Mass-dependent fractionation is corrected out of reported data using 13 C Isotopes of C contain independent information

23 Stable C isotope – 13 C Stable isotope ( 13 C) fractionation: Kinetic effects: 13 C reacts more slowly than 12 C 13 CO 2 diffuses more slowly than 12 CO 2 Equilibrium effects: 12 CO 2 + 13 CO 3 2- +H 2 O = 13 CO 2 + 12 CO 3 2- +H 2 O 13 C will partition into the species where overall energy is lowest (strongest bond or phase with least randomness). Reaction rates and equilibrium partitioning coefficients are dependent on state variables like T, P

24 Isotope data are expressed as ratios (e.g. rare isotope/abundant isotope) compared to a standard It is nearly impossible to measure the absolute abundances of isotopes accurately, but differences in relative abundance from one sample to another are easier to measure For measurements made by different laboratories to be comparable, data are expressed as the ratio to a universally accepted standard

25 ElementStandardR CarbonPee Dee Belemnite (calcium carbonate) 13 C/ 12 C = 0.0112372 18 O/ 16 O = 0.002671 Carbon-13 nomenclature

26 By definition, the standards have = 0 A leaf with 13 C value of –28 has an isotope ratio (R plant ) of (-28/1000) + 1, or 0.972. Calcium carbonate with 18 O PDB of +2 has Of (+2/1000) +1 = 1.002

27 Typical range of 13 C values in nature

28 Fossil fuel has 13 C of -21 to -27 per mil If all emissions are taken up by the biosphere, the 13 C of atmospheric CO 2 should not change. Dissolution in the ocean preferentially removes 12 C more than 13 C, so we would expect a decline in 13 C of atmospheric CO 2

29 Decline in O 2 is faster than increase in CO 2 Stoichiometry says O 2 /CO 2 for fossil fuel burning/biosphere should be ~-1.1 O 2 is less soluble than CO 2 – so it also provides a way to constrain relative ocean and land sink strengths (i.e. it is like another isotope of C )

30 Unlike stable isotopes, which can be moved around but are always conserved, radiocarbon is constantly created and destroyed Total number of 14 C atoms (N) Production in the stratosphere Loss by radioactive decay - N The total amount of radiocarbon on Earth can (and does) vary (Fridays lecture)

31 14 C is continually produced in the upper atmosphere by nuclear reaction of nitrogen with cosmic radiation. A smaller amount is produced by cosmic rays interacting with atoms in minerals at the Earths surface – we will ignore that in this class Cosmic ray spallation products thermal neutron proton 14 N nucleus 14 C nucleus Oxidation, mixing 14 CO 2 stratosphere troposphere Ocean/biosphere exchange

32 Amount of carbon (x10 16 moles) 1.0 6 1.0 1.7-2.0% typical pre-industrial ratio of 14 C/ 12 C divided by the Modern (i.e. atmospheric) 14 C/ 12 C ratio per cent of total 14 C in the major global C reservoirs Atmosphere (CO 2 ) 0.84 280 0.84 65-78% Deep Ocean (DIC) 0.6 10 0.6 2% DOC 0.95 30 0.95 8-10% Surface Ocean (DIC) 0.97 6 0.97 1.6-2% Terrestrial Biota 0.90 13 0.90 3-4% Soil Organic Matter 0.95 7-70 0.95 2-18% Coastal / Marine Sediment Where the 14 C is depends on (1) how much C is there (2) how fast it exchanges with the atmosphere

33 Radiocarbon is made a second way – from high energy in nuclear explosions bomb 14 C


35 Radiocarbon is useful on several timescales Cosmogenic 14 C (Radiocarbon dating) >300 years to ~60,000 years (± 20-100 years) Model residence time based on comparison of 14 C with Modern C Bomb 14 C Produced by atmospheric thermonuclear weapons testing ~ 1950 to present (± 1-2 years) Compare 14 C to known record of change in atmosphere Purposeful tracer 14 C follow added radiocarbon Minutes to years, depending on activity of tracer Allows tracing of specific pathways of allocation and resource use

36 Radiocarbon data are reported as the ratio of 14 C/ 12 C with respect to a standard with known 14 C/ 12 C ratio: Ninety-five percent of the activity of Oxalic Acid I from the year 1950 (Modern is 1950)

37 Why does radiocarbon data reported as Fraction Modern or 14 C have a correction for mass dependent fractionation? Leaf 13 C = -28 CO 2 in air 13 C = -8 14 C - 12 C mass difference is twice that of 13 C – 12 C Therefore a 20 difference in 13 C means ~ 40 difference in 14 C Expressed in 14 C years this is an apparent age difference of -8033*ln(.96) = 330 years

38 From Stuiver and Polach 1977

39 The 14 C standard : Oxalic Acid I The principal modern radiocarbon standard is N.I.S.T Oxalic Acid I (C 2 H 2 O 4 ), made from a crop of 1955 sugar beets. Ninety-five percent of the activity of Oxalic Acid I from the year 1950 is equal to the measured activity of the absolute radiocarbon standard which is 1890 wood (chosen to represent the pre-industrial atmospheric 14 CO 2 ), corrected for radioactive decay to 1950. This is defined as Modern, which is ~1.12 14 C atom for every trillion 12 C atoms A range of standards with different 14 C/ 12 C ratios is maintained by the International Atomic Energy Agency (IAEA).

40 The ways we use radiocarbon to study the carbon cycle: Determining the age of C in a closed system age of pollen, foraminifera, seeds As a source tracer: mixing of sources with different 14 C signatures For open systems, a measure of the rate of exchange of C with other reservoirs As a (purposeful) tracer tracing pathways (allocation) or rates We use different ways of expressing radiocarbon data for each of these applications

41 Different ways to report 14 C data depend on the application (Stuiver and Polach 1977) Expressions that do not depend on the year you make the measurement or take the sample: Fraction Modern(FM) 0.80 Per cent modern (100*FM) 80% D = (FM – 1) * 1000 -200 (this is equivalent to the stable isotope notation) Radiocarbon age (calculated using FM)

42 Radioactivity = number of decays per unit time = dN/dt dN/dt = - 14 N, where N is the number of 14 C atoms; dN/N = - 14 dt T = (-1/ 14 )ln (N(t)/N(0)) If radiocarbon production rate and its distribution among atmosphere, ocean and terrestrial reservoirs is constant, Then N(0) = atmospheric 14 CO 2 value. Note that N(t)/N(0) is then the Fraction Modern (F) [Prove to yourself: 1/2 = ln(2)/ 14 ] F Years 1/2 Drops to 0.5 in 5730 years ( 1/2 ) Drops to 0.25 in 2* 1/2 years Radiocarbon Age: used for closed systems in which carbon has resided for hundreds to thousands of years

43 Radiocarbon Age = -(1/ 14 )*ln(F) Where F is Fraction Modern and 14 is the decay constant for 14 C The half life ( 1/2 = ln(2)/ 14 ) used to calculate radiocarbon ages is the one first used by Libby (5568 years). A more recent and accurate determination of the half-life is 5730 years. To convert a radiocarbon age to a calendar age, the tree ring calibration curve is used (well do a problem on this tomorrow). More on this in the History lecture later …..

44 Time 1950 Sample made or collected before 1950 14 C/ 12 C ratio 0.95*OXI 2007 Fraction Modern, D, 14 C age all report the ratio in the year of measurement, which will not vary as time goes on because radiodecay in standard and sample occurs at the same rate ( ). Ages are always reported as years before 1950, but the ratio will be the same as that measured in 2007

45 Time 1950 Sample made or collected in year T 14 C/ 12 C 0.95*OXI in 1950 2008 One of the applications is to figure out past changes in the 14 C of atmospheric CO 2 using known-age samples; for this we use the same as C pre-1950, but not after 1950 Correct for decay of 14 C between T and 1950 decay correction Sample can be measured any time after 1950, and will have the same ratio as 1950

46 known-age corrected samples ) expresses the radiocarbon signature relative to Modern had the sample been measured in 1950. This is useful for studies attempting to show how the radiocarbon signature of air (tree rings) and water (corals) changes with time. It is the basis for creating the calibration curves used to calculate calendar age from radiocarbon age Corrects for decay of 14 C in the sample from the year of growth (x) to 1950)

47 Past Changes in Atmospheric 14 C recorded in tree rings T = known age (years before 1950); = ln(2)/5730 yr (actual half-life) This is equal to 1/8267yr (we refer to this as the mean life) 1950 1950 - T Decay correction

48 Past Changes in Atmospheric pre-1950 14 C) recorded in tree rings Year BP (also 14 C) of atmosphere Calendar Year If we know the year the sample was formed, we can correct for radiodecay from that year to 1950 to determine what the 14 C of the atmosphere was in the past. Note that 8000 yrs ago, 14 C was about 10% higher than in 1950; Higher production rates or different distribution of radiocarbon among atmosphere, ocean and land?

49 Tree-ring calibration curve The 14 C value measured in tree rings of known age is used to determine the 14 C value of the atmosphere for the year of tree growth 14C age

50 Calibration curve for radiocarbon ages shows lack of ability to determine differences in calendar ages using 14 C in the past ~300 years. Radiocarbon age: 120 +/-50 years Yields calibrated ages of 270-160 and 150-50 years BP (Present is always 1950)

51 For geochemical modeling, especially involving the distribution of bomb 14 C, we need a way or reporting the absolute amount of 14 C in the sample: Absolute per cent Modern or 14 C -Requires defining a standard that does not change with time: decay-correct the oxalic acid standard to what it would have been in 1950 (i.e. add back in the radiocarbon that decayed in the standard since 1950) -The value will therefore depend on the year in which the measurement was made (as long as the measurement was made since 1950).

52 Most commonly used is 14 C (geochemical reporting) Corrects for decay of OX1 standard since 1950 Value of this term is 1.0074 in 2011 14 C of a sample measured in 2011 will be 7 less than if it was measured in 1950 (because 14 C in the sample has undergone radioactive decay but the standard has a fixed value). Difference from is that there is no correction for radiodecay in the sample

53 Time 1950 14 C/ 12 C 0.95*OXI Correction for decay of standard since 1950 2009 Standard value doesnt change with time 14 C reports the 14 C/ 12 C ratio in the year of measurement compared to the standard measured in 1950. 14 C will change for the same sample measured in different years.

54 Why on earth would you want to use 14 C? To perform mass balance – this is called thegeochemical notation Total number of 14 C atoms in 1963 (bombs) All produced in atmosphere Atmosphere Ocean Land Fate of bomb 14 C atoms in 2011 Radio-decay Models that trace the fate of bomb 14 C require a common standard that does not change – those models track radiodecay directly and therefore can be directly compared to measurements 1963 2011Future year

55 14 C decay 14C [C 1 F 1 +C 2 F 2 +C 3 F 3 ] Closed System – Buried CaCO 3 crystal Open System, heterogeneous F atm *I F DIC *O 14 C decay F DIC *[DIC]*Vol* 14C 14 C decay C CaCO3 * 14C Open System, homogeneous I*F leaf CaCO 3 sediment S*F DIC Gas Exchange Litterfall Decomposition k 1 C 1 F 1 +k 2 C 2 F 2 +K 3 C 3 F 3 More on Modeling in Thursdays Lecture…

56 Closed System – Buried CaCO 3 crystal 14 C decay C CaCO3 * 14C What does 14 C tell you? Complications: Hard water effect DIC may not be in equilibrium with the atmosphere i.e. FM DIC FM atm Calibrated radiocarbon age can give the time since C in the CaCO 3 was buried, assuming the initial FM CaCO3 was zero. Even if the core top 14 C age is not zero, a plot of age versus depth may give sedimentation rate

57 F atm *I F DIC *O 14 C decay F DIC *[DIC]*Vol* 14C Open System, homogeneous Gas Exchange CaCO 3 sediment S*F DIC Radiocarbon age of DIC gives reflects the relative rates of gas exchange, sedimentation and radio-decay of 14 C

58 14 C Bomb radiocarbon – cannot assume F atm is constant

59 14 C decay 14C [C 1 F 1 +C 2 F 2 +C 3 F 3 ] Closed System – Buried CaCO 3 crystal Open System, heterogeneous F atm *I F DIC *O 14 C decay F DIC *[DIC]*Vol* 14C 14 C decay C CaCO3 * 14C Open System, homogeneous I*F leaf CaCO 3 sediment S*F DIC Gas Exchange Litterfall Decomposition k 1 C 1 F 1 +k 2 C 2 F 2 +K 3 C 3 F 3 More on Modeling in Thursdays Lecture…

60 The lab portion of the course

61 Two ways to measure 14 C (1)Beta-decay counting ( 14 C 14 N + - ): Measure radioactivity (decay constant times the number of 14 C atoms) directly (compare activity to oxalic acid). (2)Accelerator mass spectrometry (AMS) Count individual 14 C atoms to get 14 C/ 12 C ratio. (some labs measure 14 C/ 13 C ratio and use 13 C/ 12 C to calculate 14 C/ 12 C) One gram of Modern" carbon produces about 14 beta-decay events per minute. To measure the age of a 1g sample to a precision of +/- 20 years one needs 160,000 counts, or about 8 days of beta-counting. AMS allows you to do the same measurement on a 1 milligram sample in a few minutes.

62 Sample preparation Two ways to measure radiocarbon Decay Counting Convert C to CO 2, then to acetylene (gas) or benzene (liquid). Requires about 3 grams of sample AMS Convert C to CO 2, then reduce catalytically to graphite using iron (Fe) catalyst. We use two methods for reduction (zinc vs. H 2 as the reductant) Gas sources (CO 2 ) are starting to be in routine use

63 How do we measure/ report our Errors? (Accuracy and Precision) Accurate and Precise C D B Precise but not Accurate Low precision and low accuracy Low precision but accurate

64 Background – Processing a sample that should contain no radiocarbon – this is a measure of 14 C added during processing (graphite production, combustion, etc.). Precision – how well do I reproduce the same sample measured more than once? (precision for replicate samples (e.g. soil CO 2 sampled in three locations) is likely is not as good as the precision of the AMS measurement (measurement of aliquots of the same CO 2 ) Accuracy – how well do I reproduce the known value of a standard material when I run it as an unknown? There are a number of standard materials for purchase from IAEA representing a range of materials and 14 C contents. Factors to consider

65 What are the stages a sample goes through when it is measured for 14 C? Taking the sample (Mon/Tues) What is the question being asked? Does the sample really allow you to answer it? Does the processing in the lab introduce artifacts? (If needed) Pretreatment and Combustion (Mon/Tues) Purification of CO2 and conversion to graphite (Wednesday/ Thursday) Measurement by AMS and data reduction (Friday) Selecting standards and blanks to test your sampling procedure (Monday) Put standard and blank materials through all processes in parallel (Is my lab 14 C clean?) (Tues-Thursday) How standards and blanks are used in data reduction (Friday)

Download ppt "Radiocarbon Course Students: These slides are provided for your reference. If you wish to use any of these slides, please make sure to contact the appropriate."

Similar presentations

Ads by Google