Radiocarbon in Ecology and Earth System Science

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Radiocarbon in Ecology and Earth System Science
W.M. Keck Carbon Cycle Accelerator Mass
Spectrometry Facility
Lab Instructors Guaciara dos Santos, Xiaomei
Xu
Lecture Instructors Sue Trumbore, Ellen
Druffel, Jim Randerson, Ted Schuur, John
Southon Logistics Cynthia Dennis
Course coordinators
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Goals of the class

• Learn about the Earths carbon cycle
• Lectures on what is learned from 14C in Ocean,
  Atmosphere, Land, Paleo C cycles
• Introduce you to the details of interpreting
  radiocarbon data
• Problem Sets
• Preparation of samples for radiocarbon dating by
  accelerator mass spectrometry (AMS)
• Laboratory

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(No Transcript)
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Outline of Todays Lecture

• Global cycles of carbon and 14C
• Nomenclature for reporting radiocarbon data
• (short break)
• Steps involved in making a 14C measurement

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Earth System Science is the study of Earth as a
coupled and interacting system of Land Atmosphere
Hydrosphere Biosphere People
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Forms of Carbon in the Earth System Atmosphere
(750) Carbon dioxide (gas) CO2
(7) Methane (gas)
CH4 Ocean (38,000) dissolved ions (bicarbonate
and carbonate) Land (650) Living
organic matter (1500) Dead organic
matter (soil)
Land, air, water
Fossil organic matter (28,000,000) coal,
petroleum, natural gas Limestone (50,000,000)
CaCO3
Solid Earth
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Charles David Keeling
Carbon dioxide is increasing in the
atmosphere 1958 0.031 2006 0.038
Keeling and Whorf (2005)
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Balancing the Carbon Budget (1990s)
Release by tropical deforestation
?
Term we kind of know
Gigatons of C per year
Fossil fuel emission
Increase in atmospheric CO2
Terms we know well
Added to atmosphere Where it goes
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Some of the emitted CO2 is dissolving in the
oceans
Surface waters equilibrate quickly CO2 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
CO2 for thousands of years
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Balancing the Carbon Budget (1990s)

Release by tropical deforestation
Land by difference
Term we kind of know
Dissolves in oceans
Term we know pretty well
Gigatons of C per year
Fossil fuel emission
Increase in atmospheric CO2
Terms we know well
Added to atmosphere Where it goes
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Some big questions for the future

• Can we count on ocean and land sinks to continue
  to take up half of the CO2 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 next lectures..
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Radiocarbon - how we tell time in the carbon
cycle
The least abundant isotope of carbon C-12
(98.8) 6 protons, 6 neutrons C-13 (1.1)
6 protons, 7 neutrons C-14 (lt 10-10 ) 6
protons, 8 neutrons 14C is the longest lived
radioactive isotope of C, and decays to 14N by
emitting a b particle (electron)
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Isotopes of C contain independent information

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

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Stable C isotope 13C

• Stable isotope (13C) Fractionation
• Kinetic effects
• 13C reacts more slowly than 12C
• 13CO2 diffuses more slowly than 12CO2
• Equilibrium effects
• 12CO2 13CO32- H2O 13CO2 12CO32- H2O

13C will partition into the species where overall
energy is lowest
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Isotopes are expressed as ratios (e.g. rare
isotope/abundant isotope) compared to a standard
ratio

• 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
• To report this ratio so it is comparable to other
  measurements, it is expressed as the ratio to a
  universally accepted standard

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Nomenclature for reporting stable isotope data
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By definition, the standards have d 0,
though the standard needs to be specified for
oxygen (SMOW versus PDB) A leaf with d13C value
of 28 has an isotope ratio
of (-28/1000) 1, or 0.972.
Calcium carbonate with d18OPDB of 2 has
Of (2/1000) 1 1.002
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Typical range of d13C values
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Unlike stable isotopes, radiocarbon is constantly
created and destroyed
Loss by radioactive decay
Production in the stratosphere
Total number of 14C atoms (N)
-l N
Total amount of radiocarbon on Earth can (and
does) vary (Fridays lecture)
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14C is continually produced in the
upperatmosphere 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
proton
thermal neutron
14N nucleus
14C nucleus
spallation products
Oxidation, mixing
Ocean/biosphere exchange
stratosphere
14CO2
troposphere
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Amount of carbon (x1016 moles)
typical ratio of 14C/12C divided by the Modern
(i.e. atmospheric) 14C/12C ratio
per cent of total 14C in the major global C
reservoirs
6 1.0 1.7-2.0
Atmosphere (CO2)
30 0.95 8-10
6 0.97 1.6-2
Surface Ocean (DIC)
Terrestrial Biota
280 0.84 65-78
13 0.90 3-4
Deep Ocean (DIC)
Soil Organic Matter
10 0.6 2
DOC
Where the 14C is depends on (1) how much C is
there (2) how fast it exchanges with the
atmosphere
7-70 0.95 2-18
Coastal / Marine Sediment
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Radiocarbon is made a second way from high
energy in nuclear explosions bomb 14C
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http//www.iup.uni-heidelberg.de/institut/forschun
g/groups/kk/14co2.html
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14C is useful on several timescales

• Hundreds of years to 60,000 years (natural
  radiocarbon) longer limit is the detection
  limit (10 half-lives)
• Last 50 years (distribution of bomb
  radiocarbon)
• Hours to years (14C used as a purposeful tracer)

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Both methods report radiocarbon as the ratio of
14C/12C with respect to a standard with known
14C/12C ratio
(Modern is 1950)
Ninety-five percent of the activity of Oxalic
Acid I from the year 1950
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The 14C standard Oxalic Acid I

• The principal modern radiocarbon standard is
  N.I.S.T Oxalic Acid I (C2H2O4), 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 14CO2), corrected
  for radioactive decay to 1950. This is Modern,
  which is roughly
• 1 14C atom for every trillion 12C atoms
• A range of standards with different 14C/12C
  ratios is maintained by the International Atomic
  Energy Agency (IAEA).

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Wait - We know 13C is fractionated by kinetic and
equilibrium processes because of its mass so
14C must be too!
Samples are corrected to a common 13C value and
therefore 14C values reported as fraction Modern,
Libby Age, or D14C do not reflect mass-dependent
fractionation of isotopes. The sample is
corrected to have the a d13C of -25 (14C is
either added or subtracted, assuming 14C is
fractionated twice as much as 13C) We will do
some problems to try to make all this more clear
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Why must there be a correction for mass dependent
fractionation?
CO2 in air d13C -8
Leaf d13C -28
14C -12C mass difference is twice that of 13C
12C Therefore a 20 difference in 13C means
40 difference in 14C Expressed as an age this
is -8033ln(.96) 330 years
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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 14C
  signatures
• As a purposeful tracer
• tracing pathways (allocation) or rapid rates
• For open systems, a measure of the rate of
  exchange of C with other reservoirs
• Mean residence time versus mean age

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Determination of age hundreds to thousands of
years for closed systems
Radioactivity number of decays per unit time
dN/dt dN/dt -l14N, where N is the number of
14C atoms dN/N -l14dt T (-1/ l14)ln
(N(t)/N(0)) If radiocarbon production rate and
its distribution among Atmosphere, ocean and
terrestrial reservoirs is constant, Then N(0)
atmospheric 14CO2 value.
F
Drops to 0.5 in 5730 years (t1/2)
Drops to 0.25 in 2t1/2 years
t1/2
Years
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Radiocarbon Age (Libby age)
Radiocarbon Age -(1/l14)ln(F) Where F is
Fraction Modern and l14 is the decay constant for
14C The half life (t1/2 ln(2)/l14) 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).
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Tree-ring calibration curve
14C age
The 14C value measured in tree rings of known age
is used to determine the 14C value of the
atmosphere for the year of tree growth
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Calibration curve for radiocarbon ages shows lack
of ability to determine differences in calendar
ages using 14C in the past 300 years.
Radiocarbon age 120 /-50 years Yields calendar
ages of 270-160 and 150-50 years BP
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There are a lot of different ways to report 14C
data, but they fall into two types of use

• (1) Ways of expressing the ratio of 14C/12C based
  on Fraction Modern, and used to determine age
  (mostly for samples fixpre-1950)
• When you want to know the age of something
• Fraction Modern 0.80
• Per cent Modern (pmc) 100 FM 80
• D14C (FM 1) 1000 -200
• (this is equivalent to the stable isotope d
  notation)

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Fraction Modern, pmC, D (where D 1000(FM-1))
Reports 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
0.95OXI
14C/12C ratio
Sample made or collected before 1950
Time
2007
1950
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One of the applications is to figure out past
changes in the 14C of atmospheric CO2 using
known-age samples
Correct for decay of 14C between T and 1950
Correction for decay of standard since 1950
0.95OXI in 1950
14C/12C
decay correction
Sample made or collected in year T
Time
2007
1950
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Past Changes in Atmospheric 14C recorded in tree
rings
1950 - T
T known age (years before 1950)
1950
Decay correction
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Past Changes in Atmospheric 14C recorded in tree
rings
Year BP
D14C 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 14C of the atmosphere was in
the past.
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(2) For tracking the fate of bomb 14C we use
nomenclature that relates the 14C/12C we measure
to an absolute (fixed in time) standard D14C
Corrects for decay of OX1 standard since
1950 Value of this term is 1.0067 in 2007
D14C of a sample measured in 2007 will be less
than if it was measured in 1950 (because 14C in
the sample has undergone radioactive decay but
the standard has a fixed value).
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D14C reports the 14C/12C ratio in the year of
measurement compared to the standard measured in
1950. Also Absolute percent Modern is used.
(Useful for tracking the fate of bomb 14C)
Correction for decay of standard since 1950
Standard value doesnt change with time
14C/12C
0.95OXI
Time
2007
1950
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Why on earth would you want to use D14C? To
perform mass balance this is called the
geochemical notation
Models that trace the fate of bomb 14C require a
common standard that does not change those
models track radiodecay directly
Total number of 14C atoms in 1963 (bombs)
Fate of bomb 14C atoms in 2007
Radio-decay
Land
Atmosphere
All produced in atmosphere
Ocean
1963
2007
Future year
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Two ways to measure 14C

• (1) Beta-decay counting (14C ? 14N b-)
  Measure radioactivity (decay constant x no. of
  14C atoms).
• Accelerator mass spectrometry (AMS)
• Count individual 14C atoms to get 14C/12C
  ratio
• 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.

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Analyzer Magnet to separate isotopes (mass 12, 13
and 14)
Injection Magnet
Accelerator with stripper at terminal
Faraday cups (detectors for mass 12 and 13)
Stripper breaks apart molecular ions (e.g. 13CH-)
C- ?C
14N does not make a negative ion, so the major
contaminating isobar is eliminated at the source
Solid state single-atom detector
Final magnet
Ion Source (makes C-)
UCI AMS uses sequential injection to measure 12C,
13C, 14C
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Sample preparation

• Decay Counting
• Convert C to CO2, then to acetylene (gas)
  or benzene (liquid). Requires about 3 grams of
  sample
• AMS
• Convert C to CO2, then reduce catalytically to
  graphite using iron (Fe) catalyst. We use two
  methods for reduction (zinc vs. H2 as the
  reductant)
• Gas sources (CO2) mostly under development
• Requires about 0.1 -1.0 mg of sample for
  routine measurement

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Accuracy, Precision and Error
A
B
C
D
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• Background When a sample that should contain no
  radiocarbon is measured, some 14C is found this
  is a measure of 14C added during processing
  (graphite production, combustion, etc.)
• Precision how well do I reproduce the same
  sample measured more than once?
• 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 14C contents.

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What are the stages a sample goes through when it
is measured for 14C?
Measurement by AMS and data reduction (Friday)
Purification of CO2 and conversion to
graphite (Wednesday/Thursday)
(If needed) Pretreatment and Combustion (Tuesday)
Taking the sample (Monday)
What is the question being asked? Does the sample
really allow you to answer it? Does the
processing in the lab introduce artifacts?
Selecting standards and blanks to test your
sampling procedure (Monday)
How standards and blanks are used in data
reduction (Friday)
Put standard and blank materials through all
processes in parallel (Is my lab 14C
clean?) (Tues-Thursday)
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Example foraminifera from a deep-sea core (for
chronology)
Acidify, purify CO2 and convert to graphite
Pretreatment treat with acid to remove outer
contaminants for stable isotopes roast sample
do procedure for standard and blank materials in
parallel
Pick out individuals by species, need enough for
the AMS measurement
Measurement by AMS and data reduction
Select a blank (forams or calcite with no 14C,
and a secondary standard
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Monday Lab Taking the sample

• 1 - Terrestrial 1 Soil and CO2 sampling
• Location (Outside)
• 2- Terrestrial 2 Laboratory Incubations and
  fractionation procedures (2222 Croul)
• 3 -Ocean Water (DIC and DOC) and corals
• (Druffel Lab 22XX Croul)

Divide into groups of five and rotate (130
215, 215 300, 300-345)
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Groups after lunch Monday
1 Tom Adams Hilary Close Ricardo de
Pol-Holz Daniel Keck Sonja Keel
2 Yongxiang Lin Megan Mobley Ryan Moyer Suni Shah
3 Tony Quach Jeremy Shakun Gyami Shrestha Amber
Ulseth Katey Walter
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Tuesday Lab - Pretreatment

• Group in AMS lab
• Swipe tests (is my lab 14C clean?)
• Acid-Base-Acid pretreatment
• Acidification of carbonates
• Combustion of organics
• Group in CH2222
• Pretreatment purification of CO2 from air
  samples and molecular seive
• Preparation of tubes and combustion of
  organics

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Wednesday Lab

• Group in AMS lab
• Swipe tests (is my lab 14C clean?)
• Acid-Base-Acid pretreatment
• Acidification of carbonates
• Preparation of graphite by H2 reduction
• Group in CH2222
• Pretreatment purification of CO2 from air
  samples and molecular seive
• Preparation of graphite (tubes, etc) by zinc
  reduction

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Thursday Lab

• Group in AMS lab
• Preparation of graphite by H2 reduction
• What goes in a wheel?
• Group in CH2222
• Preparation of graphite by Zn reduction
• What goes in a wheel?

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Friday Lab

• AMS measurement
• (all together) Reduction of data