Outline

1
Outline 1. Geochemistry of 14C
2. Examples, with emphasis on scaling and
testing models
For additional detail, see notes from Radiocarbon
in Ecology and Earth System Science Short
Course https//webfiles.uci.edu/setrumbo/public/s
hortcourse/radiocarbon_short_course.html
2
Radiocarbon is how we tell time in the carbon
cycle
The least abundant naturally occurring isotope of
carbon C-12 (98.8) C-13 (1.1)
14C (lt10-10 ) or 1 14C 1 trillion 12C 14C is
the longest lived radioactive isotope of C, and
decays to 14N by emitting a b particle
(electron)
3
14C is continually produced in the
upperatmosphere by nuclear reaction of nitrogen
with cosmic radiation.
Cosmic ray
proton
thermal neutron
14N nucleus
14C nucleus
spallation products
14CO
Oxidation, mixing
Ocean/biosphere exchange
stratosphere
14CO2
troposphere
4
Unlike stable isotopes, radiocarbon is constantly
created and destroyed
Loss by radioactive decay
Production in stratosphere
Total number of 14C atoms (N) in Earths C
reservoirs
-l N
l Radioactive decay constant, 1/8267 years
Total amount of radiocarbon on Earth can (and
does) vary with factors that influence cosmic ray
interaction with upper atmosphere
5
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
6
Reporting of 14C data 1 Fraction Modern (FM)
Corrected to a common d13C value
(Modern is 1950)
The 14C standard Ninety-five percent of the
activity of Oxalic Acid I
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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 atmosphere 14CO2), corrected
  for radioactive decay to 1950. This is Modern, or
  a 14C/12C ratio of 1.18x10-12, which decays at a
  rate of 13.6 dpm per gram carbon.

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Reporting of 14C data 2 Radiocarbon Age
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 (i.e. Modern).
F
Drops to 0.5 in 5730 years (t1/2)
Drops to 0.25 in 2t1/2 years
t1/2
Years
9
Radiocarbon Age (Libby age)
Radiocarbon Age -(1/l14)ln(FM) Where FM 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. Remember that the age reported by 14C
labs uses an incorrect half-life for
geochemical purposes that age is NOT a
residence time!
10
The second way to make radiocarbon - bomb 14C-
makes 14C a useful tracer of the global C cycle
over the last 50 years
11
http//www.iup.uni-heidelberg.de/institut/forschun
g/groups/kk/14co2.html
12
For tracking bomb 14C we use yet another way of
expressing 14C data
Deviation in parts per thousand (per mil, ) from
the isotopic ratio of an absolute standard (like
stable isotope notation)
Corrects for decay of OX1 standard since
1950 This gives an absolute value of radiocarbon
that does not change with time
13
Wait - We know 13C is fractionated by kinetic and
equilibrium processes because of its mass so
14C must be too! How does that affect ages, etc?
Remember FM 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 d13C of -25 (14C is
either added or subtracted, assuming 14C is
fractionated twice as much as 13C)
14
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
13C12C Therefore a 20 difference in 13C means
40 difference in 14C Expressed as an age
this is -8033ln(.96) 330 years
To correct using 14C/12C
15
Examples of using radiocarbon for spatial
extrapolation/model testing

• The Suess effect and isodisequilibrium
• A direct test for ecosystem carbon cycle models
  (how many soil pools?)
• Partitioning soil respiration sources

16
SUESS HERADIOCARBON CONCENTRATION IN MODERN
WOOD, SCIENCE, 122 (3166) 415-417 1955
The Suess Effect Atmosphere - Carbon dioxide
(gas) CO2
Methane (gas) CH4 Ocean - dissolved
ions (bicarbonate and carbonate) organic
matter Land - Organic matter - Carbon is a
constituent of all living things
Land, air, water
Fossil organic matter (coal, petroleum, natural
gas)? OLD, NO RADIOCARBON Limestone (solid)
CaCO3
Solid Earth
17
Suess effect in 13C Depletion of Atmospheric
d13C by Fossil Fuels AND Deforestation (land C
source to atmosphere)
d13C (per mil)
CO2 (ppm)
Francey et al. 1999
18
What makes us sure CO2 increase is caused by
humans? Suess effect in radiocarbon - depletes
14C

Tree rings

Broecker et al. 1983
19
Because the atmosphere is changing with time in
13C and 14C, Isotopic reservoirs in ocean or land
reservoirs that are not in steady state with the
contemporary atmosphere degree of
isodisequilibrium varies with size of gross
exchange with atmosphere and mean age of respired
CO2
Gba
Gab
Fung et al. 1997 GBC
-6.5
Atm. d13C ()
tb
Isotopic Disequilibrium
-8.0
time
tb Mean Residence Time
20
Example of a mass balance What is the 14C
signature of CO2 being respired from soil and
accumulating in a chamber?
40 minutes
1000 ppm D14C 95
380 ppm D14C 60
CO2 mass balance 380 ppm X 1000 ppm
X 620 ppm 14C mass
balance 380ppm 60 620ppmY 1000ppm95
Y
116
21
Radiocarbon of soil-respired CO2 provides a
direct measure of isodisequilibrium mean age
of several years up to a decade
D14C
D14C
22
Model Prediction of 14C in atmospheric CO2 in
current boundary layer
Lows in northern hemisphere from fossil fuel
burning
Max. at equator biosphere recycling (large GPP
and lag of several years)
Krakauer et al. Tellus (in press)
See also Randerson et al. 2002 GBC
23
Continental Variations in Atmospheric D14C
measured using annual plants
?14C measurements of corn from the continental
U.S. during the summer of 2004 Hsueh et al
Geophy. Res. Lett. (2007)
24
Tests many aspects of carbon cycle, tracer
transport models Boundary layer
ventilation Spatial distribution of fossil fuel
sources Mean of respired CO2
Hsueh et al. GRL 2007
Annual plants are imperfect recorders (biased to
am hours?, spring season)
25
Examples of using radiocarbon for spatial
extrapolation/model testing

• The Suess effect and isodisequilibrium
• A direct test for ecosystem carbon cycle models
  (how many soil pools?)
• Partitioning soil respiration sources

26
Examples of using radiocarbon for spatial
extrapolation/model testing

• The Suess effect and isodisequilibrium
• A direct test for ecosystem carbon cycle models
  (how many soil pools?)
• Partitioning soil respiration sources

27
Simplified soil C cycle
CO2
Key factors climate, vegetation
mineralogy, time.
Plant Litter
Microbes
Microbial Byproducts
Stabilized SOM
Carbon Pools in Models
DOC
Metabolic and Resistant Plant Material
Microbial
Active
Passive
Slow
Days Years Decades Centuries
Millennia
Time
28
Approach 1. Attempt to match model pools to
physically and chemically isolated fractions in
soils
Problem We do not yet have fractionation
methods that unequivocally isolate homogeneous
fractions analogous to those in models
Plant Litter
Microbes
Low density gt Silt size
Microbial Byproducts
Stabilized SOM
PLFA incubations
Low density lt Silt size
High density
Metabolic and Resistant Plant Material
Microbial
Active
Slow
Passive
29
Physical and chemical separation of soils can
help isolate pools with different turnover
timesHowever, even these pools contain both
faster- and slower-cycling material
Bulk soil 70
30
14C signature of terrestrial carbon pools
1000
800
3 yr
600
With only one data point, non-unique solution
?14C ()
400
30 yr
200
80 yr
0
-200
1950
1960
1970
1980
1990
2000
Year
Turnover time 1/k
C(t) R(p) I R(atm) C(t-1) R(p-1) - k
C(t-1) R(p-1) - ? C(t-1) R(p-1)
31
Approach 2. Use CO2 derived from microbial
respiration as a direct measure of the time lag
between fixation and decomposition
CO2
Allows more direct comparison with ecosystem
model predictions
Plant Litter
Microbes
Microbial Byproducts
Stabilized SOM
Metabolic and Resistant Plant Material
Microbial
Active
Slow
Passive
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Data from these field sites
Boreal forest, central Manitoba (NOBS)
Temperate Mixed (Harvard) and conifer (Howland)
forests
Sierra Nevada Elevation gradient (temperature and
vegetation change with elevation)
Tropical Forest (Manaus, Santarem, Brazil)
Heterotrophic Respiration is measured by putting
litter and 0-10 cm soil cores in sealed jars,
then measuring the rate of CO2 evolution and the
isotopic signature of evolved CO2. Short-term
incubations large roots removed, all at 23 C and
field moisture, except boreal soils (incubated at
average in situ temperatures)
33
Data for O horizon (surface layer) Incubations
for four forest types
5 years
D14C
D14C
Year
34
DD14C (D14CCO2 - D14Catm) of respired CO2
Measurements suggest strong temperature
sensitivity Latitudinal gradient compared to
Sierra Nevada
Litter/O horizon
Mineral Soil
DD14C
Site Mean Annual Temperature
35
DD14C (D14CCO2 - D14Catm) of respired CO2
Measurements suggest strong temperature
sensitivity Latitudinal gradient compared to
Sierra Nevada
Litter/O horizon
0-5 cm Mineral Soil
15 years
DD14C
15 years
gt 50 years
3 years
2-3 years
Site Mean Annual Temperature
36
Estimate age of respired CO2 using a
pulse-response experiment for CASA
Tropical forest
Temperate forest
CO2 respired
Boreal forest
Thompson,and Randerson, Global Change Biol., 1999.
Years since pulse
37
CASA pulse response function provides a
prediction of the 14C of heterotrophically
respired CO2
400
S
Amount of C respired in year i
Atmosphere D14C in year i
X
i0
400
S
Amount of C respired in year i
i0
38
Comparison to CASA Prediction Example for the
tropics
Litter/O horizon
Mineral Soil
Control
No Wood
DD14C
Site Mean Annual Temperature
39
Comparison to CASA Prediction CASA has shorter
lag at low temperature Longer lag at high
temperature
Litter/O horizon
Mineral Soil
Control
No Wood
DD14C
Site Mean Annual Temperature
40
Comparison to CASA Prediction Removing inputs
from coarse wood debris improves agreement in the
tropics
Litter/O horizon
Mineral Soil
Control
No Wood
DD14C
Site Mean Annual Temperature
41
Isotopes of C contain independent information

• ? 13C integrates multiple physiological
  processes
• 14C time since C assimilation includes time in
  the plant! See Radiocarbon Short Course for more!