Outline 1. Geochemistry of 14 C 2. Examples, with emphasis on scaling and testing models For...
-
Upload
naomi-lloyd -
Category
Documents
-
view
213 -
download
0
Transcript of Outline 1. Geochemistry of 14 C 2. Examples, with emphasis on scaling and testing models For...
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/shortcourse/radiocarbon_short_course.html
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 (<10-10 %) or 1 14C : 1 trillion 12C
14C is the longest lived radioactive isotope of C, and decays to 14N by emitting a particle
(electron): )(147
146 energyNC
14C is continually produced in the upperatmosphere by nuclear reaction of nitrogen with
cosmic radiation.
Cosmicray
spallationproducts
thermalneutron
proton
14Nnucleus
14Cnucleus
Oxidation,mixing
14CO2
stratosphere
troposphere
Ocean/biosphereexchange
14CO
Unlike stable isotopes, radiocarbon is constantly created and
destroyed
Total number of 14C atoms (N) in
Earth’s C reservoirs
Production in stratosphere
Loss by radioactive decay
- N
Total amount of radiocarbon on Earth can (and does) vary with factors that influence cosmic ray interaction with
upper atmosphere
= Radioactive decay constant, ~1/8267 years
Amount of carbon (x1016 moles) 6 1.01.0 1.7-2.0%
typical ratio of typical ratio of 1414C/C/1212C C divided by the Modern (i.e. divided by the Modern (i.e. atmospheric) atmospheric) 1414C/C/1212C ratioC ratio per cent of total per cent of total
1414C in the major C in the major global C global C reservoirsreservoirs
Atmosphere (CO2)
280 0.840.84 65-78%Deep Ocean (DIC)
10 0.60.6 2%DOC
30 0.950.95 8-10%Surface Ocean (DIC) 6 0.970.97 1.6-2%
Terrestrial Biota
13 0.900.90 3-4%
Soil Organic Matter
7-70 0.950.95 2-18%Coastal / Marine Sediment
Where the 14C is depends on (1) how much C is there (2) how fast it exchanges with the atmosphere
Reporting of 14C data #1: Fraction Modern (FM)
95.0
ModernFraction
19- OX1,12
14sample,-25
12
14
C
C
C
C
The 14C standard: Ninety-five percent of the activity of Oxalic Acid I
(“Modern” is 1950)
Corrected to a common 13C value
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.
0
0.2
0.4
0.6
0.8
1
0 20000 40000
Radioactivity = number of decays per unit time = dN/dt
dN/dt = -14N,where N is the numberof 14C atoms;
dN/N = -14dt
T = (-1/ 14)ln (N(t)/N(0))
If radiocarbon production rate and its distribution amongAtmosphere, ocean and terrestrial reservoirs is constant,Then N(0) = atmospheric 14CO2 value (i.e. Modern).
F
Years1/2
Drops to 0.5 in 5730 years (1/2)
Drops to 0.25 in 2*1/2 years
Reporting of 14C data #2: Radiocarbon Age
Radiocarbon Age (Libby age)
Radiocarbon Age = -(1/14)*ln(FM)
Where FM is Fraction Modern and 14 is the decay constant for 14C
The half life (t1/2 = 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.
Remember that the “age” reported by 14C labs uses an ‘incorrect’ half-life for geochemical purposes; that “age” is NOT a residence time!
The second way to make radiocarbon - “bomb 14C”- makes 14C a useful tracer of the global C cycle over the last 50 years
http://www.iup.uni-heidelberg.de/institut/forschung/groups/kk/14co2.html
For tracking bomb 14C we use yet another way of expressing 14C data:
1000 1
95.0 82671950)-(y-
19- OX1,12
14
sample,-2512
14
14
eC
C
C
C
C
Corrects for decay of OX1 standard since 1950This gives an absolute value of radiocarbon that does not change with time
Deviation in parts per thousand (per mil, ‰) from the isotopic ratio of an absolute standard (like stable isotope notation)
Remember FM are corrected to a common 13C value and therefore 14C values reported as fraction Modern, Libby Age, or 14C do not
reflect mass-dependent fractionation of isotopes.
The sample is corrected to have 13C of -25 ‰(14C is either added or subtracted, assuming 14C
is fractionated twice as much as 13C)
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
Why must there be a correction for mass dependent
fractionation?
Leaf13C = -28 ‰
CO2 in air13C = -8 ‰
14C-12C mass difference is ~twice that of 13C–12CTherefore a 20 ‰ difference in 13C
means ~ 40 ‰ difference in 14CExpressed as an ‘age’ this is -8033*ln(.96) = 330 years
2
1000δ
1
100025
1
δ‰sample,C12C14
25‰]sample[C12C14
To correct using 14C/12C:
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
The Suess EffectThe Suess Effect
AtmosphereAtmosphere - Carbon dioxide (gas) CO - Carbon dioxide (gas) CO22
Methane (gas) CH4
Ocean - dissolved ions (bicarbonate and carbonate) + organic matter
LandLand - Organic matter - Carbon is a constituent - Organic matter - Carbon is a constituent of all living thingsof all living things
Solid Earth
Land, air, water
Fossil organic matterFossil organic matter (coal, petroleum, (coal, petroleum, natural gas)natural gas) OLD, NO RADIOCARBONOLD, NO RADIOCARBONLimestone (solid) CaCO3
SUESS HERADIOCARBON CONCENTRATION IN MODERN WOOD, SCIENCE, 122 (3166): 415-417 1955
Suess effect in 13C: Depletion of Atmospheric 13C by Fossil Fuels AND Deforestation (land C source to atmosphere)
Francey et al. [1999]
350
340
330
320
310
300
290
280
198019601940192019001880186018401820180017801760174017201700
-7.8
-7.6
-7.4
-7.2
-7.0
-6.8
-6.6
-6.4
13 C
(pe
r m
il)
CO
2 (p
pm)
What makes us sure CO2 increase is caused by humans?
Suess effect in radiocarbon - depletes 14C
Tree rings
Broecker et al. 1983
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
Atm.13C (‰)
time
Gba Gab
Isotopic Disequilibrium
b
b = Mean Residence Time
-6.5
-8.0
Fung et al.1997 GBC
Example of a mass balance: What is the 14C signature of CO2 being respired from soil and accumulating in a chamber?
380 ppm 14C = 60‰
CO2 mass balance: 380 ppm + X = 1000 ppm X = 620 ppm14C mass balance:380ppm* 60‰ + 620ppm*Y‰ = 1000ppm*95‰ Y = 116‰
1000 ppm 14C = 95‰
40 minutes
Radiocarbon of soil-respired CO2 provides a direct measure of isodisequilibrium “mean age” of several years up to a decade
50
150
250
350
450
550
650
1960 1970 1980 1990 2000
14 C
50
100
150
200
1998 2000 2002 2004
Harvard Forest MAatmosphere
Howland Forest, METapajos Forest, BrazilBonanza Creek, Alaska
Manitoba, Canada
14 C
Model Prediction of 14C in atmospheric CO2 in current boundary layer
Krakauer et al.Tellus (in press)
Max. at equator; biosphere recycling (large GPP and lag of several years)
Lows in northern hemisphere from fossil fuel burning
See also Randerson et al. 2002 GBC
Δ14C measurements of corn from the continental U.S. during the summer of 2004
Hsueh et al Geophy. Res. Lett. (2007)
Continental Variations in Atmospheric 14C measured using
annual plants
Hsueh et al. [GRL 2007]
Tests many aspects of carbon cycle, tracer transport models:Boundary layer ventilation**Spatial distribution of fossil fuel sources**Mean of respired CO2
Annual plants are imperfect recorders
(biased to am hours?, spring season)
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
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
Stabilized SOM
Microbial Byproduc
ts
Microbes
Plant Litter
CO2
Days Years Decades Centuries MillenniaTime
Metabolic and Resistant Plant
MaterialActive
MicrobialSlow Passive
Carbon Pools in Models DOC
Simplified soil C cycle
Key factors: climate, vegetation mineralogy, time.
Stabilized SOM
Microbial Byproduc
ts
Microbes
Plant Litter
Metabolic and Resistant Plant
MaterialActive
MicrobialSlow Passive
Approach 1. Attempt to match model pools to physically and chemically isolated fractions in soils
Low density> Silt size
PLFAincubations
Low density< Silt size
High density
Problem: We do not yet have fractionation methods that unequivocally isolate homogeneous fractions analogous to those in models
Physical and chemical separation of soils can help isolate pools with different turnover times
However, even these pools contain both faster- and slower-cycling material
Bulk soil +70 ‰
Flotation,Flotation,sievingsieving
Detritus +200‰
Microbially altered material
(humus) +90‰
Density separationDensity separation
Low density
+100‰
High density+50‰
Hydrolyzate
Residue -180‰
Extraction with Extraction with acids and basesacids and bases
14 C
(‰
)
Year
-200
0
200
400
600
800
1000
1950 1960 1970 1980 1990 2000
atmosphere
turnover time
14C signature of terrestrial carbon pools
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)
With only one data point, non-unique solution
3 yr
30 yr
80 yr
Turnover time = 1/k
Stabilized SOM
Microbial Byproduc
ts
Microbes
Plant Litter
CO2
Approach 2. Use CO2 derived from microbial respiration as a direct measure of the time lag between fixation and decomposition
Allows more direct comparison with ecosystem model predictions
Metabolic and Resistant Plant
MaterialActive
MicrobialSlow Passive
Tropical Forest (Manaus, Santarem, Brazil)
Temperate Mixed (Harvard) and conifer (Howland) forests
Boreal forest, central Manitoba (NOBS)
Sierra Nevada Elevation gradient (temperature and vegetation change with elevation)
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)
Data from these field sites:
0
100
200
300
400
500
600
700
800
900
1960 1980 2000
tropical forest
temperate forest (HW)
boreal forest
temp/conif
50
100
150
200
1990 1995 2000 2005
14 C
14C
Data for O horizon (surface layer)Incubations for four forest types
~5 years
Year
0
50
100
150
-10 0 10 20
MAT
14C (14CCO2 - 14Catm) of respired CO2
14
C
0
50
100
150
-10 0 10 20
MATSite Mean Annual Temperature
Measurements suggest strong temperature sensitivityLatitudinal gradient compared to Sierra Nevada
Litter/O horizon Mineral Soil
0
50
100
150
-10 0 10 20
MAT
14C (14CCO2 - 14Catm) of respired CO2
14
C
0
50
100
150
-10 0 10 20
MATSite Mean Annual Temperature
Measurements suggest strong temperature sensitivityLatitudinal gradient compared to Sierra Nevada
Litter/O horizon 0-5 cm Mineral Soil
~2-3 years
~ 15 years
~3 years
~15 years
> 50 years
Estimate age of respired CO2 using a pulse-response experiment for CASA
0
25
50
75
100
125
0 5 10 15 20 25
Manaus, Brazil
Santarem, Brazil
Harvard Forest, MA
Bear Brook, ME
NOBS, Manitoba
Years since pulse
CO
2 r
espi
red
Tropical forest
Temperate forest
Boreal forest
Thompson,and Randerson, Global Change Biol., 1999.
-200
0
200
400
600
800
1000
1950197019902010
1
4C
(p
er m
il)
0
25
50
75
100
125
0 5 10 15 20 25
X
CASA pulse response function provides a prediction of the 14C of heterotrophically respired CO2
Amount of C respired in year i
Atmosphere 14Cin year i
Amount of C respired in year i
400i=0
400
i=0
0
50
100
150
-10 0 10 20
MAT
14
CComparison to CASA Prediction: Example for the
tropics
Control
No Wood
Litter/O horizon Mineral Soil
0
50
100
150
-10 0 10 20
MATSite Mean Annual Temperature
0
50
100
150
-10 0 10 20
MAT
14
CComparison to CASA Prediction –
CASA has shorter lag at low temperatureLonger lag at high temperature
Control
No Wood
Litter/O horizon Mineral Soil
0
50
100
150
-10 0 10 20
MATSite Mean Annual Temperature
0
50
100
150
-10 0 10 20
MAT
14
CComparison to CASA Prediction –
Removing inputs from coarse wood debris improves agreement in the tropics
Control
No Wood
Litter/O horizon Mineral Soil
0
50
100
150
-10 0 10 20
MATSite Mean Annual Temperature
13C = integrates multiple
physiological processes
14C = time since C assimilation; includes time in the plant! See Radiocarbon
Short Course for more!
Isotopes of C contain independent information