outline 1. geochemistry of 14 c

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