Assigning carbon fluxes to processes using

measurements of the isotopic abundance of

carbon-14Nir Y Krakauer

Department of Earth and Planetary Science

University of California at Berkeley

niryk@berkeley.edu

Isotopes as process tracers

CO2

13C

∆14C

14C (radiocarbon) in the Holocene

• 14C (λ1/2 = 5730 years) is produced in the upper atmosphere at ~6 kg / year

• Notation: Δ14C = 14C/12C ratio relative to the preindustrial troposphere (19th century tree rings)

• Tropospheric Δ14C was lowest in the south, where exchange with deep ocean water is most intense

Stratosphere +80‰90 Pg C

Troposphere 0‰500 Pg C

Shallow ocean –50‰600 Pg C

Deep ocean –170‰37000 Pg C

Land biota –3‰1500 Pg C

Sediments –1000‰1000000 Pg C

14N(n,p)14C

Air-sea gas exchange

The bomb spike: atmosphere and surface ocean Δ14C since 1950

• Massive production in nuclear tests ca. 1960 (“bomb 14C”)

• Through air-sea gas exchange, the ocean took up ~half of the bomb 14C by the 1980s

bomb spikedata: Levin & Kromer 2004; Manning et al 1990; Druffel 1987; Druffel 1989; Druffel & Griffin 1995

The contemporary budget for atmospheric Δ14C

Contribution (‰/yr)Biosphere +4Fossil fuels −10Cosmogenic +6Ocean −6

Total −6

Cf. Krakauer et al 2006

Modeled gradients in atmospheric Δ14C

Fossil-fuelburning

Ocean uptake

(‰)

Forestrelease

Growing-season Δ14C over the USA

Hsueh et al., Regional patterns of radiocarbon and fossil fuel-derived CO2 in surface air across North America, GRL (2007)

How might Δ14C help carbon-flux inversions?

1) Transport model testing: what’s the N-S gradient due to fossil-fuel burning?

• “The ratio of largest fossil fuel interhemispheric difference to smallest IHD is 1.5 compared to 2.0 for the fossil simulation results from TransCom 1” (Gurney et al 2003)

• Ongoing analysis of Scripps air archive (Heather Graven)

• Combine with earlier measurements to fix fossil vs. ocean contributions

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Levin and Hesshaimer 2000

2) Vertical dispersion of fossil fuel CO2

• Comparison of actual with modeled vertical CO2 profiles suggests that most TransCom 3 models keep too much fossil-fuel CO2 near the surface, creating a bias in flux estimation from predominantly surface data (Yang et al, New constraints on Northern Hemisphere growing season net flux, submitted to GRL)

• Vertical profiles of Δ14C enable distinguishing biotic from fossil CO2 fluxes

Turnbull et al 2006

3) Verification of fossil-fuel burning figures

• Given transport errors, regional fossil-fuel burning can be estimated from Δ14C measurements only to within ~20%; is this ever useful?

• Time series of Δ14C depletion can reveal trends in fossil fuel use more accurately, though, with accuracy of up to perhaps ~5%

Levin et al 2003

Acknowledgements• Advisors and collaborators: Inez Fung, Jim Randerson,

Tapio Schneider• Δ14C measurements and interpretation: Stanley Tyler,

Sue Trumbore, Xiaomei Xu, John Southon, Jess Adkins, Paul Wennberg, Yuk Yung, Heather Graven, François Primeau, Dimitris Menemenlis, Nicolas Gruber

• NOAA, NASA, and the Betty and Gordon Moore Foundation for fellowships

Extra slides

Global Observing Network