Assigning carbon fluxes to processes using measurements of the isotopic abundance of carbon-14 Nir Y...
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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
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