1. Assigning carbon fluxes to processes using measurements of the isotopic abundance of carbon-14 Nir Y Krakauer Department of Earth and Planetary Science University of California at Berkeley niryk@berkeley.edu

2. Isotopes as process tracers CO 2  13 C ∆ 14 C

3. 14 C (radiocarbon) in the Holocene 14 C (λ 1/2 = 5730 years) is produced in the upper atmosphere at ~6 kg / year Notation: Δ 14 C = 14 C/ 12 C ratio relative to the preindustrial troposphere (19th century tree rings) Tropospheric Δ 14 C 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 14 N(n,p) 14 C Air-sea gas exchange

4. The bomb spike: atmosphere and surface ocean Δ 14 C since 1950 Massive production in nuclear tests ca. 1960 (“bomb 14 C”) Through air-sea gas exchange, the ocean took up ~half of the bomb 14 C by the 1980s bomb spike data: Levin & Kromer 2004; Manning et al 1990; Druffel 1987; Druffel 1989; Druffel & Griffin 1995

5. The contemporary budget for atmospheric Δ 14 C Contribution (‰/yr) Biosphere +4 Fossil fuels−10 Cosmogenic +6 Ocean −6 Total −6 Cf. Krakauer et al 2006

6. Modeled gradients in atmospheric Δ 14 C Fossil-fuel burning Ocean uptake (‰) Forest release

7. Growing-season Δ 14 C over the USA Hsueh et al., Regional patterns of radiocarbon and fossil fuel-derived CO 2 in surface air across North America, GRL (2007)

8. How might Δ 14 C help carbon-flux inversions?

9. 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 Levin and Hesshaimer 2000

10. 2) Vertical dispersion of fossil fuel CO 2 Comparison of actual with modeled vertical CO 2 profiles suggests that most TransCom 3 models keep too much fossil- fuel CO 2 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 Δ 14 C enable distinguishing biotic from fossil CO 2 fluxes Turnbull et al 2006

11. 3) Verification of fossil-fuel burning figures Given transport errors, regional fossil-fuel burning can be estimated from Δ 14 C measurements only to within ~20%; is this ever useful? Time series of Δ 14 C depletion can reveal trends in fossil fuel use more accurately, though, with accuracy of up to perhaps ~5% Levin et al 2003

12. Acknowledgements Advisors and collaborators: Inez Fung, Jim Randerson, Tapio Schneider Δ 14 C 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

13. Extra slides

14. Global Observing Network