Towards Reconciling Iron Supply and Demand in the Southern Ocean

1. Towards Reconciling Iron Supply and Demand in the Southern Ocean Alessandro Tagliabue1,2 J-B Sallée3, P.W. Boyd4, A.R. Bowie5, M. Lévy6, S. Swart2 1University of Liverpool, UK 2CSIR, South Africa 3British Antarctic Survey, UK 4University of Otago, New Zealand 5University of Tasmania, Australia 6LOCEAN-IPSL, France a.tagliabue@liverpool.ac.uk

2. Outline • Importance of physical processes • The Ferricline • Methods • Results • Ferricline distributions and relation to MLD • Estimating Fe inputs • Generalised View of Seasonal Fe Cycle • Summary and Conclusions

3. Importance of physical processes • Southern Ocean productivity is Fe limited • Variability in production should be connected to changing degrees of Fe limitation • Much attention on external supplies • Large reservoirs below the mixed layer • Physical processes crucial in mediating transfer of Fe to the biota Boyd and Ellwood, 2010 Tagliabue et al., 2010

4. Importance of physical processes • Two main physical mechanisms: • Winter Entrainment • Diapycnal Diffusion • Fe stock down to MLDMAX • Some ‘detrained’ during shallowing • dFe/dz at MLD • Kz (±10-5-10-4, m2 s-1)

5. Importance of physical processes • Two main physical mechanisms: • Winter Entrainment • Diapycnal Diffusion • Sensitive to different processes • Fe stock down to MLDMAX • Some ‘detrained’ during shallowing • dFe/dz at MLD • Kz (±10-5-10-4, m2 s-1) Buoyancy vs momentum • Relative Roles unknown, implies that we don’t well know the climate sensitivity of Fe vertical supply

6. The Ferricline • Key control on the vertical input of dFe • Similar to the ‘nitracline’ • dFe has • Longer remineralisation length scale • Particle Scavenging • Variable biological demand • Relation to MLD at basin scale unknown MLD ZFe Klunder et al. (2011)

7. Methods • 3 complementary datasets: • New compilation of dFe measurements • ARGO co-location • Satellite estimates of iron utilisation

8. Results

9. Ferricline Depths 328±198m

10. Ferricline Depths s0 at ferricline -Strong latitudinal trend -Modification to isopycnals drives much variability in ZFe

11. Relation to MLD ZFe – MLD (m) 236±200m ZFe <MLD in 11 (8%) Or 4-19 cases at ±2s

12. Vertical Gradients • The “ferricline” is the largest Fe source • Gradients sharper in the Atlantic Basin • Some regions do show some vertical gradient at the MLD

13. Diapycnal Diffusion mmol m-2yr-1 Across ±2s and Kz estimates: 2-10 nmol m-2 d-1 OR 0.6-7.7 mmol m-2 yr-1

14. Winter Entrainment • MLDs deepen in winter • ARGO provides us this information • But ZFe determinations generally from summer • Assume conservation of density • Use Fe, s measured to ‘project’ Fe onto s profile at time of MLDMAX

15. Relation to MLDMAX ZFeW – MLDMAX (m) ~210m ZFe <MLD in 22 (17%) Or 9-40 cases at ±2s

16. Entrainment mmol m-2yr-1 Across ±2s: 9.1-30.2 mmol m-2 yr-1 (0.6-7.7 mmol m-2 yr-1) “detrainment” dFe stock during shallowing accounted for from ARGO

17. Supply versus Demand? • Iron utilisation includes recycled Fe • ‘feratios’ can be as low as 0.1 • Range of different Kz estimates • Sensitivity to Detrainment of winter dFe stock Boyd et al. 2012

18. Supply vs Demand? Only - when kz is very high - fe-ratio very low can diapycnal supply meet demand at >50% of locations

19. Supply vs Demand? Only - when detrainment losses are very high - fe-ratio very high Does entrainment NOT meet demand at >50% of locations

20. Seasonal Cycling

21. Summary and Conclusions • The ferricline is robustly decoupled from the MLD by ~200m • The ferricline depth has a strong relation to density • Only entrainment during winter is able to supply appreciable amounts of Fe over much of the S.O. • Low diapycnal inputs during summer result in a large reliance on recycled Fe in many locations • PP likely sensitive to processes that modulate winter mixing rather than summer stratification a.tagliabue@liverpool.ac.uk

22. Density and the Ferricline