Towards Reconciling Iron Supply and Demand in the Southern Ocean Alessandro Tagliabue 1,2 J-B Sallée 3, P.W. Boyd 4, A.R. Bowie 5, M. Lévy 6, S. Swart 2 1 University of Liverpool, UK 2 CSIR, South Africa 3 British Antarctic Survey, UK 4 University of Otago, New Zealand 5 University of Tasmania, Australia 6 LOCEAN-IPSL, France

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

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

Importance of physical processes Two main physical mechanisms: – Winter Entrainment – Diapycnal Diffusion Fe stock down to MLD MAX Some ‘detrained’ during shallowing dFe/dz at MLD Kz (± , m 2 s -1 )

Importance of physical processes Two main physical mechanisms: – Winter Entrainment – Diapycnal Diffusion Sensitive to different processes Fe stock down to MLD MAX Some ‘detrained’ during shallowing dFe/dz at MLD Kz (± , m 2 s -1 ) Buoyancy vs momentum Relative Roles unknown, implies that we don’t well know the climate sensitivity of Fe vertical supply

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 Klunder et al. (2011) ZFe MLD

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

Results

Ferricline Depths 328±198m

Ferricline Depths -Strong latitudinal trend -Modification to isopycnals drives much variability in ZFe  0 at ferricline

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

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

Diapycnal Diffusion  mol m -2 yr -1 Across ±2  and Kz estimates: 2-10 nmol m -2 d -1 OR  mol m -2 yr -1

Winter Entrainment MLDs deepen in winter – ARGO provides us this information But ZFe determinations generally from summer Assume conservation of density Use Fe,  measured to ‘project’ Fe onto  profile at time of MLD MAX

Relation to MLD MAX ZFe W – MLD MAX (m) ~210m ZFe <MLD in 22 (17%) Or 9-40 cases at ±2 

Entrainment  mol m -2 yr -1 Across ±2  :  mol m -2 yr -1 (  mol m -2 yr -1 ) “detrainment” dFe stock during shallowing accounted for from ARGO

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

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

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

Seasonal Cycling

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

Density and the Ferricline