1. 228 Ra-derived ocean mixing and nutrient fluxes in the Cape Basin Yu-Te (Alan) Hsieh 10-09-2012 Kz Kx Data: Malcolm Woodward D357

2. Sea surface temperature ( o C) Salinity ( o / oo ) Study regions: Cape Basin (UK-GEOTRACES D357) Oct-Nov 2010

3. Uranium- Thorium-series isotopes

4. Radium-228 as a tracer for ocean mixing 232 Th 228 Ra T 1/2 : 5.75 years One dimensional diffusion model assumptions: 1. 228 Ra at steady-state 2.One dimensional input 3.No particulate removal Horizontal mixing (Kx) 228 Ra decay Vertical mixing (Kz) 228 Ra decay

5. Results: 226 Ra and 228 Ra profiles (1976) Methods: MC-ICP-MS (Hsieh and Henderson, 2011; Foster et al., 2004)

6. D357

7. A (A) (B) stn2 Stn4.5 (Matano and Beier, 2003) SST Hanfland, 2002 This study

8. stn2 stn1 stn0 stn3 stn4 stn4.5 ACL AAIW NADW AABW

9. Eddy High Ra, S and T From Indian Ocean coast High Ra, S and T Coast Open Ocean

10. K x = 3.8 K x = 1.3 K x (10 7 cm 2 s -1 ) Horizontal mixing (K x ) in the surface ocean

11. Vertical mixing (K z ) in the surface ocean 0.9 ± 0.2 Kz (cm 2 s -1 ) 2.1 ± 0.4 1.3 ± 0.3 Average Kz in the surface South Atlantic (based on tritium profiles): Kz = 1.5 cm 2 S -1, Li et al. (1984) (ACL eddy)

12. Vertical nutrient fluxes Vertical mixing (Kz) 228 Ra decay Nitrate (dNO 3 - /dz) Fick’s first law F nitrate = Kz (dNO 3 - /dz)

13. Nutrient profiles and vertical fluxes 0.50 ± 0.12 0.46 ± 0.13 0.51 ± 0.13 N flux (mmol m -2 d -1 ) (ACL eddy)

14. Convert nitrogen fluxes to carbon fluxes (ACL)

15. Conclusions 1. 228Ra-derived horizontal and vertical ocean mixing coefficients in the surface Cape Basin: Kx = 3.8 ± 0.8 (10 7 cm 2 d -1 ) and Kz = 0.9 ± 0.2 to 2.1 ± 0.4 (cm 2 d -1 ) 2. 228 Ra-derived vertical nutrient and carbon fluxes show comparable results with other techniques in the Cape Basin (N: 0.46 – 0.51 mmol m - 2 d -1 ; C: 3.1- 3.3 mmol m -2 d -1 ). 3. Agulhas current leakage enhances ocean mixing in the Cape Basin but not nutrient inputs.