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228Ra-derived ocean mixingand nutrient fluxes in the Cape Basin
Yu-Te (Alan) Hsieh10-09-2012
Kz
KxData: Malcolm Woodward
D357
Sea surface temperature ( oC)
Salinity ( o/oo )
Study regions: Cape Basin (UK-GEOTRACES D357)Oct-Nov 2010
Uranium- Thorium-series isotopes
Radium-228 as a tracer for ocean mixing
232Th228Ra228Ra
T1/2: 5.75 years
One dimensional diffusion model assumptions:1. 228Ra at steady-state2. One dimensional input3. No particulate removal
Horizontal mixing (Kx)
228Radecay
Vertical mixing (Kz)
228Ra
decay
Results: 226Ra and 228Ra profiles
(1976)
Methods: MC-ICP-MS (Hsieh and Henderson, 2011; Foster et al., 2004)
D357
A
(A)
(B)
stn2
Stn4.5
(Matano and Beier, 2003)
SST
Hanfland, 2002This study
stn2
stn1stn0
stn3stn4
stn4.5
ACL
AAIWNADW
AABW
EddyHigh Ra, S and TFrom Indian Ocean coast
High Ra, S and T
High Ra, S and T
Coast
Open Ocean
Kx = 3.8
Kx = 1.3
Kx (107 cm2 s-1)
Horizontal mixing (Kx) in the surface ocean
Vertical mixing (Kz) in the surface ocean
0.9 ± 0.2Kz (cm2 s-1)
2.1 ± 0.4 1.3 ± 0.3
Average Kz in the surface South Atlantic (based on tritium profiles): Kz = 1.5 cm2 S-1, Li et al. (1984)
(ACL eddy)
Vertical nutrient fluxes
Vertical mixing (Kz)
228Ra
decay
Nitrate
(dNO3-/dz)
Fick’s first law
Fnitrate = Kz (dNO3-/dz)
Nutrient profiles and vertical fluxes
0.50 ± 0.12 0.46 ± 0.13 0.51 ± 0.13N flux (mmol m-2 d-1)
(ACL eddy)
Convert nitrogen fluxes to carbon fluxes
(ACL)
Conclusions
1. 228Ra-derived horizontal and vertical ocean mixing coefficients in the surface Cape Basin: Kx = 3.8 ± 0.8 (107 cm2 d-1) and Kz = 0.9 ± 0.2 to 2.1 ± 0.4 (cm2 d-1)
2. 228Ra-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.
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