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.