Olivier MarchalWoods Hole Oceanographic Institution

Roger FrancoisEarth and Ocean Sciences, U. British Columbia

Jan ScholtenMarine Environment Laboratory, Monaco

An Inverse Method to Combine of 230Th Data and a Circulation Model:

Application to the North Atlantic

Thanks to Carl Wunsch (MIT)

• Half-life = 76 kyr

• In ocean waters: 230Th/234U = O(10-4)

230Th

CENTRAL NORTH PACIFIC 23oN, 158oW

Data: Roy-Barman et al. (EPSL, 1996)

r2 = 0.99 (n=8)

unfiltered samples

filtered samples

SUBANTARCTIC ZONE 47oS, 142oE

r2 = 0.99 (n=12)

Data: R. Francois (pers. comm.)

filtered samples

SOME STATIONS WITH DISSOLVED 230Th MEASUREMENTS

Data: R. Francois (pers. comm.)

DISSOLVED 230Th IN NORTH ATLANTIC

Data: R. Francois (pers. comm.)

Can 230Th data complement physical oceanography data?

QUESTION

Use of an inverse method to assess dynamical information in 230Th data:

• Inversion # 1: Hydrographic data + circulation model

• Inversion # 2: Hydrographic data + circulation model + 230Th

STRATEGY

MODEL GRID

Total 230Th

Dissolved 230Th

HORIZONTAL SPACING:

3o x 3o

VERTICAL LEVELS:

1000 m2000 m3000 m4000 m5000 m

INVERSION # 1

HYDROGRAPHIC DATA + CIRCULATION MODEL

HYDROGRAPHIC DATA

CIRCULATION MODEL

= 0

Tarantola & Valette (1982)Mercier (1986)

HORIZONTAL FLOW BETWEEN 1000-4000 m

ERRORS IN (U,V)

Ui + Vj

HORIZONTAL FLOW BETWEEN 4000-5000 m

ERRORS IN (U,V)

Ui + Vj

INVERSION # 2

HYDROGRAPHIC DATA + CIRCULATION MODEL + 230Th

230Th MODEL

FRACTION OF 230Th IN PARTICULATE FORM (K = Cp/C)

SED.

SED.

SED.

Mean 01.015.0K (n = 75)

Cochran et al. (EPSL, 1987)Colley et al. (DSR, 1995)Moran et al. (EPSL, 1997, 2002)Vogler et al. (EPSL, 1998)

EFFECT OF 230Th IN ESTIMATION OF INTEGRATED TRANSPORTS

EFFECT OF 230Th IN ESTIMATION OF INTEGRATED TRANSPORTS

WHY DO 230Th LEAD TO HIGHER ESTIMATES OF VOLUME TRANSPORT?

IN SITU DENSITY AT 3000 m IN NORTH ATLANTIC

– 1000 (kg/m3)

< 41.66 41.66> 41.66

C.I. = 0.02 kg/m3

Data: HydroBase2 (R. Curry)

u = 0 is a possible solution

u = 0 is NOT a possible solution unless C(z) is linear

CONCLUSIONS

In 1st inversion (230Th data not considered), the flow displays a WBC and coherent structures in the interior with low statistical significance

In 2nd inversion, the flow is further constrained by 230Th data:

- Addition of 230Th leads to the estimation of a larger amplitude of the integrated meridional transports below 1000 m (by 2-9 Sv)

- Correction by 230Th for a tendency of inverse geostrophic models to lead to the inference of a vanishing flow when horizontal gradients are insignificant

Yes, 230Th data can complement physical oceanography data

PENDING QUESTIONS

How comparable are 230Th data generated in different laboratories?

How do particle fluxes and composition influence Th scavenging?

- Need for 230Th measurements on both dissolved and particulate phases (K) - Need for Th scavenging experiments with various particle types (e.g., Chase et al., 2002)

Should we aim for more sophisticated scavenging models?

COMPARISON BETWEEN INTEGRATED TRANSPORTS

Roemmich & Wunsch (1985)

Bryden et al. (2005)

Roemmich & Wunsch (1985)

Error in radiochemical balance: <C

2>1/2 = 0.75 x 105 dpm s-1

EFFECT OF 230Th IN ESTIMATION OF INTEGRATED TRANSPORTS

FLOAT TRAJECTORIES

Data from Hogg and Owens (1999)

Marchal and Nycander (J. Phys. Oceanogr., 2004)

Floats

Theory

MODEL GRID

Total 230Th

Dissolved 230Th

HORIZONTAL SPACING:

3o x 3o

VERTICAL LEVELS:

1000 m2000 m3000 m4000 m5000 m

DATA & MODEL RESIDUALS

i,o

i,oi

c

xx.r.n

i,f

i

c

)x(f.r.n

INVERSION # 2: DENSITY AND 230Th

• x0 extended to include 230Th

• C0 extended to include 230Th errors

DATA & MODEL RESIDUALS

i,o

i,oi

c

xx.r.n

i,f

i

c

)x(f.r.n

(w* = Kws = 115 m yr-1)