Le néodyme (Nd)Le néodyme (Nd)
Le Nd est une terre rare (REE)
14 éléments, remplissage couche 4f
Fractionnement entre les REE (anomalie cerium)
REE: their oceanic pattern
German et al,1995
Nd :
Masse atomique A = 144,24Numéro atomique Z = 60
7 isotopes :- 142 : 27,13 %- 143 : 12,18 %- 144 : 23,8 %- 145 : 8,3 %- 146 : 17,19 %- 148 : 5,76 %- 150 : 5,64 %
Les 7 isotopes sont stables
Le 143Nd provient de la désintégration du 147Sm :
Composition isotopique du Nd :
147Sm 143Nd + T1/2=1011 yr
4
CHUR144
143SAMPLE
144
143
Nd 101
Nd
Nd
Nd
Nd
C I
C
I
Concentration et composition isotopique : des partenaires complémentaires
Concentrations reflètent une BALANCE
Composition isotopiqueun ECHANGE
Ieau>Ipart
Dissolution
Re-precipitation
valeur initiale 143Nd/144Nd CHUR = 0.50684
valeur actuelle 143Nd/144Nd CHUR = 0.512638
Fractionnement Nd / Sm lors de la différentiation manteau/ croûte continentale
LREE préfèrent le liquide résiduel = Nd ira plus dans la croûte que Sm
Sm/ Nd appauvri dans le magma crustal et enrichi dans les basaltes
Modèle CHUR : « Chondritic Uniform Reservoir »
La croûte continentale est peu radiogénique :
143Nd/144Nd < 0.512638
MORB, volcans récents très radiogéniques :
143Nd/144Nd > 0.512638
Jeandel et al, 2007
1050 Nd
Oldcontinent
Volcanicmaterial
Earth
Nd distribution on the earth surface:heterogeneity
Thermohaline Circulation
Nd oceanic distribution follows the general circulation(in the modern ocean and in the sediment)
-15 -5
The Atlantic-Pacific Nd variation follows the thermohaline circulation
Its residence time is of the order of 500-1000 y(Tachikawa et al., 2003)
When reaching the oceans, Nd is redistributed by the circulation
Num
ber
of d
ata
Nd: Present day oceanic distribution
-13 -4Lacan et al, 2012
Conservativity of Nd:
(von Blanckenburg 1999)
NorthSouth
.
NADWNd = -13.5Far from any lithogenic input: Nd behaves conservatively
Goldstein and Hemming,2003
High resolution Nd High resolution Nd recordrecord
Nd: Piotrowski et al. , EPSL 2004; Science 2005 13C: Charles et al. 1996, Ninnemann et al. 2000
Nd as paleo-tracer
-3 ± 2.5
-4 .3±2.0
-4 .6±1.0
-5.5±1.7
-9.1±0.1
-8.5±0.4
-11.2±1.1
-10.1±1.8
-11.1±2.1
(-14.7±1.2)
- 1 1 .2- 8 .6
- 1 1 .3
- 1 0 .9 - 8 .3
- 8 .3
- 6 .7
- 6 .3- 6.8
- 6 .7
North Atl. Deep Atl.
(Tachikawa, Athias, Jeandel, 2003)
Indo-PacificAustral
The Nd paradox: when concentration budget is balanced, Nd budget is not
nd distribution calculated using all the dissolved riverine fluxes and 2-30% aeolian flux (Goldstein et al, 1987; Greaves, Elderfield…)
Exchange processes are required. Margins?
A negative term is missing
A positive term is
missing
Nd=-4.3 ±0.3
Nd=-1.6 ±0.3
Nd=+7
Lacan et Jeandel 2001 EPSL
Coale et al. 1996
Dissolved iron (Coale et al, 1996)
Input from the Papouasie-New Guinea margins
Nd (and Fe) subsurface enrichments originate from the PNG
Constant Nd concentration
Kilo Moana EUC Fe Cruise, 2006
Coupling Nd concentration and isotopes:Boundary Exchange
"BOUNDARY EXCHANGE« - quantifiable using Nd IC- invisible with concentrations only
Lacan and Jeandel, 2005
(Arsouze, 2007, 2008 and Arsouze et al., 2009.)
• But: résoudre «Nd paradox » afin de réconcilier concentrations Nd et distributions IC=> representation explicite :
- sources : source reliée BE ? - processus : échange dissous /particules ? (Bertram and
Elderfield, 1993; Tachikawa et al, 1999; Nozaki and Alibo, 03 ; Oka et al, 08)
• Simulation simultanée Nd concentration et εNd : Modèle couplant dynamique / biogéochimie NEMO/PISCES.
• Approche plus réaliste va permettre
Modéliser le cycle océanique du Nd Améliorer la quantification des apports CO
Modélisation cycle Nd etNd
Existing Nd IC data(stations with at least 3 data)
13
0
-10
-20
-30
-42
Nd interpolated along the oceanic margin
Nd data compilation
(Jeandel et al, Chem. Geol., 07)
Nd cycle
3 Dissolved/particles interactions, requiring particle fields
Marine particle fieldsDetermine the sources
sources : dissolved rivers, atmospheric dusts, sediment remobilisation (BE)1
Sedimentary fluxes ( = Σ Sources at steady state) 2
Nd only3000m
Marge continentale
3Scavenging réversible
Poussières atmosphériquesApports fluviaux
Remobilisation sédimentaire
1
11
Sédimentation
2
Nd and [Nd]
Explicit simulation :
PISCES
3 m/d 50-300 m/d
PISCES: Two classes of particules • Small particles (POCs) settling velocity : 3m/d• Large particles (POCb, CaCO3, SIO2, litho)
settling velocity: 50-300 m/d
PISCES biogeochemical model
WS
Dissolved phase Particles
A
D
R
Reversible scavenging model
A: AdsorptionD: DesorptionR: RemineralisationWS: Particle settling velocity
• Partition coefficients : POCsmall : APOCs/Ad = 1.10-3
POCb / CaCO3 / bSi / litho : Ap/Ad = 2.10-5
• Sources :Dissolved rivers : FR = 2.6 108 g(Nd)/aDusts : FA = 1.0 108 g(Nd)/a
Sedimentary dissolution: FBE = 1.1 1010 g(Nd)/a
800 - 5000 m
Nd
Nd
Data
Mod
el
[Nd]
After many sensitivity tests: synthesis(Arsouze, 2008; Arsouze et al, 2009)
Mod
el
800 - 5000 m
Nd
Model coupling OGCM and Nd cycle (all sources&sinks)(Arsouze, 2008; Arsouze et al, 2009)
.•BE is THE major source term (>95% of the total : 1.1 1010 g(Nd)/an).•Dust and dissolved river inputs significant in the Atlantic surface waters.•Reversible scavenging required in order to reconciliate the Nd and εNd with depth and along the thermohaline circulation •(in agreement with H. Elderfield and Ed Sholkovitz earlier hypothesis)
Continental inputs & margins: major role in the Nd oceanic cycle.…
And for the other chemical elements?
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