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(slightly more than..) 25 years of marine geochemistry
Why the geochemical approach essen2al in oceanography
Catherine Jeandel CNRS, LEGOS, Toulouse
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Major ques6ons in oceanography
• Quan:fying the circula:on
• Quan:fying the flux and fate of the elements: sources, sink and internal cycling
Fate of the chemical species?
Which tracer for what?
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Outline
• 4 tracers across 4 main temporal steps
GEOSECS legacy
70s-‐80s
Clean lab, mass spect.
WOCE&JGOFS
90s 2000-‐20s
Futurs
….s
• The 4 temporal steps
Ø 14C : the mixing rate of the oceans Ø 234U/230Th: D-‐P exchange & sePling veloci:es of the par:cles Ø Nd isotopes : D-‐P exchange, origin of waters & elements Ø Fe and isotopes: specia:on and quan:fica:on of the sources
« and the winners..» are
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First box model to calculate deep water residence 6me (Broecker et al. 1960)
Atmosphere
Pacific +Indian
Atlan:c
14C (radioac6ve, T1/2= 5700 y) • Chronometer of the deep waters • Beta coun:ng: more than 400l, long coun:ng :mes • First deep water residence :me and ven:la:on rates
GEOSECS legacy
70s-‐80s
Pacific advec6on-‐diffusion 14C model (Craig et al, 1969)
W=3 to 6 m/y τ = 800 y
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• Change of scale: mass spec, few liters, high resolu:on • WOCE : sec:ons and maps, ages of all water masses
14C
Schlosser et al, 2001
2250 y
Beau6ful tracer for circula6on quan6fica6on
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14C
• Ancillary parameter • Highly recommended
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U/Th story: first profiles
• Developed and measured during GEOSECS • Tons of water, first in situ pumps
Krishnaswami et al, 1976 Nozaki et al, 1981
GEOSECS legacy
70s-‐80s
Par6culate (dpm/104 kg)
Depth (m
)
D-‐230Th
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U soluble
U/Th: the mechanism
230Th, T1/2 75 000y: deep processes
Particles Solution
Th insoluble
GEOSECS legacy
70s-‐80s
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234U 230Th
A230>A234
230Th
230Th
230Th
230Th
230Th
230Th
230Th
230Th
230Th 230Th 230Th
230Th 230Th 230Th 234U
234U
234U
234U
230Th 230Th 230Th 230Th
Sinking Particles
Carry 230Th
Downward
230Th 230Th 230Th 230Th 230Th
kads kdes
What is observed and hypothesized? GEOSECS legacy
70s-‐80s
Courtesy of Bob Anderson
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Low Flux
High Flux
Diss. 230Th (fg/kg) K = kads/kdes
Linearity of solid and dissolved 230Th Reversible scavenging
GEOSECS legacy
70s-‐80s
Courtesy of Bob Anderson
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SAFe Aloha
QuickTime™ and a decompressor
are needed to see this picture.
INOPEX
Hayes et al., EPSL 2013
INOPEX
SAFe
Aloha
Higher particle flux (K) in NW Pacific lowers 230Th
Concentration. N
PP
(gC
m-2
yr-1
)
Aloha data from Roy-Barman et al., 1996
1D reversible scavenging model is working in quite areas
Beau6ful tracer: P-‐D processes and par6cle dynamics
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Depleted dissolved 230Th in the North Atlantic is reflecting enhanced scavenging in the nepheloids
Barely the ventilation
Hayes & Anderson, 2015
GEOTRACES IDP 2014: high resolu6on 230Th data
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Nd isotope story
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Nd isotope story
Piepgras and Wasserburg, 1987
?
GEOSECS legacy
70s-‐80s
εNd=-‐13.5
εNd=-‐9.5
-‐13.5 -‐5 Off US Atlan:c
Coast
εNd = Thermohaline circula6on tracer?
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Goldstein and Hemming,2003
εNd varia6ons imprinted in the sediments
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Nd parameters: Data from the Pacific
Lacan & Jeandel, 2001; 2005
Nd isotopic composi6on changes while Nd concentra6on remains the same
Boundary exchange
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Jeandel et al, 2007 1050 +≤≤− NdεOld
con6nent Young volcanic
Heterogeneous distribu6on of εNd
-‐15
-‐11 -‐8
-‐5
What is controlling the εNd gradient ?
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800 - 5000 m εNd
Coupling OGCM and Nd cycle (Arsouze, 2008; 2009)
FR FA FBE
?
• 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: reconciliate the Nd and εNd with depth and along the thermohaline Nd: a beau6ful tracer of sources; Warn us on the roles of the margins;
Boundary Exchange quan6fied
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Iron: an essen6al micro-‐nutrient
Varia6ons of iron inputs to the ocean
Climate Varia6ons
Fe could limit primary produc6on in ~1/2 of the ocean (Moore et al 2001)
Ø Con:nental crust : 5.6% Ø Surface of the ocean: 0.000 000 005%
ü Electron carrierà photosynthesis, respira:on ü Enzyme cofactor à Nitrogen assimila:on
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Paucity of information about deep Fe distribution limits understanding of upwelling supply and internal cycling.
2003: Sta6ons with Fe concentra6ons at depth
P. Parekh (MIT)
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Main DFe sources? Early 2000
Dust? Jickells et al, 2005
Reducing Sediments?
Others?
Elrod et al, 2004
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hPp://www.egeotraces.org/scenes/Atlan:c_Fe_D_CONC_BOTTLE_large.html
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Tagliabue et al, 2014
Role of the sediments confirmed by modelling
On the DFe inventory
On the carbon export
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Unexpected hydrothermal inputs (EPZT GEOTRACES cruise)
DFe
DMn
DAl
3He
Resing et al, Nature July 2015
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Futurs
• Contrasted distribu:ons (ex dZn&dFe) • Land-‐ocean exchange processes (ex REE, Nd&Ra)
• Quan:fying all the sources (SGD; sediment release)
• Quan:fying the internal cycling&exits • Refining paleo-‐proxies
Futurs
….s
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What was (almost) not discussed…
• Carbonate chemistry
• Carbon cycle and flux
• Par:cles in the Ocean (except when needed…)
Jeandel, Lam, Marchal, Anderson Edits
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Thank you!
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Lead and lead isotopes
Schaule and PaPerson first profiles
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Transient Tracers, 1900-‐1990
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Near-‐Bermuda Pb profiles, 1979-‐2011
Boyle et al. (2014) Oceanography 27:69
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Distance, km Distance, km
Depth, m
GEOTRACES GA03: US GT10,11 North Atlantic Transect Pb