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Among the objectives of KEOPS - Identification
of the mechanisms of natural iron fertilization
on the Kerguelen Plateau - Processes explaining
this fertilization - Flux studies in contrasting
environments

Contribution of the geochemical tracers ?
(LEGOS, LSCE, CEREGE, ANCH, IPGP)
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Questions that will be addressed using the
geochemical tracers

• Origin of the iron enrichment ?
• Contact with the margin when deep waters
  circulate around the plateau (release from the
  shelf sediment, dissolution of particles)
• Remineralization of sinking biogenic material
  after a massive bloom

• What is the major mechanism of the upward
  transfer of iron from deep waters to the surface
  layer?
• tidal currents bottom topography ? internal
  tides transfering deep water to the surface.

What kind of tracers ? Coupled with the physics
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Radium isotopes 226Ra 1602 a 228Ra
5.75 a

• tracer of water masses
• signature of the water masses that were in
  contact with the shelf
• input from the sediments tracer of iron
  enrichment
• information on the transit time of the water
  masses since they detached from the shelves
• 226Ra is expected to behave conservatively but
  not 228Ra we need to interact with physicists
• tracer of particles
• 226Ra/ 228Ra ratio of particles compared with
  that of seawater origin of particles that form
  in the water column ( flux studies)

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REE patterns, trace elements, Nd and Pb
isotopes Source tracers

• The isotopes
• Kerguelen Archipelago intraplate islands
• Geological material outcropping characterised by
  a specific elemental composition and isotopic
  ratios.
• - Rare Earth Element (REE) patterns completely
  OPPOSITE from the seawater REE pattern
• - eNd values range between 2 to 2 (seawater
  around 8)
• - same for lead isotopic composition
• The Elemental Composition
• Kerguelen fields different from that of the
  marine sediments (eg richer in Ti, in Fe, poorer
  in Ba)
• Analysis of sediments and interstitial water
  content will contribute to characterize the iron
  content and also to identify the source of this
  material.

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Fe isotopes (F. Lacans project)
?56Fe (56Fe/54Fe ECHANTILLON) /
(56Fe/54Fe REFERENCE) -1 x 103
MC/ICPMS precision today 0.05 with 20 ng of Fe
Hypothesis FeOx reduction on the continental
margins could yield an isotopic fractionation

• IN VITRO studies (Butler et al. 2003 and Beard et
  al, 2003)
• Bacterial reduction could yield an isotope
  deviation of -1.3
• FeS precipitation -0.3
• This allows us to assume that the dissolved iron
  released by the margins have distinct iron
  signatures compared with the continental one.
• Hypothesis verified by recent works shown in
  Portland (OS 2004) on the Californian margin
  (Severmann et al, 2004).

KEOPS

• To determine Fe signatures in the water and
  particles before and after the contact in the
  sediment cores
• Sediment and interstitial waters Fe
  fractionation factor
• Iron fluxes between the water masses and the
  sediments
• To follow the evolution of this signature when
  leaving the margin

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Chronometers of particle and water mass
transport Th and Pa isotopes

• 231Pa / 230Th ratios
• both decay products of soluble and conservative
  U (ie. 235U and 234U)
• produced at a fixed known rate in the ocean and
  sensitive to scavenging
• 230Th (t 50 years) more reactive than 231Pa (t
  200 years) trace more rapid and recent
  processes
• no efficient subtraction or ventilation
  regular increase of activities with depth
• when distributions of these tracers are not
  linear particle scavenging or advection of a
  water mass which is depleted (or enriched) in
  these tracers
• study of 230Th / 231Pa ratios (dissolved
  particulate phases)  story  of the water
  masses sinking particles ( flux studies)

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Multi-tracers approach
Cosmogenics 3He
Litho. sources Nd, lead isotopes, REE patterns,
elemental abundances
238U
234Th
Biogenic flux Ba, Si, POC
226Ra
238U
234Th
234U
230Th
226Ra
Water mass pathways 228/226Ra, Pa,Th, lead and
Nd isotopes
226Ra
232Th
228Ra
228Ra
228Th
230Th
234U
235U
231Pa
210Pb
Fe isotopes
Particle transport Pa, Th, 228/226Ra in barite,
lead and Nd isotopes, 3He
Simultaneous analysis of all these tracers on
sediment samples, marine particles and
seawater Information on sources and pathways of
the analysed material
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surface-bottom profiles at stations D6, D4 / C1,
C4, C11 B4, B11 A5, A11
- track the water mass enriched in Fe (origin,
transport ) - study of the downward particle
flux (origin, sinking rate )
A11
B11
C11
A5
B4
- water samples 24 x Niskin bottles -
particles 7 LVP (4 x 293mm 3 x 142 mm)
Tom pump for surface samples (water
particles) sediment traps Ba, Sr 30
ml REE 500 ml 234Th 5l Fe isotopes 4 l Nd
isotopes 10 l Pa/Th 20 l Ra isotopes 50 l
Cerege is missing!
D1
M2
A1
C4
D2
B1
D3
C1
D4
D5
D6