Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

Chlorophyll (mg m-3)

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)

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

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.

Questions that will be addressed

using the geochemical tracers

What kind of tracers ?Coupled with the physics…

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

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 shelves226Ra 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)

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

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

- εNd 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.

REE patterns, trace elements, Nd and Pb isotopes:

Source tracers

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

Fe isotopes (F. Lacan’s project)56Fe ‰ = [ (56Fe/54Fe ECHANTILLON) / (56Fe/54Fe REFERENCE) -1 ] x 103

MC/ICPMS: precision today = 0.05‰ with 20 ng of Fe

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).

Hypothesis: FeOx reduction on the continental margins could yield an isotopic fractionation

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 factorIron fluxes between the water masses and the sedimentsTo follow the evolution of this signature when leaving the margin

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

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 ( =50 years) more reactive than 231Pa ( = 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)

Chronometers of particle and water mass transport:

Th and Pa isotopes

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

Cosmogenics3He

238U 234Th 234U 230Th 226Ra 226Ra

232Th 228Ra 228Ra 228Th 230Th234U

238U 234Th

235U 231Pa

Litho. sources: Nd, lead isotopes, REE patterns, elemental abundances

Water mass pathways: 228/226Ra, Pa,Th, lead and Nd isotopes

Biogenic flux: Ba, Si, POC

Particle transport : Pa, Th, 228/226Ra in barite, lead and Nd isotopes, 3He

210Pb226Ra

Fe isotopes

Simultaneous analysis of all these tracers on sediment samples, marine particles and seawater

Information on sources and pathways of the analysed material

Multi-tracers approach

Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

M2

D6D5

D4D3

D2D1

A5

A1

A11 B11

C11

C1B1

C4

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 mlREE : 500 ml234Th : 5lFe isotopes : 4 lNd isotopes : 10 lPa/Th : 20 lRa isotopes : 50 l Cerege is missing!

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 …)