Chlorophyll (mg m -3 )

1. 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

2. 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… Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

3. 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) Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

4. 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 - 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. Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

5. 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 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 Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

6. 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) Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

7. 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 234U 230Th 210Pb 235U 231Pa 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 Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS

8. 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 C4 A1 D2 B1 D3 C1 D4 D5 D6 Catherine Jeandel, Marseille 2004: Geochemistry in KEOPS