KEOPS I & II

by TIM — last modified Apr 03, 2015 07:21 PM

KEOPS I & II

Objectives:

The major objective of the KEOPS project is to understand which processes are naturally fertilizing the Kerguelen plateau, an « HNLC » area. Besides, the efficiency of this fertilization on the seasonal CO₂ pump as well as on the trophic chain differentiations will be closely evaluated.
KEOPS I cruise occurred at the end of the austral summer (Marion-Dufresne, jan-feb.2005), KEOPS II in spring 2011 (Marion-Dufresne, oct-nov 2011)
The contribution to KEOPS of the Toulouse Marine Isotopy group is to identify the sources, transport and fate of the natural fertilizing factors with the help of key tracers (REE and Nd, Fe & Ra isotopes…)

Main tasks

The Southern Ocean is regarded as a key region in understanding the role of biogeochemical cycling on the variation of global climate. Although this Ocean is characterized by HNLC (High Nutrient Low Chlorophyll) conditions, areas of high biomass do occur including the Kerguelen-Heard one. These bloom occurrences are attributed to natural fertilization due to micronutrient inputs from the Kerguelen archipelago and plateau, which is therefore an ideal laboratory to study the mechanisms of natural iron fertilization in the Ocean. Better defining these mechanisms was the main aim of the KErguelen Ocean and Plateau compared Study project (KEOPS, PI S. Blain). For this purpose, a good understanding of the parameters forcing the biological activity, the particle dynamics and advection processes in this area was required. Two cruises were realized onboard R/V Marion-Dufresne (IPEV): KEOPS 1 at the end of summer and centered on the plateau and KEOPS 2 at the beginning of spring and which track was extended to the fertilized plume in the wake of Kerguelen.

Toulouse Marine Isotopy group strategy as part of KEOPS

The Toulouse Marine Isotopy group (TIM) proposed several complementary tracers that contribute to solve the issues raised by KEOPS. Below: tracers measured by TIM are in black, by other groups are in grey

4 Ra isotopes: Chronometers allowing us to estimate the time spanned between the instant waters left the contact with the coast and that of its analysis. Radium isotopes display 4 contrasting half-lives allowing the determination of rapid water fluxes (²²³Ra, ²²⁴Ra: short, measured on board) or slow and long distance ones (²²⁸Ra, ²²⁶Ra: long, measured back to the laboratory)
Rare Earth Elements & radiogenic Nd isotopes: Tracers of sources, more specifically of the origin of the material and of dissolved/particle exchange in the water column.
Iron concentrations and isotopes: Iron is a micro-nutrient considered as the major natural fertilizing agent. Its stable isotopes are fractioning from each other during biological uptake and the oxydo-reduction reactions. Measuring Fe isotope fractionation allows tracing the Fe sources and studying these processes
²³²Th , Mn concentrations: These tracers allows the characterization of the lithogenic sources and oxydo-reduction processes.
Pa/ Th couple, Al, Ba concentrations: these tracers are particle settling chronometers or help to identify where and how remineralization occurs: they are measured in collaboration with M. Roy-Barman, F. Dehairs, S. Jacquet etc…
All these tracers are further constrained in models in order to exploit the richness of their complementarity to constrain processes on and off the plateau.

Some selected results

Pa/Th (Venchiarutti et al, 2008, 2011)

In the framework of her thesis, Celia Venchiarutti demonstrate that

The ratio between Pa and Th was around 1 in the southern waters, a factor 10 larger than in the remaining ocean reflecting the preferential absorption of Pa on the biogenic silica, dominant in this area.

Modelling the particle settling velocity couldn’t hold without considering an external contribution of tracers due to lateral advection of waters to the plateau (in other words, the new model is in 2D); this contribution is superimposed to the scavenging processes, modifying therefore the settling speed simulation.

The resulting settling velocities estimated with the 2D modelling for the 2 tracers Th and Pa are of 3000 m/y on the Kerguelen plateau, that is 10 times larger than in the subtropical gyres and central oceanic basins (Venchiarutti et al, 2008, 2011)

The 2 tracers are strongly scavenged and depleted along the eastern margin of the plateau, likely due to the occurrence of an Intermediate Nepheloid Layer (INL, 1000-3000m)

Ra isotopes

In the framework of her thesis, V. Sanial uses radium isotopes released by the shelves of the Crozet and the Kerguelen Islands to trace the water masses that contribute to fertilize offshore waters. Radium isotopes also provide information on the rates and timescales of the transport between the shelves and offshore waters.


	A transect conducted during the KEOPS 2 project across the phytoplankton bloom that develops north of the Crozet Island allowed us to show that the transport between the shelves of the Crozet Islands was relatively fast (65 km within 10 days). Radium isotopes also provide information on the origin of the Fe fertilization (eastern Plateau versus western Plateau). Timing information provided by the radium tracer was compared with physical observations (drifter ; ADCP ; model based on altimetry data ; Sanial et al., JGR, in press). The 4 radium isotopes were also analyzed off the Kerguelen Islands (KEOPS 2 project) in order to provide information on (i) the origin of the Fe fertilization (Kerguelen Islands and/or Heard Island) and (ii) on the timescales of the transport between the shelves and offshore waters.

A transect conducted during the KEOPS 2 project across the phytoplankton bloom that develops north of the Crozet Island allowed us to show that the transport between the shelves of the Crozet Islands was relatively fast (65 km within 10 days).
Radium isotopes also provide information on the origin of the Fe fertilization (eastern Plateau versus western Plateau).
Timing information provided by the radium tracer was compared with physical observations (drifter ; ADCP ; model based on altimetry data ; Sanial et al., JGR, in press).
The 4 radium isotopes were also analyzed off the Kerguelen Islands (KEOPS 2 project) in order to provide information on (i) the origin of the Fe fertilization (Kerguelen Islands and/or Heard Island) and (ii) on the timescales of the transport between the shelves and offshore waters.

Perspectives

Nd, REE : The analysis and publication of the REE and Nd isotopes (Grenier, Garcia-Solsona, Bouvier, Lemaitre, Jeandel) are underway. Mélanie Grenier is currently doing her post doc in Hobart with Tom Trull, Francesco d’Ovidio and Andrew Bowie in order to model the sources of the fertilizing tracers.

Participants: P. van Beek, C. Jeandel, F. Lacan, M. Souhaut, V. Bouvier, C. Pradoux, M. Bourquin, C. Venchiarutti, V. Sanial, E. Garcia Solsona, M. Grenier, N. Lemaître, B. Lansard, S. Cockenpot.

Funding : CNRS-INSU, UPS/OMP, ANR, IPEV

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