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KEOPS

by TIM last modified Nov 21, 2019 02:50 PM

Published by François Lacan, November 2019

 

Study of the natural iron fertilization offshore Kerguelen Island, Southern Ocean



Objectives:

The major objective of the KEOPS project is to understand which processes are naturally fertilizing the Kerguelen plateau, which is a region where an annual phytoplankton bloom takes place, although it is located in HNLC waters of the Southern Ocean. The efficiency of this fertilization on the seasonal CO2 pump as well as on the trophic chain differentiations will be closely evaluated.
The KEOPS I cruise took place at the end of the austral summer (Marion-Dufresne, Jan-Feb. 2005), while the KEOPS II cruise was in spring 2011 (Marion-Dufresne, Oct-Nov 2011). The contribution of the Toulouse Marine Isotopy group to the project was to identify the sources, transport and fate of the natural fertilizing factors with the help of key tracers (REE and Nd, Fe & Ra isotopes…).


Scientific Rationale

 

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 as it is the case off the coast of Kerguelen and Heard Islands. These phytoplankton bloom occurrences are attributed to natural fertilization due to micronutrient inputs from the Kerguelen archipelago and plateau. This region 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 Islands.

 

Strategy/organization


The Toulouse Marine Isotopy group (TIM) proposed to use several complementary tracers that contribute to solve the issues raised by KEOPS. Below: tracers measured by TIM are in black and by other groups in gray :
Ra isotopes (223Ra, 224Ra, 226Ra, 228Ra): Tracers of ocean circulation and horizontal/vertical mixing. Chronometers that allow us to estimate the transit time of waters that interacted with sediments deposited onto continental shelves. Radium isotopes display various half-lives (ranging from several days to 1600 y) that allow us to study different time/space scales. Radium isotopes were used to trace the input of iron released by the sediments (deep sediments and coastal sediments).
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 allow 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 have been measured in collaboration with M. Roy-Barman, F. Dehairs, S. Jacquet.

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. Pa, Th, Ba and part of REE and Ra results have been published before 2015. Here are highlighted the most recent results only.

 

Results and perspectives


Ra isotopes (Sanial et al., 2014 ; Sanial et al., 2015 ; Bourquin et al., 2008 ; van Beek et al., 2008)

 

Rare earth elements - Nd isotopic compositions (Grenier et al., 2018; Zhang et al., 2008)

The differences in the REEs patterns (eg. Eu/Eu*) enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident (Fig. 2).

 

Participants:

Pieter van Beek, Catherine Jeandel, François Lacan, Marc Souhaut, Catherine Pradoux, Moustafa Belhadj, Virgnie Sanial, Mélanie Grenier, Nolwenn Lemaître, Lise Artigue, Michael Bourquin, Célia Venchiarutti, Ester Garcia-Solsona, Bruno Lansard, Sabine Cockenpot


Collaborations:

LOMIC, MIO, LEMAR, LOCEAN, LSCE, VUB


Fundings:

ANR; INSU; IPEV; UPS/OMP


Peer review articles related to this project (papers related to KEOPS 2 only) 

Grenier M., Garcia-Solsona E., Lemaitre N., Trull T.W., Bouvier V., Nonnotte P., van Beek P., Souhaut M., Lacan L., Jeandel C., Differentiating lithogenic supplies, water mass transport and biological processes on and off the Kerguelen Plateau using rare earth element concentrations and neodymium isotopic compositions, Frontiers in Marine Science, in press

Sanial V., van Beek P., Lansard B., Souhaut M., Kestenare E., d’Ovidio F., Blain S., 2015. Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen, Biogeosciences, 12, 1415-1430, doi:10.5194/bg-12-1415-2015.

Sanial V., van Beek P., Lansard B., d'Ovidio F., Kestenare E., Souhaut M., Zhou M., Blain S., 2014. Study of the phytoplankton plume dynamics off the Crozet Islands (Southern Ocean): A geochemical-physical coupled approach. Journal of Geophysical Research Oceans 119, 2227-2237.


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