Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen

Type Article
Date 2015-03-05
Language English
Author(s) Sanial V.1, Van Beek P.1, Lansard B.1, 2, Souhaut M.1, Kestenare E.1, D'Ovidio F.3, Zhou M.4, Blain S.5, 6
Affiliation(s) 1 : UPS, Lab Etud Geophys & Oceanog Spatiales, Observ Midi Pyrenees, CNRS,CNES,IRD,LEGOS, F-31400 Toulouse, France.
2 : UVSQ, Lab Sci Climat & Environm, LSCE, IPSL,CEA,CNRS,IPSL, F-91198 Gif Sur Yvette, France.
3 : Univ Paris 06, Univ Paris 04, CNRS, LOCEAN,IPSL, F-75005 Paris, France.
4 : Univ Massachusetts, Dept Environm Earth & Ocean Sci, Boston, MA 02125 USA.
5 : Univ Paris 06, Univ Paris 04, Lab Oceanog Microbienne, Observ Oceanol,UMR7621, F-66650 Banyuls Sur Mer, France.
6 : Observ Oceanol, Lab Oceanol Microbienne, CNRS, UMR7621, F-66650 Banyuls Sur Mer, France.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2015-03-05 , Vol. 12 , N. 5 , P. 1415-1430
DOI 10.5194/bg-12-1415-2015
WOS© Times Cited 18
Note Special issue KEOPS2: Kerguelen Ocean and Plateau Study 2
Abstract The Southern Ocean is known to be the largest high-nutrient, low-chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually downstream of the Kerguelen Islands, a bloom which is sustained partly by iron released from the sediments deposited onto the shelves. In the framework of the KEOPS-2 project, we used radium isotopes (Ra-224, T-1/2 = 3.66 d; Ra-223, T-1/2 = 11.4 d; Ra-228, T-1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sedimentderived inputs (including iron and other micronutrients) towards offshore waters. Significant Ra-224 and Ra-223 activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant Ra-224 and Ra-223 activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the polar front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales on the order of several days up to several weeks) and (ii) that the polar front is not a physical barrier for the chemical elements released from the sediments of the Kerguelen Plateau. The Ra data set suggests that iron and other micronutrients released by the shelves of the Kerguelen Islands may contribute to fueling the phytoplankton bloom downstream of the islands, despite the presence of the polar front. However, the heterogeneous distribution of the Ra-224 and Ra-223 activities in surface waters suggests that this supply across the front is not a continuous process but rather a process that is highly variable in space and time.
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