Dissolved rare earth elements tracing lithogenic inputs over the Kerguelen Plateau (Southern Ocean)

摘要

Dissolved rare earth elements (REEs) were measured in the water masses upstream from and over the Kerguelen Plateau (Indian sector of the Southern Ocean). Upstream from the plateau, the REE profiles were consistent with published data from the Southern and Indian Oceans. Over the plateau ( < 500m) the following features are observed: (1) higher REE concentrations at most stations, (2) weak Ce anomalies and HREE/LREE fractionations and (3) positive Eu anomalies similar to those of the Kerguelen and Heard basalts. This suggests that the high dissolved-REE contents observed in these waters result from the dissolution of lithogenic material from Kerguelen and/or Heard islands. REE pattern differences between the north and the south of the plateau suggest that both sources could be involved: Kerguelen Island influence in the north, through sporadic mesoscale intrusion of waters through the Polar Front, and Heard Island for the remaining stations, consistent with the main current direction over the Kerguelen Plateau. A one-box model yields a lithogenic-Nd flux, from the dissolution of lithogenic material, of 89 ± 20tNdy~(-1), and a corresponding dissolved-iron input larger than 100,0001 Fey~(-1). However, despite evidence of this significant lithogenic input, dissolved Fe is strongly depleted in the upper layers, contrasting with the enriched REE concentrations. This contrast stresses the different behaviours of REE and iron in the surface waters and the fact that strong fertilization does not imply dissolved Fe-replete waters.