Approche de la dynamique des particules dans le sillage des Kerguelen, à l'aide des traceurs géochimiques : 230Th et 231Pa

During the Kerguelen Ocean and Plateau compared Study Project, particle dynamics (settling velocities, residence times) and dissolved/particle exchanges (boundary scavenging) were investigated to understand the mechanisms responsible for the natural iron fertilisation observed over the Kerguelen plateau. This work proposes to use two oceanic radionuclides 230Th and 231Pa - both decay products of Uranium and submitted to particle removal - as particle dynamic tracers. More than a hundred 230Th and 231Pa concentrations in dissolved and particulate phases were obtained, resulting in 10 characteristic and relevant oceanic profiles. The plateau, in spite of its high C export revealed low 230Th derived particle settling velocities (S= 500 m/y) whereas the off-plateau stations, considered as HNLC area, exhibit higher settling speeds (S= 800 m/y). This unexpected result raises a question on the relevance of 230Th reversible scavenging model applied. The influence of lateral advection on the plateau was highlighted by a "2D" reversible scavenging model which seems more appropriate to represent the coast-open ocean system.On the plateau, high 231Pa dissolved concentrations suggest the release of 231Pa in solution, likely linked to the re-mineralisation of opal rich aggregates (efficient Pa scavenger).On the eastern escarpment of the plateau, evidence of an intensive boundary scavenging has been shown in the deep waters by conspicuous depletions in both dissolved 230Th and 231Pa concentrations with a concomitant enrichment in the particles, due to the presence of nepheloid layers or re-suspension from the shelf or bottom opal rich sediment.Lors du projet Kerguelen Ocean and Plateau compared Study (KEOPS), la dynamique des particules (vitesses de chute, temps de résidence) et les interactions entre dissous et particules (" boundary scavenging ") ont été étudiées afin de mieux comprendre les mécanismes responsables de la fertilisation naturelle en fer observée sur le plateau des Kerguelen. Notre étude utilise deux radionucléides le 230Th et le 231Pa - tous deux produits de désintégration de l'uranium et très réactifs vis-à-vis des particules - comme traceurs de la dynamique des particules.L'étude des vitesses de chute des particules, estimées à partir des distributions de 230Th a montré que, paradoxalement à son fort export de C, le plateau avait des vitesses de chute des particules plus faibles (S= 500 m.an-1) que les stations du large dans les eaux HNLC (S=800 m.an-1). Ce résultat, assez surprenant, soulève une question quant à la limitation du modèle de scavenging 1D dans le cadre de l'étude de la dynamique des particules des systèmes côte-large et suggère l'utilisation d'un modèle 2D pour reproduire le scavenging de ces régions.Sur le plateau, les fortes concentrations de 231Pa dissous suggèrent que du 231Pa a été relâché lors de la dégradation bactérienne d'agrégats, riches en opale (phase pour laquelle le Pa a une forte affinité).Sur l'escarpement à l'Est du plateau, la diminution des concentrations de 230Th et de 231Pa dissous, concomitante à l'augmentation de leur concentration dans les particules, a montré un intense boundary scavenging le long de la pente du plateau, sous l'effet de couches néphéloïdes ou de re-suspension depuis les sédiments, riches en opale

Approche de la dynamique des particules dans le sillage des Kerguelen, à l'aide des traceurs géochimiques (230Th et 231Pa)

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF

(Table 2) 230Th, 230Th xs and 232Th concentrations in dissolved, particulate and total (dissolved+small particles) for all KEOPS stations, Marion Dufresne cruise MD145

In the context of the KErguelen Ocean and Plateau compared Study (KEOPS, 19 January-13 February 2005), particle dynamics were investigated using thorium isotope measurements over and off the Kerguelen plateau. Dissolved and particulate 230Th and 232Th samples were collected at nine stations. Dissolved excess 230Th concentrations (230Thxs) vary from 0.5 to 20.8 fg/kg and particulate 230Thxs concentrations from 0.1 to 10.0 fg/kg. Dissolved and particulate 232Th concentration ranges are 16.8-450.2 pg/kg and 3.8-502.8 pg/kg, respectively. The 230Thxs concentrations increase linearly with depth down to the bottom at most of the plateau stations and down to 1000 m at the off-plateau stations. This linear trend is observed down to the bottom (1550 m) at Kerfix, the open-ocean "upstream" station located west of the Kerguelen plateau. A simple reversible scavenging model applied to these data allowed the estimation of adsorption rate constant (k1~=0.2-0.8 per year), desorption rate constant (k-1~=1-8 per year) and partition coefficients (average K=0.16±0.07). Calculated particle settling velocities S deduced from this simple model are ca. 500 m/year at most of the plateau stations and 800 m/year at all the off-plateau stations. The plateau settling velocities are relatively low for such a productive site, compared to the surrounding HNLC areas. The difference might reflect the fact that lateral advection is neglected in this model. Taking this advection into account allows the reconstruction of the observed 230Thxs linear distributions, but only if faster settling velocities are considered. This implies that the 1D model strongly underestimates the settling velocity of the particles. In the deep layers, the occurrence of intense boundary scavenging along the escarpment due to bottom sediment re-suspension and interaction with a nepheloid layer, yielding a removal of ~50% of the Th stock along the northwestward transect, is suggested

Nd and Hf isotopic compositions in the Atlantic sector of the Southern Ocean

AB: Radiogenic hafnium (Hf) and neodymium (Nd) isotopes have been used as tracers for past continental weathering regimes and ocean circulation. To date there is only very few data available on dissolved Hf and Nd isotope compositions of the modern Southern Ocean (SO). During expedition ANTXXIV/3 (February to April 2008) filtered (0.45 µm) seawater samples (20-120 liters) were collected in the Drake Passage, the Weddell Sea and the Subantarctic Atlantic Ocean (near the Zero Meridian) to determine the dissolved isotopic composition of Hf and Nd of the major water masses in the SO. The dissolved seawater isotopic compositions of both Hf and Nd in the open Drake Passage show only minor variability (εHf = 4.2 to 4.7 and εNd = -8.8 to -7.6). However, in the Weddell Sea and at stations from the southernmost part of the Drake Passage close to the Antarctic Peninsula, the Nd isotopic composition ranged from εNd = -8.9 to -6.4. At one station near the Subtropical Convergence we measuered εNd = -15.9 in surface waters and -8.9 and -9.9 in intermediate and deep waters, respectively. Compared with the previously measured particulate fraction, the dissolved Nd isotope distribution indicates that particle leaching contributes unradiogenic Nd to near surface waters, whereas the isotopic compositions of deep waters are controlled by water mass mixing. Furthermore, we infer that the Nd isotope signature of Antarctic Intermediate Water (AAIW), which is formed in the Polar Frontal Zone, has been mixed with relatively unradiogenic surface water signatures before it is subducted. Our data therefore suggest that the variability of the surface water Nd isotope composition is relevant for the final Nd isotope signature of the submerging intermediate waters in the SO