Study Of The Phytoplankton Plume Dynamics Off The Crozet Islands (Southern Ocean): A Geochemical-Physical Coupled Approach

2169-9291The Crozet Archipelago, in the Indian sector of the Southern Ocean, constitutes one of the few physical barriers to the Antarctic Circumpolar Current. Interaction of the currents with the sediments deposited on the margins of these islands contributes to the supply of chemical elements--including iron and other micro-nutrients--to offshore high-nutrient, low-chlorophyll (HNLC) waters. This natural fertilization sustains a phytoplankton bloom that was studied in the framework of the KEOPS-2 project. In this work, we investigated the time scales of the surface water transport between the Crozet Island shelves and the offshore waters, a transport that contributes iron to the phytoplankton bloom. We report shelf-water contact ages determined using geochemical tracers (radium isotopes) and physical data based on in situ drifter data and outputs of a model based on altimetric Lagrangian surface currents. The apparent ages of surface waters determined using the three independent methods are in relatively good agreement with each other. Our results provide constraints on the time scales of the transport between the shelf and offshore waters near the Crozet Islands and highlight the key role played by horizontal transport in natural iron fertilization and in defining the extension of the chlorophyll plume in this HNLC region of the Southern Ocean

Seasonal cycle of suspended barite in the mediterranean sea

0016-703

Measuring the radium quartet (Ra-228, Ra-226, Ra-224, Ra-223) in seawater samples using gamma spectrometry

ISI Document Delivery No.: 609LE Times Cited: 13 Cited Reference Count: 51 Cited References: BOURQUIN M, ANAL CHEM UNPUB Bourquin M, 2008, MAR CHEM, V109, P226, DOI 10.1016/j.marchem.2008.01.002 Burnett WC, 2008, ESTUAR COAST SHELF S, V76, P501, DOI 10.1016/j.ecss.2007.07.027 Charette MA, 2001, LIMNOL OCEANOGR, V46, P465 Dimova N, 2008, MAR CHEM, V109, P220, DOI 10.1016/j.marchem.2007.06.016 Dulaiova H, 2008, MAR CHEM, V109, P395, DOI 10.1016/j.marchem.2007.09.001 Dulaiova H, 2006, CONT SHELF RES, V26, P1971, DOI 10.1016/j.csr.2006.07.011 ELSINGER RJ, 1982, ANAL CHIM ACTA, V144, P277, DOI 10.1016/S0003-2670(01)95545-X ELSINGER RJ, 1983, EARTH PLANET SC LETT, V64, P430, DOI 10.1016/0012-821X(83)90103-6 Foster DA, 2004, MAR CHEM, V87, P59, DOI 10.1016/j.marchem.2004.02.003 Garcia-Solsona E, 2008, MAR CHEM, V109, P198, DOI 10.1016/j.marchem.2007.11.006 Ghaleb B, 2004, J ANAL ATOM SPECTROM, V19, P906, DOI 10.1039/b402237h KAUFMAN A, 1973, J GEOPHYS RES, V78, P8827, DOI 10.1029/JC078i036p08827 VanderLoeff MMR, 1995, DEEP-SEA RES PT II, V42, P1533 Kim G, 2003, GEOPHYS RES LETT, V30, DOI 10.1029/2003GL017565 KOCZY F. 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Souhaut, M. Reyss, J. -L. 13 ELSEVIER SCI LTD OXFORD J ENVIRON RADIOACTIV SIRadium isotopes are widely used in marine studies (eg. to trace water masses, to quantify mixing processes or to study submarine groundwater discharge). While Ra-228 and Ra-228 are usually measured using gamma spectrometry, short-lived Ra isotopes (Ra-224 and Ra-223) are usually measured using a Radium Delayed Coincidence Counter (RaDeCC). Here we show that the four radium isotopes can be Ra-228 Ra-224 223 analyzed using gamma spectrometry. We report Ra-226 Ra activities measured using low-background gamma spectrometry in standard samples, in water samples collected in the vicinity of our laboratory (La Palme and Vaccares lagoons, France) but also in seawater samples collected in the plume of the Amazon river, off French Guyana (AMANDES project). The Ra-223 and Ra-224 activities determined in these samples using gamma spectrometry were compared to the activities determined using RaDeCC. Activities determined using the two techniques are in good agreement. Uncertainties associated with the Ra-224 activities are similar for the two techniques. RaDeCC is more sensitive for the detection of low Ra-223 activities. Gamma spectrometry thus constitutes an alternate method for the determination of short-lived Ra isotopes. (C) 2009 Elsevier Ltd. All rights reserved

Ra-228/Ra-226 and Ra-226/Ba ratios to track barite formation and transport in the water column

0016-703

The influence of particle composition on Thorium scavenging in the Mediterranean Sea

International audienceSediment trap data are crucial for the study of marine biochemical cycles but they need careful validation by Thorium (Th) isotopes. In the present study, 230Th, 232Th, Uranium (U), Aluminum (Al), Barium (Ba) and Manganese (Mn) were analyzed in sinking particles collected by moored sediment traps in the Ligurian sea (DYFAMED site) in order to determine the collection efficiency of the traps and to constrain which particulate phase(s) carries Th isotopes in a region with strong lithogenic inputs. The 230Thxs evaluation was based on the U content of each sample rather than on the 232Th content of each sample and the average 238U/232Th ratio of the continental crust because the latter method introduced too much uncertainty in the calculation. High trapping efficiencies (187 ± 85% at 200 m and 87 ± 11% at 1000 m) indicated no evidence of particle under-collection by the traps. The lack of correlation between 230Thxs and the carbonate or the organic matter fraction suggests that these phases are not major 230Thxs-carrying phases in the deep ocean. For most samples of the time series, the 230Thxs concentration is correlated with the lithogenic fraction and the Mn concentration, but pulses of particles with high lithogenic and Mn content and low 230Thxs concentration occur in winter. Assuming that 230Thxs is only carried by lithogenic particles, we estimate Kd_lithoTh ranges from (0.42 ± 0.04) × 107 ml/g to (0.8 ± 0.2) × 107 ml/g. Assuming that 230Thxs is only carried by authigenic Mn oxide precipitates (MnO2), we estimate that Kd_MnO2Th ranges from (0.6 ± 0.1) × 1010 ml/g to (1.1 ±0.4) × 1010 ml/g. The relative variation of Kd_MnO2Th (a factor 7) between different oceanic sites is significantly lower than the relative variation of Kd_lithoTh (a factor 50), suggesting that 230Thxs has a tighter link with MnO2 rather than with lithogenic particles and hence that 230Thxs may be more likely scavenged by MnO2 than by lithogenic particles. The unambiguous determination of the particles carrying 230Thxs remains to be done

Influence of intense scavenging on Pa-Th fractionation in the wake of Kerguelen Island (Southern Ocean)

Dissolved and particulate excess 230Th and 231Pa concentrations (noted 230Thxs and 231Paxs respectively) and 231Paxs/230Thxs activity ratios were investigated on and out of the Kerguelen plateau (Southern Ocean) in the framework of the Kerguelen Ocean and Plateau compared Study project in order to better understand the influence of particle flux and particle chemistry and advection on the scavenging of 231Pa. In the wake of Kerguelen, particulate 231Paxs is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between 230Th and 231Pa (FTh/Pa ranging from 0.06 ± 0.01 to 1.6 ± 0.2) reflects the domination of the biogenic silica in the particle pool. Along the eastern escarpment of the Kerguelen plateau, the strong 231Paxs horizontal gradient in the deep waters highlights the intense removal of 231Pa at depth, as already observed for 230Thxs. This local boundary scavenging was attributed to re-suspension of opal-rich particles by nepheloid layers, resulting in fractionation factors FTh/Pa ≤ 1 along the Kerguelen plateau slope. Therefore, both the composition (biogenic opal) and the flux (intense along the margin) of particles control the scavenging of the two radionuclides in the Kerguelen wake. The modelling of 231Pa distribution with an advection-scavenging model demonstrates that lateral advection of open ocean water on the Kerguelen plateau could supply most of the 231Pa, which is then efficiently scavenged on the highly productive plateau, as previously proposed for 230Thxs. It stresses that lateral advection can play a significant role in the overall budget of particle reactive trace elements in a coastal-open ocean system

228Ra/226Ra and 226Ra/Ba ratios to track barite formation and transport in the water column

International audienc