Sediment trap data

Constraining the seasonal particle flux in the eastern North Atlantic with Thorium isotopes M. Roy-Barman (1), R. El Hayek (1), I. Voege (2), M.Souhaut (3), N. Leblond (4), C. Jeandel (2). (1) LSCE, Avenue de la Terrasse, Gif sur Yvette, France, (2) AWI, PO Box , Bremerhaven, Germany, (3) LEGOS, 14 Avenue E. Belin, 31400, Toulouse France, (4) LOV, BP 28, Villefranche sur mer, France Poster Area Esplanade poster board number P0600

Principle of the sediment trap calibration. Problems… =trapping efficiency 230 Th production above the trap 230 Thtrappedflux 230 Thfixedonparticles 234 U  230 Th Problems… =trapping efficiency 230 Th production above the trap 230 Thtrappedflux 230 Thfixedonparticles 234 U  230 Th

Time dependent of the sediment trap calibration.

Eddy structure of the sampling area (M. Assenbaum et al., 2003) Pomme 1Pomme 2 Pomme 3 (winter) (spring) (Automn)

1000 m samples

Constraining the seasonal particle flux in the eastern North Atlantic with Thorium isotopes M. Roy-Barman (1), R. El Hayek (1), I. Voege (2), M.Souhaut (3), N. Leblond (4), C. Jeandel (2). (1) LSCE, Avenue de la Terrasse, Gif sur Yvette, France, (2) AWI, PO Box , Bremerhaven, Germany, (3) LEGOS, 14 Avenue E. Belin, 31400, Toulouse France, (4) LOV, BP 28, Villefranche sur mer, France Introduction: Constraining the present and past oceanic biological pump requires a good understanding of the marine particle dynamics and of their interaction with the surrounding water column. Particle fluxes are studied mainly with sediment traps, so that it is critical to evaluate their efficiency. The average efficiency of moored sediment traps is calibrated by the 230 Th method. 230 Th is produced uniformely in the ocean by radioactive decay of 238 U and it is rapidely removed from the water column by scavenging on settling particles. The trapping efficiency is obtained by comparing the 230 Th xs trapped flux with the 230 Th production in the overlying watercolumn. This was previously done with trap data obtained over a year in order to avoid the problem of estimating the change of 230 Th inventory in the water column. The idea: Here we propose to extend this method to estimate the efficiency of moored traps on a seasonal scale. It requires high precision data to measure the change of 230 Th inventory in the water column. By combining sediment trap data with the change of 230 Th inventory, it is possible to obtain the trapping efficiency for a short time period. Water column results: While the 230 Th xs decreases in the surface waters from POMME 1 to POMME 3, there is a significant increase of 230 Th xs in the deep waters that cannot be solely due to the remineralisation of the settling particles. The increasing rate of the 230 Th xs inventory from 0 to 400 or 1000 m in larger than the in situ production rate suggesting that advective inputs of 230 Th are significant. The effect of water mass mixing is supported by the correlation between the 230 Th xs content and the salinity of the water masses at 1000 m. Trapping efficiency: For short periods, the flux/production ratio (apparent efficiency) may be different from the trapping efficiency. However during a high flux period, when particulate export must dominate over in situ production, it provides an upper bound of the trapping efficiency. The uncertainties on the change of 230 Th xs inventory are large so that the corrected efficiencies are not well constrained. Negative efficiencies are found when the 230 Th xs inventory increases faster than the in situ production. If we neglect the effect water mass mixing, we can obtain a lower bound for the corrected trapping efficiency. Conclusion: By comparing apparent and corrected trapping efficiencies, a good braketing of the trapping efficiency can be obtained (eg. at 1000 m between P 1 and P 2 or at 400 m between P 2 and P 3). An explicit treatment of the mixing effect is required to improve the the method. Principle of the sediment trap calibration. Sampling: During the POMME program, we have measured Thorium isotopes in seawater, small particles and large particles collected in early March 2001 (POMME 1), mid April 2001 (POMME 2) and September 2001 (POMME 3) in the eastern North Atlantic. Here, we report results obtained on large particles collected with moored sediment traps (at 400 and 1000 m) and dissolved and small particles samples collected between 10 and 1000 m in anticyclonic eddies closely associated to the sediment trap at the time of sampling. All these samples were analysed for 230 Th and 232 Th by ID-TIMS. Trap results: During the spring bloom (from POMME 1 to POMME 2), strong variations of total mass flux and of the 230 Th xs flux are recorded by the sediment traps. Over this period, the trapped 230 Th xs flux represents 125% of the overlying production at 400 m and 65% at 1000 m. From POMME 2 to POMME 3, the 230 Th xs fluxes are more constant and they represent 30% of the overlying production only. Beside the trap calibration aspect, Th isotopes provide important information on the particle dynamics: the increase of 230 Th/ 232 Th ratio in the trapped material between 400 and 1000 m implies (independently of the flux data) that small particles have been aggregated to the sinking particles between these depths. Eddy structure of the sampling area (M. Assenbaum et al., 2003) Sediment trap data Water column data

Constraining the seasonal particle flux in the eastern North Atlantic with Thorium isotopes M. Roy-Barman (1), R. El Hayek (1), I. Voege (2), M.Souhaut (3), N. Leblond (4), C. Jeandel (2). (1) LSCE, Avenue de la Terrasse, Gif sur Yvette, France, (2) AWI, PO Box , Bremerhaven, Germany, (3) LEGOS, 14 Avenue E. Belin, 31400, Toulouse France, (4) LOV, BP 28, Villefranche sur mer, France Introduction: Constraining the present and past oceanic biological pump requires a good understanding of the marine particle dynamics and of their interaction with the surrounding water column. Particle fluxes are studied mainly with sediment traps, so that it is critical to evaluate their efficiency. The average efficiency of moored sediment traps is calibrated by the 230 Th method. 230 Th is produced uniformely in the ocean by radioactive decay of 238 U and it is rapidely removed from the water column by scavenging on settling particles. The trapping efficiency is obtained by comparing the 230 Th xs trapped flux with the 230 Th production in the overlying watercolumn. This was previously done with trap data obtained over a year in order to avoid the problem of estimating the change of 230 Th inventory in the water column. The idea: Here we propose to extend this method to estimate the efficiency of moored traps on a seasonal scale. It requires high precision data to measure the change of 230 Th inventory in the water column. By combining sediment trap data with the change of 230 Th inventory, it is possible to obtain the trapping efficiency for a short time period. Water column results: While the 230 Th xs decreases in the surface waters from POMME 1 to POMME 3, there is a significant increase of 230 Th xs in the deep waters that cannot be solely due to the remineralisation of the settling particles. The increasing rate of the 230 Th xs inventory from 0 to 400 or 1000 m in larger than the in situ production rate suggesting that advective inputs of 230 Th are significant. The effect of water mass mixing is supported by the correlation between the 230 Th xs content and the salinity of the water masses at 1000 m. Trapping efficiency: For short periods, the flux/production ratio (apparent efficiency) may be different from the trapping efficiency. However during a high flux period, when particulate export must dominate over in situ production, it provides an upper bound of the trapping efficiency. The uncertainties on the change of 230 Th xs inventory are large so that the corrected efficiencies are not well constrained. Negative efficiencies are found when the 230 Th xs inventory increases faster than the in situ production. If we neglect the effect water mass mixing, we can obtain a lower bound for the corrected trapping efficiency. Conclusion: By comparing apparent and corrected trapping efficiencies, a good braketing of the trapping efficiency can be obtained (eg. at 1000 m between P 1 and P 2 or at 400 m between P 2 and P 3). An explicit treatment of the mixing effect is required to improve the the method. Principle of the sediment trap calibration. Sampling: During the POMME program, we have measured Thorium isotopes in seawater, small particles and large particles collected in early March 2001 (POMME 1), mid April 2001 (POMME 2) and September 2001 (POMME 3) in the eastern North Atlantic. Here, we report results obtained on large particles collected with moored sediment traps (at 400 and 1000 m) and dissolved and small particles samples collected between 10 and 1000 m in anticyclonic eddies closely associated to the sediment trap at the time of sampling. All these samples were analysed for 230 Th and 232 Th by ID-TIMS. Trap results: During the spring bloom (from POMME 1 to POMME 2), strong variations of total mass flux and of the 230 Th xs flux are recorded by the sediment traps. Over this period, the trapped 230 Th xs flux represents 125% of the overlying production at 400 m and 65% at 1000 m. From POMME 2 to POMME 3, the 230 Th xs fluxes are more constant and they represent 30% of the overlying production only. Beside the trap calibration aspect, Th isotopes provide important information on the particle dynamics: the increase of 230 Th/ 232 Th ratio in the trapped material between 400 and 1000 m implies (independently of the flux data) that small particles have been aggregated to the sinking particles between these depths. Eddy structure of the sampling area (M. Assenbaum et al., 2003) Sediment trap data Water column data