References

BGD

Biogeosciences Discussions

BGD

Biogeosciences Discuss.

1810-6285

Göttingen, Germany

10.5194/bgd-6-143-2009

<sup234Th measured particle export from surface waters in north-western Mediterranean: comparison of spring and autumn periods

Schmidt

¹ Goutx

² Raimbault

³ Garcia

² Guibert

³ Andersen

⁴

UMR 5805 – EPOC, Université de Bordeaux – CNRS, 33 405 Talence Cedex, France

Laboratoire de Microbiologie, de Géochimie et d'Ecologie Marines, Campus de Luminy, Case 907, 13 288 Marseillecedex 9, France

UMR5060 – IRAMAT-CRP2A, Maison de l'Archéologie, Université de Bordeaux – CNRS; Esplanade des Antilles; 33 607 Pessac Cedex, France

Observatoire Océanologique, BP 28, 06 234 Villefranche-sur-Mer Cedex, France

06 01 2009

6 1 143 161

2009

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234Th was used to quantify the short-term variability of particle dynamics and of POC export during transition periods in surface waters over the north-western Mediterranean. As a part of DYNAPROC I and II cruises, two intensive time-series of 234Th were carried out near the DYFAMED station (43° 25´N–7°51´E) during late spring (May, 1995), when the system changes towards oligotrophy, and during autumn (October, 2004), when the stratification is disturbed by wind. Particulate fluxes derived from 234Th measured in the upper water column and in drifting sediment trap showed large differences between the two situations: the flux decreased from high to low values during late spring, at the difference of the autumnal situation where the fluxes were always low. 234Th-derived POC fluxes, calculated from the 234Th238U disequilibrium in the water column and POC/234Th ratio on trapped material, and export ratios (ThE: ratio of 234Th-derived POC export to primary production) showed a large range, from 8 to 110 mgC m−2 d−1 and 3–55%; the lowest values were observed at the end of the productive period (end May) and in automn. The 234Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity. From these experiments during transition periods, it is not obvious that renewal of nutrients by wind events is strong enough to sustain significant export after the end of the productive period or to initiate significant export in autumn.

References 1

Andersen, V. and Prieur, L.: One-month study in the open NW Mediterranean Sea (DYNAPROC experiment, May~1995), Deep-Sea Res. I, 47, 397–422, 2000.

Andersen, V., Goutx, M., Prieur, L., and Dolan, J. R.: Short-scale temporal variability of physical, biological and biogeochemical processes in the NW Mediterranean Sea: an introduction, Biogeochemistry, submitted, 2008.

Benitez-Nelson, C., Buesseler, K. O., Karl, D. M., and Andrews, J.: A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on $^234$Th: $^238$U disequilibrium, Deep-Sea Res. I, 48, 2595–2611, 2001.

Béthoux, J. P., Prieur, L., and Bong, J.-H.: The Ligurian current off the French Riviera, Oceanol. Acta, SP, 59–67, 1988.

Buesseler, K. O.: The decoupling of production and particulate export in the surface ocean, Global Biogeochem. Cy. 12, 297–310, 1998.

Buesseler, K. O., Bacon, M. P., Cochran, J. K., and Livingston, H. D.: Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from $^234$Th: $^238$U disequilibria, Deep Sea Res., A, 39, 1115–1137, 1992.

Buesseler, K. O., Michaels, A. F., Siegel, D. A., and Knap, A. H.: A 3-Dimensional Time-Dependent Approach to Calibrating Sediment Trap Fluxes, Global Biogeochem. Cy., 8, 179–193, 1994.

Buesseler, K. O., Benitez-Nelson, C. R., Moran, S. B., Burd, A., Charette, M., Cochran, J. K., Coppola, L., Fisher, N. S., Fowler, S. W., Gardner, W. D., Guo, L. D., Gustafsson, O., Lamborg, C., Masque, P., Miquel, J. C., Passow, U., Santschi, P. H., Savoye, N., Stewart, G., and Trull, T.: An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of $^234$Th as a POC flux proxy, Mar. Chem., 100, 213–233, 2006.

Buesseler, K. O. and Lampitt, R. S.: Introduction to "Understanding the Ocean's biological pump: Results from VERTIGO", Deep-Sea Res. II, 55, 1519–1521, 2008.

Charette, M. A., Moran, S. B., and Bishop, J. K. B.: $^234$Th as a tracer of particulate organic carbon export in the subarctic northeast Pacific Ocean, Deep-Sea Res. II, 46, 2833–2861, 1999.

Chen, J. H., Edwards, R. L., and Waserburg, G. J.: $^238$U, $^234$U and $^232$Th in seawater, Earth Planet. Sc. Lett., 80, 241–251, 1986.

Coale, K. H. and Bruland, K. W.: Th-234 / U-238 disequilibria within the California Current, Limnol. Oceanogr., 32, 22–33, 1985.

Cochran, J. K., Buesseler, K. O., Bacon, M. P., Wang, H. W., Hirschberg, D. J., Ball, L., Andrews, J., Crossin, G., and Fleer, A.: Short-lived thorium isotopes ($^234$Th, $^228$Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean, Deep-Sea Res. II, 47, 3451–3490, 2000.

Cochran, J. K. and Masqué, P., 2003: Short-lived U/Th series radionuclides in the ocean: Tracers for scavenging rates, export fluxes and particle dynamics, In: Uranium-Series Geochemistry, edited by: Bourdon, B., Henderson, G. M., Lundstrom, C. C.,Turner, S. P., Reviews in Mineralogy and Geochemistry, 52, 461–492, 2003.

Cochran, J. K., Miquel, J. C., Armstrong, R., Fowler, S. W., Masqué, P., Gasser, B., Hirschberg, D., Szlosek, J., Rodriguez y Baena, A. M., Verdeny, E., and Stewart, G. M.: Time-series measurements of $^234$Th in water column and sediment trap samples from the northwestern Mediterranean Sea, Deep-Sea Res. II, in press, 2008.

Coppola, L., Roy-Barman, M., Wassmann, P., Mulsow, S., and Jeandel, C.: Calibration of sediment traps and particulate organic carbon export using $^234$Th in the Barents Sea, Mar. Chem., 80, 11–26, 2002.

Giuliani, S., Radakovitch, O., Frignani, M., and Bellucci, L. G.: Short time scale variations of $^234$Th/$^238$U disequilibrium related to mesoscale variability on the continental slope of the Gulf of Lions (France), Mar. Chem., 106, 403–418, 2007.

Marty, J. C., Goutx, M., Guigue, C., Leblond, N., and Raimbault, P.: Short-term particulate fluxes measured by drifting sediment traps during end summer oligotrophic in the NW Mediterranean Sea, Biogeosciences, accepted, 2008.

Michaels, A. F., Bates, N. R., Buesseler, K. O., Carlson, C. A., and Knap, A. H.: Carbon-cycle imbalances in the Sargasso Sea, Nature, 372, 537–540, 1994.

Moran, S. B. and Buesseler, K. O.: Short residence time of colloids in the upper ocean estimated from $^238$U–$^234$Th disequilibria, Nature, 359, 221–223, 1992.

Rutgers van der Loeff, M., Sarin, M. M., Baskaran, M., Benitez-Nelson, C., Buesseler, K. O., Charette, M., Dai, M., Gustafsson, Ö., Masque, P., Morris, P., Rodriguez y Baena, A., Savoye, N., Schmidt, S., Vöge, I., and Waples, J.: A review of present techniques and methodological advances in analyzing $^234$Th in aquatic systems, Mar. Chem., 100, 190–212, 2006.

Savoye, N., Benitez-Nelson, C., Burd, A., Cochran, K., Charette, M., Buesseler, K. O., Jackson, G., Roy-Barman, M., Schmidt, S., and Elskens, M.: $^234$Th sorption and export models in the water column: a review, Mar. Chem., 100, 213–233, 2006.

Schmidt, S. and Reyss, J. L.: Uranium concentrations of Mediterranean seawaters with high salinities, C. R. Acad. Sci., 312, 379–384, Paris, France, 1991.

Schmidt, S. and Reyss, J.-L.: Radium as internal tracer of Mediterranean Outflow Water, J. Geophys. Res., 101, 3589–3596, 1996.

Schmidt, S. and Reyss, J.-L.: Improvement in radiochemical and γ-counting procedures for the determination of $^234$Th in seawater, Radiochim. Acta, 88, 459–463, 2000.

Schmidt, S., Nival, P., Reyss, J.-L., Baker, M., and Buat-Ménard, P.: Relation between Th-234 scavenging and zooplankton biomass in Mediterranean surface waters, Oceanol. Acta, 15, 227–231, 1992.

Schmidt, S., Andersen, V., Belviso, S., and Marty, J. C.: Strong seasonality in particle dynamics of north-western Mediterranean surface waters (JGOFS France DYFAMED time-series station) as revealed by $^234$Th/$^238$U. Deep-Sea Res. I, 49, 1507–1518, 2002.

Stewart G., Cochran, J. K., Miquel, J. C., Masque, P., Szlosek, J., Rodriguez y Baena, A. M., Fowler, S. W., Gasser, B., and Hirschberg, D. J.: Comparing POC export from $^234$Th/$^238$U and $^210$ Po/$^210$ Pb disequilibria with estimates from sediment traps in the northwest Mediterranean, Deep-Sea Res. I, 54, 1549–1570, 2007.

Vidussi, F., Marty, J.-C., and Chiavérini, J.: Phytoplankton pigment variations during the transition from spring bloom to oligotrophy in the northwestern Mediterranean sea, Deep-Sea Res. I 47, 423–445, 2000.