Particle fluxes in the deep Eastern Mediterranean basins: the role of ocean vertical velocities
1Centro Euro-Mediterraneo per i Cambiamenti Climatici (CMCC), Viale Aldo Moro 44, 40 127 Bologna, Italy
2Laboratorio SINCEM, Università di Bologna, Via S. Alberto 163, 48 100 Ravenna, Italy
3Dipartimento di Scienze Geologiche e Geotecnologie, Università Milano-Bicocca, Piazza della Scienza 4, 20 126 Milano, Italy
4Istituto Nazionale di Geofisica e Vulcanologia (INGV), Viale Aldo Moro 44, 40 127 Bologna, Italy
5Istituto di Scienze dell'Atmosfera e del Clima–C. N. R., Via del Fosso del Cavaliere 100, 00 133 Rome, Italy
Abstract. This paper analyzes the relationship between deep sedimentary fluxes and ocean current vertical velocities in an offshore area of the Ionian Sea, the deepest basin of the Eastern Mediterranean Sea. Sediment trap data are collected at 500 m and 2800 m depth in two successive moorings covering the period September 1999–May 2001. A tight coupling is observed between the upper and deep traps and the estimated particle sinking rates are higher than 200 m day−1. The current vertical velocity field is computed from a high resolution Ocean General Circulation Model simulation and from the wind stress curl. Current vertical velocities are larger and more variable than Ekman vertical velocities, yet the general patterns are alike. Current vertical velocities are generally smaller than 1 m day−1: we therefore exclude a direct effect of downward velocities in determining high sedimentation rates. However, we find that upward velocities in the subsurface layers of the water column are positively correlated with deep particle fluxes. We thus hypothesize that upwelling would produce an increase in upper ocean nutrient levels – thus stimulating primary production and grazing – a few weeks before an enhanced vertical flux is found in the sediment traps. By analyzing the delayed effects of ocean vertical velocities on deep particle fluxes we envisage a spectrum of particle sinking speeds ranging from about 100 m day−1 to more than 200 m day−1. High particle sedimentation rates may be attained by means of rapidly sinking fecal pellets produced by gelatinous macro-zooplankton. Other sedimentation mechanisms, such as dust deposition, are also considered in explaining large pulses of deep particle fluxes. The fast sinking rates estimated in this study might be an evidence of the efficiency of the biological pump in sequestering organic carbon from the surface layers of the deep Eastern Mediterranean basins.