Abstract
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 more than 200 m day−1. The current vertical velocity field is computed from a 1/16°×1/16° 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. 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.
Highlights
The Eastern Mediterranean Sea is characterized by an antiestuarine basin scale circulation through the Sicily Channel, with the entrance of surface nutrient-depleted waters and the exiting of nutrient-enriched intermediate waters (MalanotteRizzoli et al, 1997; Pinardi and Masetti, 2000)
The open areas of the Ionian Sea, which is the deepest basin of the Eastern Mediterranean Sea, are highly oligotrophic, as surface chlorophyll concentrations are generally lower than 0.5 mg m−3 and a deep chlorophyll maximum is found at around 80–100 m depth (Boldrin et al, 2002; Malinverno et al, 2003; Ignatiades, 2005)
Particle flux data analyzed in this work were collected by means of conical Technicap PPS5/2 sediment traps deployed at 500 m and 2800 m depth in two offshore sites located in the southeastern Ionian Sea, the Urania and Bannock stations (Fig. 1)
Summary
The Eastern Mediterranean Sea is characterized by an antiestuarine basin scale circulation through the Sicily Channel, with the entrance of surface nutrient-depleted waters and the exiting of nutrient-enriched intermediate waters (MalanotteRizzoli et al, 1997; Pinardi and Masetti, 2000). Superimposed to this large scale circulation pattern, a seasonal wind-forced sub-basin scale circulation is evident (Pinardi and Navarra, 1993; Molcard et al, 2002), with the dominant north-westerly wind component giving a characteristic separation between the southern regions, dominated by anticyclonic motion, and the northern regions, dominated by cyclonic motion (Pinardi et al, 2005). Klein et al (1999) measure in the year 1987 a 300–400 m deep nutricline in the southeastern Ionian Sea, Published by Copernicus Publications on behalf of the European Geosciences Union
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