Dynamics of particles along the western margin of the Southern Adriatic: Processes involved in transferring particulate matter to the deep basin

Abstract

The Southern Adriatic is an area of dense shelf water (DSW) cascading and open-ocean convection. The impact of DSW cascading events in transferring organic matter to the deep benthic community and in producing a wide range of bedforms along the continental margin has been highlighted in recent years. In order to improve our knowledge on the interannual variability of the DSW cascading with the ultimate goal to understand timing of DSW formation and transport as well as the cascading process, its variability in intensity and duration, and the ultimate impact of the dense water on the deep sea, in March 2009 we deployed an instrumented mooring in a field of sediment waves (860 m depth) located down current to the Bari canyon. In March 2010, a second mooring was installed in the northern channel of the canyon. Winters 2009, 2010 and 2011 were mild and particularly wet and the Po river discharge remained relatively high throughout the whole winter. Hence, we expected weak dense shelf water formation associated with a shallow shelf water overflowing off the Adriatic shelf. By contrast, in winter 2012, the North Adriatic experienced a severe cold wave with NE Bora winds and reduced fresh water input. These weather conditions allowed the formation of extremely dense shelf water. Three additional moorings were quickly deployed during this cold event in the Southern Adriatic Sea to investigate the spatial variability of particle and organic matter fluxes exiting from the Adriatic continental shelf under the influence of a strong DSW cascading event. Slow near-bottom currents, never exceeding 40 cm s− 1, were recorded between March 2009 and February 2012. Water temperatures depicted minor negative shifts. Total mass fluxes (TMFs) were low (annual avg., 1.7–2.8 g m− 2 d− 1 and 0.3–0.6 g m− 2 d− 1 in the canyon and in the sediment wave field, respectively; and peak values ≤ 6.8 g m− 2 d− 1), but showed significant seasonal and interannual variability. Fluxes in the canyon were higher than those measured in the sediment wave field at deeper water depth. Mass flux peaks during the 2112-DSW cascading were up to 5 times higher than the peaks of previous years (up to 18.70 g m− 2 d− 1), with a spatial variability mainly driven by the localized pathways of DSW cascading. In the canyon, near-bottom currents exceeded 70 cm s− 1 and temperature dropped to 12.2 °C while current speeds were high also at 1200 m depth (~ 60 cm s− 1) in the moat surrounding the Dauno seamount. Surprisingly, mass flux peaks occurred from the 16th February to the 1st of March, 3–4 weeks ahead of the usual DSW occurrence, suggesting an early arrival of the DSW. The deep DSW cascading was the main process driving the particle transfer across the southern Adriatic margin during late winter–spring 2012. Mooring data showed a NW–SE gradient of temperature and kinetic energy from upslope to basin floor, indicating slope transverse flow modulated by local obstruction caused by the rugged seafloor topography. Bari canyon is one of the sites of DSW flow and in this area we have the possibility to extend observations back in time using continuous mooring data since 2009 and previous published materials; this approach allowed evaluation of additional mechanisms of particle transport (e.g., open-ocean convection, storm-driven downward transport, shallow dense water cascading) that are particularly relevant in years when the DSW formation is less vigorous and cascading processes are sluggish. The small amplitude of total mass flux peaks, the weak currents and the relatively high and constant temperatures recorded during 2009, 2010 and 2011 springs are consistent with an enhanced vertical particle rain from a mid-water nepheloid layer, triggered in turn by a shallow cascading of not-particularly dense shelf water detaching from the seafloor when reaches its neutral buoyancy. Thus, the intensity of DSW cascading (shallow vs. deep) plays a first order control on the particulate fluxes through the western margin of the Southern Adriatic, while storm-induced sediment transport can occasionally be relevant too.