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26CasCades iN MediterraNeaN subMariNe GraNd CaNyoNs H e r M e s s P e C i a L i s s u e F e at u r e This article has been published in Oceanography, Volume 22, Number 1, a quarterly journal of The oceanography society.

Natural episodic events, such as gravity flows, submarine landslides, and benthic storms can determine severe modifications in the structure and functioning of deep-sea ecosystems. Here, we report and compare the ecosystem effects produced by dense water formation events that occurred in the Gulf of Lions (NW Mediterranean) and the Aegean Sea (NE Mediterranean). In both regions, the rapid sinking of cold dense waters, driven by regional meteorological forcings, results in important immediate modifications that can be summarised in: (i) increased organic matter content in the deep basin; (ii) diminished benthic abundance; and (iii) changes of benthic biodiversity. At longer time scale the analysis reveals, however, different resilience times in the two regions. The Gulf of Lions is characterized by a very fast (months) recovery whereas the Aegean Sea shows much longer (45 years) resilience time. New long-term studies are further needed to identify the potential effects that changes in the duration, intensity and frequency of episodic events could have on the structure, biodiversity and functioning of the deep Mediterranean Sea under environmental and climate change scenario

International audienceSurface sediments collected from deep slopes and basins (1018–4087 m depth) of the oligotrophic Eastern Mediterranean Sea have been analysed for bulk elemental and isotopic composition of organic carbon, total nitrogen and selected lipid biomarkers, jointly with grain size distribution and other geochemical proxies. The distribution and sources of sedimentary organic matter (OM) have been subsequently assessed and general environmental variables, such as water depth and currents, have been examined as causative factors of deep-sea sediment characteristics. Lithogenic and biogenic carbonates are the dominant sedimentary fractions, while both bulk and molecular organic tracers reflect a mixed contribution from autochthonous and allochthonous sources for the sedimentary OM, as indicated by relatively degraded marine OM, terrestrial plant waxes and anthropogenic OM including degraded petroleum by-products, respectively. Wide regional variations have been observed amongst the studied proxies, which reflect the multiple factors controlling sedimentation in the deep Eastern Mediterranean Sea. Our findings highlight the role of deep Eastern Mediterranean basins as depocentres of organic-rich fine-grained sediments (mean 5.4 ± 2.4 μm), with OM accumulation and burial due to aggregation mechanisms and hydrodynamic sorting. A multi-proxy approach is hired to investigate the biogeochemical composition of sediment samples, which sheds new light on the sources and transport mechanisms along with the impact of preservation vs. diagenetic processes on the composition of sedimentary OM in the deep basins of the oligotrophic Eastern Mediterranean Sea

Atmospheric forcing during 2009–2010 and 2010–2011 winter months showed differences in both intensity and persistence that led to distinct oceanographic responses. Persistent dry northern winds caused strong heat losses (14 211 W m⁻²) in winter 2009–2010 that triggered a pronounced sea surface cooling compared to winter 2010–2011 (1597 W m⁻² lower). As a consequence, a large volume of dense shelf water formed in winter 2009–2010, which cascaded at high speed (up to ∼ 1 m s⁻¹) down Cap de Creus canyon, as measured by current-meters in mooring lines deployed inside the canyon at 300 m and 1000 m water depth. The lower heat losses recorded in winter 2010–2011, together with an increased river discharge, resulted in lowered density waters over the shelf, thus preventing the formation of dense shelf water. Particle fluxes were concurrently measured by using sediment traps at the same mooring stations. High total mass fluxes (up to 84.9 g m⁻² d⁻¹) recorded in winter 2009–2010 indicate that dense shelf water cascading resuspended and transported sediments at least down to 1000 m deep within the canyon. Sediment fluxes were lower (28.9 g m⁻² d⁻¹) under the quieter conditions of winter 2010–2011. The dominance of the lithogenic fraction in mass fluxes during the two winters points to a resuspension origin for most of the particles transported down canyon. The variability in organic matter and opal contents relates to seasonally controlled inputs associated to the plankton spring bloom during March and April of both years. Our measurements of particle fluxes (including major components and grain size distribution), together with meteorological and oceanographic parameters such as wind speed, turbulent heat flux, near-bottom water temperature, current speed and suspended sediment concentration, during winters 2009–2010 and 2010–2011 along the Cap de Creus submarine canyon, show the important role of atmospheric forcings in transporting particulate matter through the submarine canyon and towards the deep sea