Effect of double diffusion processes in the deep ocean on the distribution and dynamics of particulate and dissolved matter: a case study in Tyrrhenian Sea

Abstract. This study examines CTD, ADCP and optical data collected during the PERLE-3 cruise in March 2020 between the surface and 2000 m depth over an east-west section of the Tyrrhenian Sea in the Mediterranean. The focus will be on the impact of double diffusion processes, in particular salt fingering, on the distribution and dynamics of particulate and dissolved matter. The staircases develop at the interface between the warm, salty Levantine Intermediate Water (LIW) and the colder, less salty Tyrrhenian Deep Water (TDW) in the centre of the basin with low hydrodynamic energy. The results show that thermohaline staircases formed by salt fingering significantly influence particle sedimentation and biogeochemical cycling in deep ocean environments by altering vertical flux patterns. These density steps create distinct vertical layers that act as physical barriers, slowing the descent of particles and facilitating their retention and aggregation. Retention of fine particles at density gradients promotes the formation of larger aggregates, affecting particle size distribution. The staircases also affect dissolved matter by creating pronounced concentration gradients of oxygen and nutrients, which can influence microbial activity and nutrient cycling.