  Like elsewhere in the deep-sea, life in the deep Mediterranean depends on
turbulent exchange across the stable vertical density stratification for supply
of nutrients and oxygen. Commonly modelled, turbulent exchange is inversely
proportional to the stratification rate. However, this proportionality depends
on the particular turbulence type, whether it is driven by vertical current
differences (shear) or by buoyancy (convection). While shear-turbulence is well
observed in stratified seas, direct observations of convection-turbulence are
limited. In this paper, high-resolution moored temperature observations show
that Mediterranean Sea waters are not stagnant in the lower 109 m above the
seafloor at 2480 m, although variations are in the range of only 0.0001-0.001
degrC. In winter, convection-turbulence is regularly observed. Fortnightly
averaged spectra show a collapse to the inertial-subrange scaling of dominant
shear-turbulence for data from about 100 m above the seafloor, and to the
buoyancy-subrange scaling of dominant convection-turbulence at about 10 m above
the seafloor. Time-depth images reveal details of convection-turbulence driven
from below, which is considered primarily due to general geothermal heating
through the Earth crust not related to volcanic vents. When its observation is
not masked by (sub-)mesoscale eddies that advect warmer waters from above, the
geothermal heat flux matches the deep-sea turbulence dissipation rate, if in
the calculations a mixing efficiency of 0.5 is taken typical for natural
convection, integration is over 250 m above the seafloor as confirmed from
shipborne CTD, and if maximum 2-m-scale buoyancy frequency replaces its
100-m-scale mean equivalent.

Atmospheric and Oceanic Physics