Oceans in Crisis - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 167

Mediterranean, down to nearly a kilometre below the sea surface, par-
ticularly around the craggy topography of the submarine canyons
that snake across the sea floor. Every day, hundreds of motorized fish-
ing boats from the Catalan sea-ports drag weighted trawl nets across
those canyon sides, to scoop up whatever lives in those deep-sea sedi-
ments. The effect on those canyons, as viewed in sonar images, is little
short of astonishing. The rugged topography has been smoothed, as
if by a giant hand, completely effacing what had once been a complex
system of tributaries leading into the main canyon floor.
It is a form of bulldozing. The effect on the sediment is a little like
what happens when running water passes over a mixture of sand and
mud: the mud is swept away, and the remaining sediment becomes
progressively more sandy. Clouds of muddy water are raised in the
wake of the dredges, and these travel downslope or downcurrent, to
eventually settle in quieter, deeper parts of the sea floor, away from
the trawled areas. On steeper slopes, things can get more violent.
Slabs of seabed can be dislodged to break up into rapidly moving
dense slurries that sweep down into the canyon floors and from there
be funnelled into yet deeper waters. It is a kind of grand, large-scale
sedimentary sorting process, creating new kinds of sedimentary
strata.
On the sea floor, where the trawls have passed over, there is com-
monly a distinct layer, a metre thick or so, in which the sediment
grains get larger nearer the sea surface because the finer particles have
been winnowed away. This 'coarsening-upwards trend', as geologists
call it, is a direct signature of the ploughing of the sea floor. Down-
slope, particularly in the floors of submarine canyons below the
trawled areas, there may be layers of sand or mud, each usually a few
centimetres thick. Each of these is the result of a dense turbulent
sediment-laden cloud—a turbidity current—that sped downslope,
hugging the sea floor, in the wake of the trawlers. Turbidity currents