upright animal tubes or slender sea pens generate local turbulence close to the
sediment and thus increase scouring. Megafaunal echinoderms (brittle stars, sea stars,
urchins and sea cucumbers) moving across the bottom produce tracks, often with a
flat bottom surface and side ridges raised from slightly to several centimeters above
the sediment surface (Plate 13.1). Benthic ecologists refer to the array of animal
marks in sediment by the German term Lebensspuren, life traces. It was once thought
that activity in the deep-sea is slow enough that Lebensspuren might have ages of
decades or longer. Time-lapse camera studies and repeated submersible visits have
made clear that this sculpturing changes and changes again on a time scale of weeks
(e.g. Wheatcroft et al. 1989).
Fig. 14.23 Sediment mounding from burrowing sea cucumbers.
(^) (After Gage & Tyler 1991.)
Sediment can be both stabilized and destabilized by animal secretions and activity.
Mucus added to sediment in gut contents can enhance its coherence at least
temporarily. In the opposite direction, formation of large numbers of clam
pseudofeces, tiny compacted balls of sediment, can make the bottom more fluid. It is
believed that burrowing activity in the rich nearshore sediments deposited on steep
slopes in submarine canyons can sometimes set off slumping and trigger turbidity
currents.