  Motivated by recent observations of plateaus and minima in the radial
abundance distributions of heavy elements in the Milky Way and some other
spiral galaxies, we propose a dynamical mechanism for the formation of such
features around corotation. Our numerical simulations show that the
non-axisymmetric gravitational field of spiral density waves generates cyclone
and anticylone gas flows in the vicinity of corotation. The anticyclones
flatten the pre-existing negative abundance gradients by exporting many more
atoms of heavy elements outside corotation than importing inside it. This
process is very efficient and forms plateaus of several kiloparsec in size
around corotation after two revolution periods of a galaxy. The strength of
anticyclones and, consequently, the sizes of plateaus depend on the pitch angle
of spiral arms and are expected to increase along the Hubble sequence.
