  We report observations of resolved C2H emission rings within the gas-rich
protoplanetary disks of TW Hya and DM Tau using the Atacama Large Millimeter
Array (ALMA). In each case the emission ring is found to arise at the edge of
the observable disk of mm-sized grains (pebbles) traced by (sub)mm-wave
continuum emission. In addition, we detect a C3H2 emission ring with an
identical spatial distribution to C2H in the TW Hya disk. This suggests that
these are hydrocarbon rings (i.e. not limited to C2H). Using a detailed
thermo-chemical model we show that reproducing the emission from C2H requires a
strong UV field and C/O > 1 in the upper disk atmosphere and outer disk, beyond
the edge of the pebble disk. This naturally arises in a disk where the
ice-coated dust mass is spatially stratified due to the combined effects of
coagulation, gravitational settling and drift. This stratification causes the
disk surface and outer disk to have a greater permeability to UV photons.
Furthermore the concentration of ices that transport key volatile carriers of
oxygen and carbon in the midplane, along with photochemical erosion of CO,
leads to an elemental C/O ratio that exceeds unity in the UV-dominated disk.
Thus the motions of the grains, and not the gas, lead to a rich hydrocarbon
chemistry in disk surface layers and in the outer disk midplane.
