  The C/O-ratio as traced with C$_2$H emission in protoplanetary disks is
fundamental for constraining the formation mechanisms of exoplanets and our
understanding of volatile depletion in disks, but current C$_2$H observations
show an apparent bimodal distribution which is not well understood, indicating
that the C/O distribution is not described by a simple radial dependence. The
transport of icy pebbles has been suggested to alter the local elemental
abundances in protoplanetary disks, through settling, drift and trapping in
pressure bumps resulting in a depletion of volatiles in the surface and an
increase of the elemental C/O. We combine all disks with spatially resolved
ALMA C$_2$H observations with high-resolution continuum images and constraints
on the CO snowline to determine if the C$_2$H emission is indeed related to the
location of the icy pebbles. We report a possible correlation between the
presence of a significant CO-icy dust reservoir and high C$_2$H emission, which
is only found in disks with dust rings outside the CO snowline. In contrast,
compact dust disks (without pressure bumps) and warm transition disks (with
their dust ring inside the CO snowline) are not detected in C$_2$H, suggesting
that such disks may never have contained a significant CO ice reservoir. This
correlation provides evidence for the regulation of the C/O profile by the
complex interplay of CO snowline and pressure bump locations in the disk. These
results demonstrate the importance of including dust transport in chemical disk
models, for a proper interpretation of exoplanet atmospheric compositions, and
a better understanding of volatile depletion in disks, in particular the use of
CO isotopologues to determine gas surface densities.
