  The elemental carbon-to-oxygen ratio (C/O) in the atmosphere of a giant
planet is a promising diagnostic of that planet's formation history in a
protoplanetary disk. Alongside efforts in the exoplanet community to measure
C/O in planetary atmospheres, observational and theoretical studies of disks
are increasingly focused on understanding how the gas-phase C/O varies both
with radial location and between disks. This is mostly tied to the icelines of
major volatile carriers such as CO and H2O. Using ALMA observations of CS and
SO, we have unearthed evidence for an entirely novel type of C/O variation in
the protoplanetary disk around HD 100546: an azimuthal variation from a
typical, oxygen-dominated ratio (C/O=0.5) to a carbon-dominated ratio
(C/O>1.0). We show that the spatial distribution and peculiar line kinematics
of both CS and SO molecules can be well-explained by azimuthal variations in
the C/O ratio. We propose a shadowing mechanism that could lead to such a
chemical dichotomy. Our results imply that tracing the formation history of
giant exoplanets using their atmospheric C/O ratios will need to take into
account time-dependent azimuthal C/O variations in a planet's accretion zone.
