  Observations of the vicinity of a variety of galactic gamma-ray sources have
indicated a local suppression of diffusivity of cosmic rays by up to three
orders of magnitude. However, the impact of these low-diffusion zones on
\emph{global} properties of cosmic-ray transport is however only poorly
understood. Here, we argue that cosmic-ray nuclear ratios, like the
boron-to-carbon ratio and relative abundances of Beryllium isotopes are
sensitive to the filling fraction of such low-diffusion zones and hence their
measurements can be used to constrain the typical sizes and ages of such
regions. We have performed a careful parameter study of a cosmic-ray transport
model that allows for different diffusion coefficients $\kappa_{\mathrm{disk}}$
and $\kappa_{\mathrm{halo}}$ in the galactic disk and halo, respectively.
Making use of preliminary data from the AMS-02 experiment on the ratio of
Beryllium isotopes, we find a $3.5 \sigma$ preference for a suppression of the
diffusion coefficient in the disk with a best-fit value of
$\kappa_{\mathrm{disk}}/\kappa_{\mathrm{halo}} = 0.20^{+0.10}_{-0.06}$. We
forecast that with upcoming data from the HELIX balloon experiment, the
significance could increase to $6.8 \sigma$. Adopting a coarse-graining
approach, we find that such a strong suppression could be realised if the
filling fraction of low-diffusion zones in the disk was $\sim 66 \, \%$. We
conclude that the impact of regions of suppressed diffusion might be larger
than usually assumed and ought to be taken into account in models of Galactic
cosmic ray transport.
