  We show a new physical phenomenon expected for the ratio R_{pA} of the
unintegrated gluon distribution of a nucleus over the unintegrated gluon
distribution of a proton scaled up by the atomic factor A^{1/3} in the
fluctuation-dominated (diffusive scaling) region at high energy. We calculate
the dependence of R_{pA} on the atomic number A, the rapidity Y and the
transverse gluon momentum k_{\perp}. We find that R_{pA} exhibits an increasing
gluon shadowing with growing rapidity, approaching 1/A^{1/3} at asymptotic
rapidities which means total gluon shadowing, due to the effect of gluon number
fluctuations or Pomeron loops. The increase of R_{pA} with rising gluon
momentum decreases as the rapidity grows. In contrast, in the geometric scaling
region where the effect of fluctuations is negligible, the ratio R_{pA} shows
only partial gluon shadowing in the fixed-coupling case, basically independent
on the rapidity and the gluon momentum.
