  We study gauge threshold corrections for local GUT models in IIB/F-theory.
Consistency with holomorphy requirements of supergravity and the
Kaplunovsky-Louis formula implies that the unification scale is enhanced by the
bulk radius R from the string scale to M_X =RM_S. We argue that the stringy
interpretation of this is via a locally uncancelled tadpole sourced by the
hypercharge flux. This sources closed string modes propagating into the bulk;
equivalently open string gauge coupling running up to the winding scale M_X.
The enhancement to R M_s is tied to GUT breaking by a globally trivial
hypercharge flux and will occur in all models realising this mechanism.
