  We calculate the ionisation fraction in protostellar disk models using two
different gas-phase chemical networks, and examine the effect of turbulent
mixing by modelling the diffusion of chemical species vertically through the
disk. The aim is to determine in which regions of the disk gas can couple to a
magnetic field and sustain MHD turbulence. We find that the effect of diffusion
depends crucially on the elemental abundance of heavy metals (magnesium)
included in the chemical model. In the absence of heavy metals, diffusion has
essentially no effect on the ionisation structure of the disks, as the
recombination time scale is much shorter than the turbulent diffusion time
scale. When metals are included with an elemental abundance above a threshold
value, the diffusion can dramatically reduce the size of the magnetically
decoupled region, or even remove it altogther. For a complex chemistry the
elemental abundance of magnesium required to remove the dead zone is 10(-10) -
10(-8). We also find that diffusion can modify the reaction pathways, giving
rise to dominant species when diffusion is switched on that are minor species
when diffusion is absent. This suggests that there may be chemical signatures
of diffusive mixing that could be used to indirectly detect turbulent activity
in protoplanetary disks. We find examples of models in which the dead zone in
the outer disk region is rendered deeper when diffusion is switched on. Overall
these results suggest that global MHD turbulence in protoplanetary disks may be
self-sustaining under favourable circumstances, as turbulent mixing can help
maintain the ionisation fraction above that necessary to ensure good coupling
between the gas and magnetic field.
