  We study the bulk viscosity of quark matter in the strong coupling regime
within the two-flavor Nambu--Jona-Lasinio model. The dispersive effects that
lead to non-zero bulk viscosity arise from quark-meson fluctuations above the
Mott transition temperature, where meson decay into two quarks is kinematically
allowed. We adopt the Kubo-Zubarev formalism and compute the equilibrium
imaginary-time correlation function for pressure in the $O(1/N_c)$ power
counting scheme. The bulk viscosity of matter is expressed in terms of the
Lorentz components of the quark spectral function and includes multi-loop
contributions which arise via re-summation of infinite geometrical series of
loop diagrams. We show that the multi-loop contributions dominate the
single-loop contribution close to the Mott line, whereas at high temperatures
the one-loop contribution is dominant. The multi-loop bulk viscosity dominates
the shear viscosity close to the Mott temperature by factors 5 to 20, but with
increasing temperature the shear viscosity becomes the dominant dissipation
mechanism of stresses as the one-loop contribution becomes the main source of
bulk viscosity.
