  Analyses of primitive meteorites and cometary samples have shown that the
solar nebula must have experienced a phase of large-scale outward transport of
small refractory grains as well as homogenization of initially spatially
heterogeneous short-lived isotopes. The stable oxygen isotopes, however, were
able to remain spatially heterogenous at the $\sim$ 6% level. One promising
mechanism for achieving these disparate goals is the mixing and transport
associated with a marginally gravitationally unstable (MGU) disk, a likely
cause of FU Orionis events in young low-mass stars. Several new sets of MGU
models are presented that explore mixing and transport in disks with varied
masses (0.016 to 0.13 $M_\odot$) around stars with varied masses (0.1 to 1
$M_\odot$) and varied initial $Q$ stability minima (1.8 to 3.1). The results
show that MGU disks are able to rapidly (within $\sim 10^4$ yr) achieve
large-scale transport and homogenization of initially spatially heterogeneous
distributions of disk grains or gas. In addition, the models show that while
single-shot injection heterogeneity is reduced to a relatively low level
($\sim$ 1%), as required for early solar system chronometry, continuous
injection of the sort associated with the generation of stable oxygen isotope
fractionations by UV photolysis leads to a sustained, relatively high level
($\sim$ 10%) of heterogeneity, in agreement with the oxygen isotope data. These
models support the suggestion that the protosun may have experienced at least
one FU Orionis-like outburst, which produced several of the signatures left
behind in primitive chondrites and comets.
