  Isotopic abundances of short-lived radionuclides such as 26Al provide the
most precise chronometers of events in the early solar system, provided that
they were initially homogeneously distributed. On the other hand, the
abundances of the three stable isotopes of oxygen in primitive meteorites show
a mass-independent fractionation that survived homogenization in the solar
nebula. As as result of this and other cosmochemical evidence, the degree of
spatial heterogeneity of isotopes in the solar nebula has long been a puzzle.
We show here that based on hydrodynamical models of the mixing and transport of
isotopic anomalies formed at, or injected onto, the surface of the solar
nebula, initially high levels of isotopic spatial heterogeneity are expected to
fall to steady state levels (~10%) low enough to validate the use of 26Al for
chronometry, but high enough to preserve the evidence for mass-independent
fractionation of oxygen isotopes. The solution to this puzzle relies on the
mixing being accomplished by the chaotic fluid motions in a marginally
gravitationally unstable disk, as seems to be required for the formation of gas
giant planets and by the inability of alternative physical processes to drive
large-scale mixing and transport in the planet-forming midplane of the solar
nebula. Such a disk is also capable of large-scale outward transport of the
thermally annealed dust grains found in comets, and of driving the shock fronts
that appear to be responsible for much of the thermal processing of the
components of primitive meteorites, creating a self-consistent picture of the
basic physical processes shaping the early solar nebula.
