  We determine the polarization of the bulk $^{13}$C nuclear spin system in
diamond produced by interaction with optically oriented nitrogen-vacancy (NV-)
defect centers. $^{13}$C nuclei are polarized into the higher energy Zeeman
state with a bulk-average polarization up to 5.2%, although local polarization
may be higher. The kinetics of polarization are temperature independent, and
occur within 5 minutes. Fluctuations in the dipolar field of the NV- center
spin bath are identified as the mechanism by which nuclear spin transitions are
induced near defect centers. Polarization is then transported to the bulk
material via spin diffusion, which accounts for the observed kinetics of
polarization. These results indicate control over the nuclear spin bath, a
methodology to study dynamics of an NV- center ensemble, and application to
sensitivity-enhanced NMR.
