The Open Cluster Chemical Abundances and Mapping Survey: IV. Abundances
  for 128 Open Clusters using SDSS/APOGEE DR16
  The Open Cluster Chemical Abundances and Mapping (OCCAM) survey aims to
constrain key Galactic dynamical and chemical evolution parameters by the
construction of a large, comprehensive, uniform, infrared-based spectroscopic
data set of hundreds of open clusters. This fourth contribution from the OCCAM
survey presents analysis using SDSS/APOGEE DR16 of a sample of 128 open
clusters, 71 of which we designate to be "high quality" based on the appearance
of their color-magnitude diagram. We find the APOGEE DR16 derived [Fe/H]
abundances to be in good agreement with previous high resolution spectroscopic
open cluster abundance studies. Using the high quality sample, we measure
Galactic abundance gradients in 16 elements, and find evolution of some of the
[X/Fe] gradients as a function of age. We find an overall Galactic [Fe/H] vs
R_GC gradient of $-0.068 \pm 0.001$ dex kpc$^{-1}$ over the range of $6 <$ R_GC
$< 13.9$ kpc; however, we note that this result is sensitive to the distance
catalog used, varying as much as 15%. We formally derive the location a break
in the [Fe/H] abundance gradient as a free parameter in the gradient fit for
the first time. We also measure significant Galactic gradients in O, Mg, S, Ca,
Mn, Cr, Cu, Na, Al, and K, some of which are measured for the first time. Our
large sample allows us to explore four well-populated age bins to explore the
time evolution of gradients for a large number of elements and comment on
possible implications for Galactic chemical evolution and radial migration.
