  We report the discovery with the Neutron Star Interior Composition Explorer
(NICER) of narrow emission and absorption lines during photospheric radius
expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820-30. NICER
observed the source in 2017 August accumulating about 60 ks of exposure. Five
thermonuclear X-ray bursts were detected of which four showed clear signs of
PRE. We extracted spectra during the PRE phases and fit each to a model that
includes a comptonized component to describe the accretion-driven emission, and
a black body for the burst thermal radiation. The temperature and spherical
emitting radius of the fitted black body are used to assess the strength of PRE
in each burst. The two strongest PRE bursts (burst pair 1) had black body
temperatures of approximately 0.6 keV and emitting radii of 100 km (at a
distance of 8.4 kpc). The other two bursts (burst pair 2) had higher
temperatures (~0.67 keV) and smaller radii (75 km). All of the PRE bursts show
evidence of narrow line emission near 1 keV. By co-adding the PRE phase spectra
of burst pairs 1 and, separately, 2 we find, in both co-added spectra,
significant, narrow, spectral features near 1.0 (emission), 1.7 and 3.0 keV
(both in absorption). Remarkably, all the fitted line centroids in the co-added
spectrum of burst pair 1 appear systematically blue-shifted by a factor of
$1.046 \pm 0.006$ compared to the centroids of pair 2, strongly indicative of a
gravitational red-shift, a wind-induced blue-shift, or more likely some
combination of both effects. The observed shifts are consistent with this
scenario in that the stronger PRE bursts in pair 1 reach larger photospheric
radii, and thus have weaker gravitational red-shifts, and they generate faster
outflows, yielding higher blue-shifts. We discuss possible elemental
identifications for the observed features in the context of recent burst-driven
wind models.
