  The broad MgII doublet has been much studied in connection with its
potentially important role as a virial estimator of black hole mass in high
redshift quasars. An important task is therefore identification of any line
components likely related to broadening by non-virial motions. High s/n median
composite spectra (binned in the "4D eigenvector 1" context of Sulentic et al.
2007) were constructed for the brightest 680 SDSS DR7 quasars in the 0.4 < z <
0.75 range where both MgII 2800 and Hbeta are recorded in the same spectra.
Composite spectra representing 90% of the quasars confirm previous findings
that FWHM(MgII 2800) is about 20% narrower than FWHM(Hbeta). The situation is
clearly different for the most extreme (Population A) sources which are the
highest Eddington radiators in the sample. In the median spectra of these
sources FWHM MgII 2800 is equal to or greater than FWHM(Hbeta) and shows a
significant blueshift relative to Hbeta. We interpret the MgII 2800 blueshift
as the signature of a radiation-driven wind or outflow in the highest accreting
quasars. In this interpretation the MgII 2800 line width -- affected by
blueshifted emission -- is unsuitable for virial mass estimation in ~ 10% of
quasars.
