A REDSHIFT 4.38 MG-II ABSORBER TOWARD BR-1202-0725

We have detected a strong (EW(obs) = 20 Angstrom) redshift 4.38 Mg II absorber toward the redshift 4.68 quasar BR 1202-0725. The identificat ion of this feature as Mg II lambda lambda 2796, 2803 is supported by the identification of a damped Ly alpha absorber and ultraviolet Si II lambda 1526 absorption at the same redshift. The strengths of Mg II l ambda lambda 2796, 2803 and Si II lambda 1526 are consistent with thos e of strong low-redshift Mg II systems, damped Ly alpha absorbers, and diffuse clouds within the Milky Way. C IV lambda lambda 1548, 1550 is not detected, indicating a lower ionization state than is seen in low er redshift Mg II absorbers and within the interstellar medium of our Galaxy. The detection of a single strong Mg II absorber at z > 4.2, am ong the three quasars observed with sufficient signal-to-noise ratio t o detect an absorber, is consistent with the statistics and evolution of lower redshift Mg II absorbers. Thus, despite being at nearly twice the redshift of any previously identified Mg II absorber, this system appears quite similar to lower redshift Mg II systems that are identi fied with normal field galaxies. Because of the saturated nature of th e Si II and Mg II lines, the metallicity of the gas is uncertain, but it is clear that some enrichment of Si and Mg, probably amounting to m ore than 1% of solar, has occurred by a redshift of 4.38.