  We report the discovery of a transient equivalent hydrogen column density
with an absorption edge at ~3.8 kiloelectron volts in the spectrum of the
prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be
satisfactorily modeled with a photoelectric absorption by a medium located at a
redshift of ~0.86 and with an iron abundance of ~75 times the solar one. The
transient behavior is attributed to the strong ionization produced in the
circumburst medium by the GRB photons. The high iron abundance points to the
existence of a burst environment enriched by a supernova along the line of
sight. The supernova explosion is estimated to have occurred about 10 years
before the burst. Our results agree with models in which GRBs originate from
the collapse of very massive stars and are preceded by a supernova event
