  In the process of producing the roughly three ionizing photons per atom
required to reionize the IGM, the same massive stars explode and eject metals
into their surroundings. While the overly sensitive Lya transition makes
Gunn-Peterson absorption of background quasar light an ineffective probe of
reionization at z > 6, strong low-ionization transitions like the MgII doublet
will give rise to a detectable "metal-line forest", if metals pollute the
neutral IGM. We measure the auto-correlation of the MgII forest transmission
using a sample of ten ground based z >= 6.80 quasar spectra probing the
redshift range 5.96 < z_MgII < 7.42 (z_MgII,median = 6.47). The correlation
function exhibits strong small-scale clustering and a pronounced peak at the
doublet velocity (768 km/s) arising from strong absorbers in the CGM of
galaxies. After these strong absorbers are identified and masked the signal is
consistent with noise. Our measurements are compared to a suite of models
generated by combining a large hydrodynamical simulation with a semi-numerical
reionization topology, assuming a simple uniform enrichment model. We obtain a
95% credibility upper limit of [Mg/H] < -3.73 at z_MgII,median = 6.47, assuming
uninformative priors on [Mg/H] and the IGM neutral fraction x_HI. Splitting the
data into low-z (5.96 < z_MgII < 6.47; z_MgII,median = 6.235) and high-z (6.47
< z_MgII < 7.42; z_MgII,median = 6.72) subsamples again yields null-detections
and 95% upper limits of [Mg/H] < -3.75 and [Mg/H] < -3.45, respectively. These
first measurements set the stage for an approved JWST Cycle 2 program (GO 3526)
targeting a similar number of quasars that will be an order of magnitude more
sensitive, making the Mgii forest an emerging powerful tool to deliver
precision constraints on the reionization and enrichment history of the
Universe.
