  We discuss the detailed composition of 28 extremely metal-poor dwarfs, 22 of
which are from the Hamburg/ESO Survey, based on Keck Echelle spectra. Our
sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat
less metal-poor than was expected from [Fe/H](HK,HES) determined from low
resolution spectra. Our analysis supports the existence of a sharp decline in
the distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So far
no additional turnoff stars with [Fe/H]}<-3.5 have been identified in our
follow up efforts. For the best observed elements between Mg and Ni, we find
that the abundance ratios appear to have reached a plateau, i.e. [X/Fe] is
approximately constant as a function of [Fe/H], except for Cr, Mn and Co, which
show trends of abundance ratios varying with [Fe/H]. These abundance ratios at
low metallicity correspond approximately to the yield expected from Type II SN
with a narrow range in mass and explosion parameters; high mass Type II SN
progenitors are required. The dispersion of [X/Fe] about this plateau level is
surprisingly small, and is still dominated by measurement errors rather than
intrinsic scatter. The dispersion in neutron-capture elements, and the
abundance trends for Cr, Mn and Co are consistent with previous studies of
evolved EMP stars. Two dwarfs in the sample are carbon stars, while two others
have significant C enhancements, all with C12/C13 ~ 7 and with C/N between 10
and 150. Three of these C-rich stars have large enhancements of the heavy
neutron capture elements, including lead, which implies a strong s-process
contribution, presumably from binary mass transfer; the fourth shows no excess
of Sr or Ba.
