  We study primordial nucleosynthesis abundance yields for assumed ranges of
cosmological lepton numbers, sterile neutrino mass-squared differences and
active-sterile vacuum mixing angles. We fix the baryon-to-photon ratio at the
value derived from the cosmic microwave background (CMB) data and then
calculate the deviation of the 2H, 4He, and 7Li abundance yields from those
expected in the zero lepton number(s), no-new-neutrino-physics case. We
conclude that high precision (< 5% error) measurements of the primordial 2H
abundance from, e.g., QSO absorption line observations coupled with high
precision (< 1% error) baryon density measurements from the CMB could have the
power to either: (1) reveal or rule out the existence of a light sterile
neutrino if the sign of the cosmological lepton number is known; or (2) place
strong constraints on lepton numbers, sterile neutrino mixing properties and
resonance sweep physics. Similar conclusions would hold if the primordial 4He
abundance could be determined to better than 10%.
