We compare measurements of absorption features in a sample of 35 near
infrared (NIR) spectra from Type Ia supernovae (SNe) and the intrinsic
brightness of the of the supernovae as characterized by the light
curve parameter Δm15(B). We find that the line depths of OI and MgII
features become deeper as Δm15(B) increases. Thus spectra from faster
declining (intrinsically dimmer) SNe Ia display stronger signatures of
OI and MgII than found in spectra from slower decliners. CaII
absorption does not exhibit this correlation. OI and MgII are nuclear
burning products created under burning conditions of lower
temperatures and densities such as may be found in the outer layers of
the progenitor. The spectra in this sample were obtained between -11d
and +6d with respect to T(Bmax) during which time a SN Ia has a well
defined photosphere in the outer layers of the explosion envelope.