  While O is often seen in spectra of Type Ia supernovae (SNe Ia) as both
unburned fuel and a product of C burning, C is only occasionally seen at the
earliest times, and it represents the most direct way of investigating
primordial white dwarf material and its relation to SN Ia explosion scenarios
and mechanisms. In this paper, we search for C absorption features in 188
optical spectra of 144 low-redshift (z < 0.1) SNe Ia with ages <3.6 d after
maximum brightness. These data were obtained as part of the Berkeley SN Ia
Program (BSNIP; Silverman et al. 2012) and represent the largest set of SNe Ia
in which C has ever been searched. We find that ~11 per cent of the SNe studied
show definite C absorption features while ~25 per cent show some evidence for C
II in their spectra. Also, if one obtains a spectrum at t < -5 d, then there is
a better than 30 per cent chance of detecting a distinct absorption feature
from C II. SNe Ia that show C are found to resemble those without C in many
respects, but objects with C tend to have bluer optical colours than those
without C. The typical expansion velocity of the C II {\lambda}6580 feature is
measured to be 12,000-13,000 km/s, and the ratio of the C II {\lambda}6580 to
Si II {\lambda}6355 velocities is remarkably constant with time and among
different objects with a median value of ~1.05. While the pseudo-equivalent
widths (pEWs) of the C II {\lambda}6580 and C II {\lambda}7234 features are
found mostly to decrease with time, we see evidence of a significant increase
in pEW between ~12 and 11 d before maximum brightness, which is actually
predicted by some theoretical models. The range of pEWs measured from the BSNIP
data implies a range of C mass in SN Ia ejecta of about (2-30) * 10^-3 M_Sun.
