Symbolism, Imagery, Allegory
Death is a frosty place in Sabriel's world. When necromancers visit Death, they wade through an icy river so cold that their physical bodies, left behind in Life, become visibly frozen. And when Sabriel finally discovers her father's body in the underground reservoir, he has been trapped in Death so long that he's like an Abhorsen Sno-Cone. The coldness of Death makes it abundantly clear that this isn't a happy-go-lucky kind of place.
Snow and ice appear in a much more tangible sense when Sabriel first journeys to the Old Kingdom. The division between the magical world on one side of the Wall and non-magical Ancelstierre is evident from the abrupt weather shift on each side:
[…] it was clear and cool on the Ancelstierre side, and the sun was shining—but Sabriel could see snow falling steadily behind the Wall […] as if some mighty weather-knife had simply sheared through the sky. (2.6)
Sabriel prepares for her trip to the Old Kingdom by packing cross-country skis. Here the division between magic and non-magic is illustrated by a walk into the snow—and like the icy river in Death, cold seems to evoke both the otherworldly and supernatural in Sabriel's world, in addition to danger.
Touchstone is frozen when we first meet him, although not in ice—he's preserved as a wooden figure on the prow of a ship. He's suspended in time, caught between worlds, just as Sabriel's father was frozen between two worlds as well, which his immobility represents. That he's frozen in a way that isn't icy cold, though, also immediately clues us into the fact that this guy is one of the good ones.