How did Hal, Charles, and Mercedes die in The Call of the Wild?

Hal, Charles, and Mercedes die when the ice on which they are running gives way, throwing them into the freezing water, where they drown.

Hal, Charles, and Mercedes had been warned numerous times that it would be foolhardy to try to cross the frozen water so late in the season, but they wouldn't listen.  Arrogant novices, they insisted on doing things their way, scoffing at those with more experience who cautioned that "the bottom's likely to drop out at any moment".  Not only did they refuse to heed the counsel of others, but they were also vicious in their treatment of their dogs, relying on the whip to push them far beyond their capacity, feeding them insufficiently, and doing nothing to lighten the load they were expected to pull.  John Thornton rescues Buck from the foolish trio when Buck is "too near dead to be of further use in hauling the sled".  The two of them watch as Hal drives the "limping and staggering" remnant of the team over the ice, with Mercedes selfishly riding the overloaded sled and Charles stumbling along behind.  Suddently, when the sled is about a half-mile away, they see "its back end drop down, as into a rut, and the gee-pole, with Hal clinging to it, jerk into the air...Mercedes's scream (comes) to their ears...then a whole section of ice give(s) way and dogs and humans disappear...a yawning hole (is) all that (is) to be seen" (Chapter V).