What clues throughout the novel have foreshadowed Ethan and Mattie’s suicide attempt?

In Chapter Nine, Ethan persuades Mattie to come sledding with him; it's their last chance to go coasting down the hill. The find a sled and go down, despite the fact that it's twilight and the light is the most confusing of any time of day. The first trip down is exhilarating; Ethan steers well, and they reach the bottom safely. They climb up the hill and realize fully that soon they will never see each other again. Both begin to cry, unable to leave each other, telling each other that they can't bear to be apart. Mattie asks Ethan to take her down the hill again, straight into the big elm. She does not want to live without him. They get into the sled for a second ride down, Ethan in front this time. He won't be able to steer, but the track will carry them down straight into the tree.

They go down the hill, but they don't die. Ethan comes to, disoriented. He is in unbelievable pain. He realizes he is holding Mattie; she, too, is still alive.