sad news today

my turtles living in a pond, yesterday i put some new blocks in the pond. This morning I woke up and check the pond, the baby turtle was lying on a rock in the sun and it looks like he was choking... I put him in a quarantaine aquarium, he died this afternoon.
How could this happened???????

I think it were the new blocks from a apple tree in the pond, but i'm not sure...... yesterday the baby turtle was swimming, eating and basking and he was very active, now he's dead......
I put them 3 months ago in the pond, beside the pond there are special trees but there not poison.
Maby some of you have an answer????