Final answer:
The death of John Thornton marks Buck's shift from absolute morality to moral nihilism in 'The Call of the Wild', as he loses his connection to human civilization.
Step-by-step explanation:
Buck is the anthropomorphized main character in Jack London's The Call of the Wild. The half Saint Bernard-half Scotch Shepherd dog goes through a massive physical and mental transformation, as he goes from being a spoiled dog from a wealthy family to a toiling sled dog to a wild wolf dog.
In Jack London's The Call of the Wild, the event that marked Buck's decline from absolute morality to moral nihilism was a) the death of John Thornton. This tragic event signifies a turning point for Buck, as he loses his last tie to human civilization and kindness.
After this loss, he fully succumbs to the call of the wild and his primal instincts, leading him to embrace a life that adheres to the natural laws of survival and dominance rather than the moral values of human society.