In the poem "thanatopsis" lines 17 30 have a sad, tragic tone. can you describe the shift in tone that occurs in line 31. what metaphors and images in this section of the poem reinforce the change in tone?

Well, most obvious shift in tone is with the caesura in line 17. Notice the dash? That tells you right away that the poet wants to draw attention. The line break accentuates that even further. Do you see this in line 31? No it is not an obvious shift here, but instead a simple word. What word do you see that starts line 31? "Yet."  There is no line break, but simply the same word, leading to a simpler shift that does not deny the sadness in tone from these lines, but instead, acknowledges it for what it is worth. It is only tragic if that is how you view it, in isolation, in loneliness. 

The almost unnoticeable shift in tone relates nicely to the images in that part of the poem. Note the indirectness of death: 
"The all-beholding sun shall see no more"
"to be resolved to earth again"
"To mix for ever with the elements"