Swap the yellow imaging unit with Cyan or Magenta and do a few prints. If the issue moves to the other color you need to replace that imaging unit. Also clean the LED window. Directions online here.
The black at the bottom looks like maybe there is toner inside the machine, not sure how it got there (cracked toner, broken developer etc etc) Try running about 20 blank pages through and see if it clears up.
We are facing some major print quality issues with our Phaser 7100N. There is a really bad yellow fade at the top of each page, and black ink splotted toward the bottom of each page. In addition, there are streaks through the colors in seemingly random places.
We are looking for any type of solution to these problems.
To show, here is our Photo Calibration Chart: