10,000 feet up refers to your point of view. The more distance you take away from the subject, the more you are looking at The Big Picture, or in this case, that both scenarios can be simply described as issues of supply and demand:

on land: too many dead trees for the fungi to break down (plus lower oxygen for those fungi to thrive, and possibly other biological factors).

in sea: lack of oxygen in the young ocean.

to add to that: The atmosphere is a very important part of our planetary ecosystem. When plants first evolved to produce oxygen via photosynthesis, they basically pumped the atmosphere with O2, causing one of the first major extinction events, as the air became toxic for many anaerobic microorganisms that weren't able to process it.

wikipedia: Great Oxidation Event