The atmosphere is like a leaky boat. Oxygen leaks out when rocks are eroded and weathered and when volcanoes release reduced gases, and burial of photosynthesized material pumps it back in. The troubling part is that there's no obvious reason why these two processes have to be balanced over periods much longer than the time constant of the system.
What? That's not what I said at all. Erosion of mountains should itself be an O2 sink, not a source (although subsequent burial of organic matter as the eroded material reaches the ocean, and fertilization of plankton in the ocean by the material, could be an O2 source).
