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I have two questions based on the history of atmospheric oxygen levels depicted below.

graph of oxygen concentration in the Earth's atmosphere over time, from wikipedia commons
Source: https://commons.wikimedia.org/wiki/File:Sauerstoffgehalt-1000mj2.png.

The notes below the chart explain most of the changes in the past Gya.

  1. What they don't explain, however, is why oxygen has declined for the last ~100 Mya. For example, is this explicable in terms of volcanism?

  2. They do explain the Permian decline in terms of volcanic activity, but why do volcanoes affect oxygen levels?

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    $\begingroup$ I would take that graph with a pinch of salt. Past atmospheric oxygen levels are notoriously difficult to determine. $\endgroup$ – bon May 17 '18 at 18:16
  • $\begingroup$ @bon The notes mention that, but both declines do appear to be real. $\endgroup$ – J.G. May 17 '18 at 18:22
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    $\begingroup$ Echillibrium is complex at CO2 Ch4 and O2 atmospheric system. The fall 100 Mya may be related with Alpine Orogeny. $\endgroup$ – Leukocyte May 18 '18 at 16:05
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    $\begingroup$ @Universal_learner Yeah; they cite the sources they averaged, but they don't, for example, say how they averaged them. (I do hope it was done in a variance-minimising way, but I suspect they weren't that smart.) $\endgroup$ – J.G. May 21 '18 at 12:33
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    $\begingroup$ @Universal_learner - The graph in the question needs a huge grain of salt. The End Permian event was 252 MYA. One conjectured consequence of the massive eruption of the Siberian traps that is widely thought to have triggered the End Permian is that the immense quantities of lava, about 4 million cubic kilometers of it, triggered fires in coal deposits. $\endgroup$ – David Hammen May 29 '18 at 18:01
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Here's a somewhat different looking graph, from Oxygen and Evolution, Robert A. Berner et al., Science 316, 557 (2007):

Graph depicting estimates of the percentage of oxygen in the atmosphere over the last 550 million years.

The graph shows three marked drops in O2 levels, each corresponding to an extinction event. These are intervals 4, 9, and 11. Interval 9, the largest such drop, represents the end Permian event about 252 million years ago. Interval 11 corresponds to the end Triassic event, 201 million years ago. The unsourced wikipedia graph linked in the question has at most a superficial resemblance to this sourced graph.

Now for your two questions.

What they don't explain, however, is why oxygen has declined for the last ~100 Mya. For example, is this explicable in terms of volcanism?

I would take even the sourced graph that I used with a grain of salt. That unsourced wikipedia graph: A very large block of salt is needed.

They do explain the Permian decline in terms of volcanic activity, but why do volcanoes affect oxygen levels?

It is very widely conjectured that the end Permian extinction event was caused by the Siberian Traps. This extreme volcanism may have triggered coal fires, thereby reducing oxygen levels. This remains conjecture.

What isn't conjecture is that the end Permian event killed off almost all life extant at that time, including much of the algae and plants that produce oxygen. Without a continuous supply of new oxygen, oxygen sinks such as unweathered rock will inevitably result in a decline in oxygen levels.

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