Not sure about clay I haven't heard anything about that. If it formed in large quantities at that time it should still be in sediments.
The ocean was acidic and anoxic and that was likely a cause of the extinction.
There definitely wasn't enough inorganic carbon released to fully account for the amount of warming that occurred (about 12°C). This was a big part of the book and highly technical (just the explanation of organic versus inorganic carbon was a chapter) so I'm afraid to try to summarize I think I'll get it wrong. But the author's theory was that methane was in the mix as well, released by cooking off carboniferous coal deposits.
The issue is not just the increase in CO2 at the P/T boundary, but the unusual persistence of high CO2 levels for 5 million years. In contrast, the CO2 spike from the CAMP flood basalts lasted only 300 thousand years. The paper proposes that the ocean became enriched in dissolved silica due to the loss of silica-secreting microorganisms (as indicated by lack of chert deposits during this time), and that this caused enhanced "reverse weathering" that kept CO2 levels high.
The ocean was acidic and anoxic and that was likely a cause of the extinction.
There definitely wasn't enough inorganic carbon released to fully account for the amount of warming that occurred (about 12°C). This was a big part of the book and highly technical (just the explanation of organic versus inorganic carbon was a chapter) so I'm afraid to try to summarize I think I'll get it wrong. But the author's theory was that methane was in the mix as well, released by cooking off carboniferous coal deposits.