"It doesn't really matter whether that carbon comes from supervolcanoes or from human emissions."

It matters a lot. Whilst I was eating dinner I've now read the article you cited, the paper that it cites (Kump, Arthur & Pavlov, 2015). I also cross checked the argument and numbers against several other papers. Ward's argument is extremely slippery. This is sadly what I'm coming to expect from academics with unverifiable models. You have to read everything they write adversarially.

"In [Kump2015]'s models, if the deepwater H2S concentrations were to increase beyond a critical threshold during such an interval of oceanic anoxia, then the chemocline separating the H2S-rich deepwater from oxygenated surface water could have floated up to the top abruptly. The horrific result would be great bubbles of toxic H2S gas erupting into the atmosphere."

Observe that his argument starts with a "critical threshold" for H2S levels, not CO2, and that he doesn't tell us what this critical level is. The obvious questions are thus: what is this level, is it realistic for our present day oceans to reach this critical level and if so, how? To get to those levels of H2S he hypothesises that:

"if ancient volcanism raised CO2 and lowered the amount of oxygen in the atmosphere, and global warming made it more difficult for the remaining oxygen to penetrate the oceans, conditions would have become amenable for the deep-sea anaerobic bacteria to generate massive upwellings of H2S."

In other words in this theory the supervolcanoes have to come first, triggering a sharp drop in oxygen levels in the atmosphere, which in turn then causes the chemocline to move, which then causes the H2S upswellings. It's all caused by a massive loss of oxygen, not increase in CO2 levels, which is simply another result of the volcanisms. Kump2015 also makes it clear that their hypothesis requires a truly massive drop in oxygen levels to occur. We'll look at how much in a moment.

But just a few paragraphs later Ward has forgotten all about the volcanoes and oxygen levels. Suddenly it's all about absolute CO2 levels - not even rates! Cause, effect and unrelated side effects have become entirely muddled, probably because he knows nobody would care about his article unless he ties it to global warming armageddon somehow and because hey, this entire field is nothing but assumptions, suppositions, and playing with numbers that can never be verified anyway so why not?

His argument is brittle in other ways. He asserts without any backing argument that whilst supervolcanoes explain all the other non-asteroid mass extinction events it doesn't for the Permian because, apparently, supervolcanoes are really great for plants on land which can "probably" survive the warming. And there was me thinking that large scale volcanic activity is supposed to be very bad for plants because it dims the atmosphere:

https://www.vulkane.net/en/volcanism/supervolcano.html#:~:te....

"Plant growth is restricted and mass extinction can be caused."

Back to the H2S claims. Turning our attention to the Kump2015 paper we find immediately that it's got an annoying structure in which they work backwards from their desired scenario to calculate the level of H2S that could trigger it:

"Thus, if the H2S of the deep sea increased during an anoxic interval beyond a critical value (1 mmol/kg), upwelling regions of the world ocean would become sulfidic, even with the modern [oxygen level at atmospheric pressure] .... A slightly more sulfidic ocean with H2S = 3 mmol/kg ... could sustain ... 2000 times the present-day flux and a critical value for the atmosphere (see following)."

But these critical values are never placed clearly in context. Is 3 millimols a lot or not much? Note that the 3 mmol/kg value is the absolute "best" case for their scenario; it can only be that low in (they estimate) 0.1% of the world's oceans because normal ocean requires levels 20x higher. They do admit that:

"The [H2S] condition is extreme, and thus likely to have been rarely achieved in Earth history. Is there any evidence that such conditions have occurred in the geologic past?"

How extreme is it? The value of the normal ocean would be important to have here but they don't give it to us. The paper "Hydrogen Sulfide in the Black Sea" by Volkov & Neretin does give values though, and bear in mind this is by far the most anoxic basin in the world: at a depth of 1km the value is ~314 micromols/kg. So these H2S levels that are claimed to trigger this process are literally many, many orders of magnitude higher than even the Black Sea.

The levels of oxygen drop needed is something they also don't seem to directly share, but in one section they're kicking around a figure like half of all today's oxygen having vanished, or for a different event, 99% of all oxygen having gone.

Even putting aside that these papers are just piles of completely unverifiable suppositions stacked like a jenga tower, there is absolutely nothing even remotely close to realistic about this scenario happening to us. It requires oxygen levels to drop so much that if it were to ever become an actual threat we'd all have died of oxygen starvation long before.