Very good proposition. This can happen next to old mines, waste rock piles especially. Not so much that the CO2 kills you, but it displaces the oxygen. You enter an oxygen deficient environment. Suffocation can happen almost instantly. Similar to confined spaces. However with weather changes, lets say a low pressure system moving in, the CO2 exhales from the surrounding porous rock. You starve for oxygen, almost immediately. And even the oxygen in your body gets sucked out. It has happened before, it took a long time for coroner's to figure out. Mine rescue personnel even get killed, thinking they can hold their breath. Have often wondered why this is isn't presented more to the public, so they can be warned. Avoid hollows, low spots, etc, especially in old mining country. (The reactions in the rock use up the oxygen pricing CO2 - But that's a longer discussion...)

They suspected that nearest mine was 3 miles away now they suspect poisonous cyanobacteria in one of the ponds

There was another article that stated the police were treating the area with hazmat suits because of possible carbon monoxide suffocation.

https://www.sacbee.com/news/california/article253588249.html

https://www.fresnobee.com/news/local/article253577919.html

In 2018, there was the family that died in Tulum, Mexico from CO poisoning as well:

https://www.nbcnews.com/news/world/iowa-family-four-found-de...

Here is an article about 4 men that died next to a waste rock pile in BC, Canada, from lack of oxygen. Another problem is almost all health and safety sensors use oxygen as a baseline for testing, so in a lack of oxygen they do not work accurately. So even experts can get caught in such an extreme situation. https://www.cbc.ca/news/canada/british-columbia/lack-of-oxyg...

CO2 asphyxiation can happen as a result of lake turnover. Perhaps the most famous case is Lake Nyos where 1700 people died in 1986.

https://www.sciencealert.com/how-this-small-lake-in-africa-o...

Didn’t know this happens outdoors. Does the CO2 rise well above the surface though? I’ve heard there are “Dog Caves” [1] around the world where CO2 is low in the cave and will kill short animals but spare adults.

[1] https://www.amusingplanet.com/2018/03/why-dogs-cant-stand-ca...

Co2 probably not...there's a very pronounced physical reaction to an exposure. CO? Or other noble gasses, maybe

CO poisoning leaves a telltale bright red skin coloration in human victims. CO prevents oxygen release from hemoglobin, making venous blood near the skin as red as arterial (oxygenated) blood. I think responders would have seen this if CO was the toxic agent.

It's puzzling - as you pointed out, CO2 poisoning causes extreme distress. You'd think victims would have shown evidence of that.

Maybe as another poster described, asphyxiation occurred not due to CO2 poisoning, but to oxygen deprivation through displacement of air by CO2. Oxygen deficit asphyxiation is very fast (seconds) and asymptomatic.

It will be interesting to see what they finally determine CoD to have been.

I think this is most likely. There are areas around Mammoth with CO2 warning signs.