Running Out of carbon Dioxide.
The oceans nearly absorbed all carbon dioxide from the air 325 million years ago. In the nick of time, volcanoes replaced some of it, but they died down, and now the CO2 is almost gone again.
There was five times as much CO2 in the air during dinosaur years, and twenty times as much when modern photosynthesis began. All biology is on the verge of becoming extinct due to a shortage of carbon dioxide in the air which is necessary for photosynthesis.
The oceans constantly absorb CO2, as it combines with calcium to form calcium carbonate and limestone. The result was almost total depletion of CO2 from the atmosphere 325 million years ago (mya). Then a significant restoration of CO2 in the atmosphere occurred. Volcanoes would have been the source of the increase.
Early on, there was a lot of CO2 in the atmosphere due to a lot of carbonates being released from minerals. But oceans have always been absorbing it and tying it up. The decline between 500 and 325 mya says, eventually, the oceans depleted the supply.
The extremely low level 325 mya makes a strong statement. Throughout the early history of the planet, there were several source for CO2 being released into the atmosphere. The early CO2 was largely absorbed into the oceans, as shown by the massive amounts of calcium carbonate and similar substances that formed in early oceans. To remove almost all of the CO2 from the atmosphere says that oceans never stop absorbing it and tying it up, until it is all gone. Biology cannot significantly change the amount of carbon on the surface of the earth, only oceans can.
The increase 300 mya also makes a strong statement. Nothing could have released the carbonates, on a large scale, that the oceans had already tied up in mineral form. There is no other explanation for the increase that began 300 mya but volcanic activity. The increase in volcanic activity is quite logical.
As the earth cooled, tectonic plates thickened, which prevented volcanoes from replacing the CO2 between 500 and 325 mya. The result was the long slow decline in atmospheric CO2. As tectonic plates collided, they would form small mountains but nothing drastic enough to create volcanoes.
As tectonic plates got thicker due to constant cooling of the earth, mountains got larger due to stronger collisions. Then the plates began to move over and under each other which promoted volcanoes and restored CO2 to the atmosphere reaching a peak during the dinosaur years.
Then CO2 levels began to decline again during the later part of the dinosaur years. The decrease would have been a result of tectonic plates getting so thick that volcanic magma could not easily get to the surface. Volcanic activity is at a very low level at this time. Yet, oceans continue to absorb CO2 and convert it into calcium carbonate and limestone.
Now there is so little CO2 left in the atmosphere that all life is on the verge of becoming extinct, because photosynthesis depends upon it. Humans are slightly improving the situation by adding a small amount through combustion of fossil fuels.