[koboll]

https://en.wikipedia.org/wiki/Canfield_ocean

Also, this: https://www.smithsonianmag.com/science-nature/no-strangelove...

"Sepúlveda's team was able to tell that the carbon in these post-impact sediments could only have been assembled into these hydrocarbons (and other compounds) in a functioning open ocean ecosystem with plenty of algae photosynthesizing away at a pretty good clip. Since these sediments were deposited right after the impact, the "Strangelove" ocean theory, with a vast lifeless sea, is highly unlikely."