I heard about this[1] recently, essentially spurring a massive plankton bloom to capture carbon where it ends up on the sea floor and becomes future oil deposits in a few millenia.

The nice thing about it is that it doesn't require global cooperation.

[1]https://www.onepercentbrighter.com/p/the-no-bullshit-way-to-...

Edit: I should probably link where I heard about it to give credit to someone who deserves it

https://uncomfortableconversations.substack.com/p/the-climat...

I was under the impression that there have been multiple large extinction events in the past caused by excessive anaerobic decomposition underwater that led to the oceans becoming swamps and giving off nasty toxic gasses.