"Twilight Zone" effect

Carbon dropped into the oceans is not automatically stored away for millennia. Instead, microorganisms living in the "twilight zone" captures much of the carbon and returns it to the biosphere. This is important for OTEC schemes that plan to sequester carbon by locking it up in algae and sinking it to the ocean bottom. From this study, it looks as if the material needs to be deposited at least 1000 meters down to get a maximum impact.