What if there was a genetically engineered bamboo that was denser than seawater, so that you could grow it and just dump it in the deep ocean... you could have trains of barges taking it to the coast and dumping it. Bamboo is also highly regular so its growth could probably be mechanized/automated to a degree.
There are some tropical fast-growing trees that are denser than water. In Costa Rica, there are rivers with logs on the bottom as a result of poorly-planned logging expeditions. The trees are so heavy partially because they take up silica from the soil - which is why they're also difficult to cut down.
