Floating flakes made from agricultural waste such as rice husks, lignin and mineral tailings are ideal method of ocean fertilisation – and cheap. The method has been estimated potentially to sequester 22-47Gt CO2 /year for an estimated $10 per tonne of CO2. At the same time it would also absorb some undersea methane releases, increase ocean albedo, boost fish stocks and restore ocean health generally. Laboratory and ship tank trials with seawater are reportedly going well, with open sea trials off the west coast of India expected to begin in early 2019:

Ocean Fertilization by Bouyant Flakes

Tech info: Rationale for Buoyant Flake Ocean Fertilisation