I wonder if dumping the biochar into a subduction zone/trench would permanently remove the carbon from the system at any reasonable rate. Or possibly back-fill

I wonder if dumping the biochar into a subduction zone/trench would permanently remove the carbon from the system at any reasonable rate. Or possibly back-fill mines with it. Either way, I completely agree. Planting trees doesn't solve the problem if those trees are allowed to naturally decompose. Trees are just really efficient carbon fixing machinery, we need to store the solid carbon somewhere.