I'm leaning towards subfracture drilling as a carbon sequester strategy.
Using an electric vibratory hammer, you can penetrate dozens of metres of clay, such as the deposit under the St. Clair river that is over sandstone which is in turn over impermiable shale.
This is different strategy than excavating clay and filling with trees and sphagnum fuscum pore water.
Ideally it will be possible to shear the layer between the shale and sandstone without cracking the way above clay or underlying shale. Ideally oxygen and microorganisms will not be a major factor.
A key here is residence time, and the depth of bedrock. Deeper is better; more likely to housing existing ice age era water.
If not, going with the clay excavation plan (likely more expensive a carbon price).