I've been thinking about how to create ecotheatre that relates to an ongoing area of carbon sequestration research. One method that interests me and that involves rivers is shown in the pictures. Aquaculture along waterways that lead to the Black sea creates biomass that floats into the sea and precipitates into the anoxic zone, where oxidation would presumably be slow. The idea has been proposed by Robert Bugg to use azolla for this purpose, in imitation of the Azolla event.

I'm not sure, but I think there might be some overlap here with what you are proposing.