I don't have a source, just some napkin math. I don't know what the timescale is for plant decay in the deep ocean. Perhaps my 'it works' is optimistic.
I've done some further googling and found some studies on the topic, though on-land: https://cbmjournal.biomedcentral.com/articles/10.1186/1750-0... and some more from a presentation https://www.feem.it/m/events_pages/20167111127315Zeng_presen...
Though the question still remains how securely the CO2 would be stored under the ocean. Maybe with a large enough layer of rock on top it would suffice.. I suppose it's high time we started field trials on all these approaches.
My own back-of-napkin calc says that we'd need to cut about .1 to .2 % of tropical rainforests per year and remove them from the carbon cycle, not sure if that's right.