So my background isn't historical, it's scientific, but I think I could potentially help with this question if you were to clarify it more. I can't think of anything off the top of my head that the mongols would/could have done to sequester 700m tonnes of carbon dioxide. If you provide a premise for how this might occur, I would do my best to elaborate on it and explain why it would/would not be the case.

My gut reaction to this question is that the mongols would semi-frequently burn and destroy the cities they encountered and as such would have actually been releasing CO2 as well as other miscellaneous chemicals in the process. Any sequestration they were doing would likely simply have been through expanding their own population as the human body is the only reservoir for CO2 they interacted with that comes to mind.