Do oceans not have bacteria that break down organic matter? Does ocean water not contain oxygen that combines with carbon during decomposition?

plants you eat get turned into shit, which can then be used as fertilizer mixed into the dirt to grow more plants.

Is animal flesh not turned into shit? Are fertilizers carbon? Or do they rather provide nitrogen, phosphorus and potassium? And even if it is carbon, does shit dumped into fields not rot?

Your last proposition implies that it’s the continuous production of shit and new plants that’s keeping the carbon bound. But it means then that the humans are actually the store of carbon. Maybe we should chuck humans into five-kilometers-deep mine shafts.