I don't know what Trump has to do with this, or the amount of O2 in the atmosphere. After googling it, it appears that 30% of the weight of the ore is o2. This means for every 100,000 tonnes of ore we send overseas the ships are displacing 30,000 tonnes of o2. This weight is how much extra water the ship needs to be displacing so it is quite a significant number burning a lot more bunker oil. These ships carry between 180,000 and 400,000 tones of ore by the looks of it. The useful part of the ore is the iron!