I must admit confusion of the use of the unit gigaton. Normally this unit is used as a measure of explosive power. A ton, no matter which side of the Atlantic you reside, is not equivalent to a tonne, which is of course generally considered to be the same as a cubic meter (metre) of (pure) water. I will have to read the original 'Nature' article now to find out what was actually intended.

ton in USA = 2000 lb
ton elsewhere = 2240 lb
tonne = 2000kg

If the original article contained that unit then so be it. If the original article specified cubic meters then why not stick with that unit of measurement.