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.
Mozzza
May 18, 2018