Iron was added to the surface ocean together with another compound - sulphur hexafluoride - which could be used as a marker for the iron-rich water even after extensive dilution.
(Iron; was added together; to the surface ocean) (another compound - sulphur hexafluoride; could be used; as a marker for the iron-rich water even after extensive dilution)
Iron another compound - sulphur hexafluoride
was added together could be used
to the surface ocean
as a marker for the iron-rich water even after extensive dilution