How would you implement this variable: "Surface water oxygen 18 isotope ratio"

[charlycou]

Hi there,
I'm struggling to implement the framework on this variable: Surface water oxygen 18 isotope ratio.
I can't make my mind on an object of interest and I'm having trouble to convince myself that "ratio" is the iop:Property here.
The confusion comes from the fact that two concentrations are being measured to build this variable (O16 and O18).
"O16 and O18" would be a good candidate to iop:ObjectOfIntesrest but it combines two elementary concepts and then it does'nt suit well my vocabulary.
What would be your suggestion?

For reference: https://en.wikipedia.org/wiki/Oxygen_isotope_ratio_cycle