Oxygen isotope mass balance between the ocean and polar ice caps: since water is conserved between these two reservoirs, the isotopic composition of the combined (whole) hydrological system (ocean plus ice) is the weighted average of the two reservoirs' isotopic values weighted by their mass fractions; melting all the ice returns its isotopically light water to the ocean, so the resulting fully mixed, ice-free ocean's delta-18O value equals this mass-weighted average of the current ocean and ice cap values.