I was reading about the Kelvin, then water, which defines it, so I ended up reading about the VSMOW. It's based on "average" ocean water.

From https://en.wikipedia.org/wiki/Vienna_Standard_Mean_Ocean_Water :

"VSMOW is a recalibration of the original SMOW definition and was created in 1967 by Harmon Craig and other researchers from Scripps Institution of Oceanography at the University of California, San Diego who mixed distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major isotopic water benchmarks in use today."

Hence the question in the title.

For further clarification more from the above link:

"The isotopic ratios of VSMOW water are defined as follows:

  • ${}^2H/{}^1H$ = $1455.76 ±0.1$ ppm (a ratio of 1 part per approximately $6420$ parts)
  • ${}^3H/{}^1H$ = $1.85 ±0.36 × 10^{−11}$ ppm (a ratio of 1 part per approximately $5.41 × 10^{16}$ parts, ignored for physical properties-related work)
  • ${}^{18}O/{}^{16}O$ = $2005.20 ±0.43$ ppm (a ratio of 1 part per approximately $498.7$ parts)
  • ${}^{17}O/{}^{16}O$ = $379.9 ±1.6$ ppm (a ratio of 1 part per approximately $2632$ parts)"

So these are averages gained from the mix, because, apparently, the components vary in their individual ratios. So that's what the question is. Why do they vary?

  • 4
    $\begingroup$ Not that this is off-topic here, but questions like this are also a good fit for Earth Science. $\endgroup$
    – user10851
    Jul 14, 2015 at 3:07

3 Answers 3


I looked up some of the original publications by Harmon Craig. There are a ton of them, and they are very highly cited. For example, in Isotopic exchange effects in the evaporation of water: 1. Low-temperature experimental results (Journal of Geophysical Research, 1963 - DOI 10.1029/JZ068i017p05079) the abstract states:

The deuterium and oxygen 18 concentrations of water evaporating into air of nonzero humidity do not follow the simple batch distillation equation but increase asymptotically to a stationary isotopic state as the mass of water decreases to zero. This effect is due to a rapid molecular exchange between liquid and vapor, which predominates over the simple separation effect of removal of vapor. In dry air the batch distillation law is obeyed, and the isotopic enrichment continually increases; however, the separation factor is larger than the equilibrium isotopic vapor pressure ratio when the evaporation rate is comparable to natural rates. The HDO and H2O18 enrichments in our experiments show a linear correlation with a slope of about 5.5, as found in the evaporation of natural water bodies. The mechanism of the molecular exchange will be described in following papers.

In other words - even the simple act of evaporation (which is a strong function of geography because of surface temperature and humidity of the air) will change the isotopic composition of the oceans over time.

I found a full (but short) pdf by the same author "Isotopic variation in meteoric waters" . Note that "meteoric waters" in this context means "water falling from the sky", i.e. rain, snow, hail etc. - not stuff that came from outer space.

Again, from this article:

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  • $\begingroup$ I think your answer is right there: "In other words - even the simple act of evaporation (which is a strong function of geography because of surface temperature and humidity of the air) will change the isotopic composition of the oceans over time." From a statistical point of view, it is clear that a heavier molecule (containing the heavier isotope) will require more energy to evaporate, thus the heavier ones will be found in higher proportion in water which underwent more evaporation. $\endgroup$
    – PhilMacKay
    Jul 14, 2015 at 13:02
  • $\begingroup$ @PhilMacKay - yes. You might consider upvoting akhmeteli's short answer as that's where the "evaporation is the key" concept first appeared (although it started with "I guess" and now we have more data). $\endgroup$
    – Floris
    Jul 14, 2015 at 13:04
  • $\begingroup$ It should probably be added that the formation of sea ice is also just slightly isotope dependent. $\endgroup$ Jul 14, 2015 at 14:41
  • $\begingroup$ @dmckee - good point. There are probably other factors in play, including perhaps some geographical / geological ones as Anna listed. $\endgroup$
    – Floris
    Jul 14, 2015 at 15:12

I guess the isotopic composition of seawater depends on temperature, among other things, as water comprising lighter isotopes evaporates more easily.


The water in the Oceans is partly

1) the original water from their formation and this will have come into equilibrium with the underlying strata of earth. These strongly depend on location for the chemicals composing them and this will apply to the isotopes in these chemicals

2) Rain water which brings down whatever dust exists in the atmosphere, and again there are different chemicals in the dust. You should see the rust rain from the winds coming from the Sahara desert to the Mediterranean.

3) drainage of rain water by the rivers from the continents surrounding the ocean

All this has been happening from the creation of the oceans and the continents , which define the oceans, are largely barriers to homogenizing currents.

Now the quantity you are asking about is for distilled water. This would reflect the isotopic composition of the ocean for chemicals that can be distilled, which according to 1-3 will be different from ocean to ocean.


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