[Physics] Why do the waters of the different oceans of the world have different ratios of their H and O isotopes

isotopeswater

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?

Best Answer

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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