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The average proportion of naturally occurring semi-heavy water or deuterated water, $\ce{^2HHO}$ or $\ce{HDO}$, is about 1 molecule in 3200. The proportion comes from the fact that one hydrogen in 6400 is in the form of deuterium. The heavy water, $\ce{D2O}$, proportion is about 1 molecule in 41 million ($\ce{6400^2}$) as part of a dynamic equilibrium between normal water, semi-heavy and heavy water (water containing 50% H and 50% D, is in fact a balance between 25% of $\ce{H2O}$ and $\ce{D2O}$ and the rest is $\ce{HDO}$). I understand that the Vienna Standard Mean Ocean Water (VSMOW) has a set proportion equal to the average proportion, but heavy water abundance changes slightly from one kind of water to another.

My question is how much does it change? What oceans have a higher proportion of heavy water? As a hydrogen bond with deuterium is stronger than a normal one, would we expect any biological effects from the different abundances?

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    $\begingroup$ That's a very interesting question that I thought I could answer by digging data from the World Ocean Database: unfortunately they do record tritium abundance but not deuterium. From a quick glance at the literature, it seems that deuterium/hydrogen-1 ratio is thought to vary in part because of temperature. One of the few dataset I could find on pangaea.de with deuterium/hydrogen-1 ratio data shows quite a wide variation in this ratio on a small scale, but the data comes from riverine/coastal samples, not open ocean. $\endgroup$
    – plannapus
    Dec 15, 2015 at 7:45

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For detailed information, see Isotopes of the Earth's Hydrosphere, especially chapter 3 which is "Isotopic Composition of Ocean Water".

At the surface, deuterium fraction is maximized in regions of high evaporation, and minimized in areas with ice.

In the deep ocean, there is about 2% above average deuterium in arctic, and 2% below average near Antarctica. There is a general increasing trend from south to north.

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