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Some oceans are very deep. But is there also current (convection) deep in the ocean so that all water molecules will ones be evaporated and fall down as rain or is it very quite at the bottom so some water molecules will stay at the bottom for decades?

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    $\begingroup$ This reminded me of a XKCD What If question (what-if.xkcd.com/74) and sure enough it contains a link to a study (?) that gives the renewal time for ocean water to be around 3100 years. You may want to examine that. esf.edu/efb/schulz/Limnology/hydrologic.html $\endgroup$ – Boluc Papuccuoglu Jun 8 '16 at 14:38
  • $\begingroup$ decades is very short in the hydrological cycle. agree with my friend boluc there. $\endgroup$ – com.prehensible Jan 21 '17 at 7:13
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    $\begingroup$ Also, maybe related, the isotopic composition of the water molecule controls how easily it evaporates from the oceans surface. So eg. water with an $^{18}O$ isotope would have a smaller chance to form rain than the more common $^{16}O$. $\endgroup$ – Tactopoda Jan 21 '17 at 17:14
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Never is a very, very long time. Almost never is a vague term.

The age of deep ocean water has been studied with both models and observations, and both indicate a lower limit age in the order of thousands of years, rather than decades. Here, age is defined as elapsed time since contact with the surface, which would be the only opportunity for evaporation.

On a geological timescale, all ocean water has ample opportunity to evaporate and fall back as rain again and again. On a human timescale, not so much: for some water in the ocean, it has been well over a thousand years since the last time it was near the surface.

For an example of a recent study into the topic:

Gebbie and Huybers (2012): The Mean Age of Ocean Waters Inferred from Radiocarbon Observations: Sensitivity to Surface Sources and Accounting for Mixing Histories. in: Journal of Physical Oceanography. http://dx.doi.org/10.1175/JPO-D-11-043.1

From the abstract (emphasis mine):

A number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700–1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds given Global Ocean Data Analysis Project (GLODAP) radiocarbon observations, and they show that the potential range of ages increases with the number of constituents or sources that are included in the analysis. (...) The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates. (...) Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200–1500 yr old, which is more in line with existing numerical model results.

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Note that individual water molecules lifetimes that are much shorter than the ventilation timescales you are inquiring about. See

http://www.livescience.com/32254-does-a-water-molecule-have-a-lifespan.html

and

https://www.reddit.com/r/askscience/comments/1gk9gm/whats_the_lifetime_of_a_water_molecule/

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  • $\begingroup$ So the question should be how long it takes before a deep oxygen atom or deep hydrogen atom appears in a raindrop? But probably the hydrogen atoms get much sooner in a raindrop than a oxygen atom because the are lighter? $\endgroup$ – Marijn Jun 9 '16 at 8:17
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Maybe I am very far from to be an earth scientist :-), but here are some laymans calculations based on the available data.

  1. The land area of the Earth is around 148million km^2 (ref).
  2. The mean yearly rain on land is around 715mm (ref).
  3. (1) and (2) means a total volume of yearly rain on land around 148e+6 * 715e-6 = 105820 km^3.
  4. The total area of the World in around 510072000 km^2 (ref).
  5. (1) and (4) means a total water area of around 362 million km^2.
  6. Average overall depth of the world Oceans is 3800 m (ref).
  7. (5) and (6) means a 1375.6 million km^3 of the total volume of the world oceans.
  8. (3) and (7) means a time constant of around 1376.6/0.105820 = 13000 years for the time constant of the exchange of waters by rainfall to the land area of the world oceans.

(8) would also mean, that after every 13000 years, around 63% (1-1/e) of the water molecules of the World's oceans evaporate and falls as rain into the land - if the evaporation would happen with the same ratio in the whole volume of the world oceans. Which is clearly not true - there are deep areas without nearly every underwater stream or any similar. But - considering the other replies - their exchange by underwater streams is much faster as this 13000 years, so we can consider them negligible.

Consider that the world oceans exists since around 4.4 billion years. It is around 338462 times of the time constant of the rain exchange. This would mean, that around 1/e^338462 part of the water molecules of the World didn't fall as rain until now. It is around 10^-146992 .

Now consider that the number of the protons in the visible Universe is around 10^85.

This 10^146992 is so many times bigger as even the number of the protons in the whole visible universe, that even considering the minor inaccuracies in this calculation, or considering the quasi-stable deep ocean regions (which were detailed by the previous answers), we can surely say:

There is no water molecule on the world oceans which wouldn't ever fallen as rain somewhere to a surface area.

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Not in geological timescales.

The average residence time of water in the oceans is around 3,000 years. This is because the world's anual precipitation is about 5e14 m3/yr, while the global ocean volume is 1.4e18 m3 [http://hypertextbook.com/facts/2008/VernonWu.shtml]. If you divide the second number by the first, you roughly get the 3000 yrs.

However, this approach assumes that water is well mixed in the oceans. Note that there are some exceptional seas that contain highly stratified waters where circulation is extremely slow, for example, in the Black Sea. Local residence times in those basins can become orders of magnitude longer.

In the other hand, the water forming the Antartica ice can spend about 20,000 years before being recycled to the atmosphere, because ice flow is slower than ocean circulation.

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Are there? Sure. For example, any water molecule that undergoes a chemical transformation. For example, the chemical equation $$\ce{CO_{2(aq)} + H_2O <--> HCO_{3}^-_{(aq)} + H^-_{(aq)}}$$ destroys the water molecule to create ions. Now since it is a reversible reaction, it is arguable whether or not the molecule is actually destroyed. However, once the reaction $$\ce{Ca^{2+}_{(aq)} + 2HCO_{3}^-_{(aq)} <--> CaCO_3 + CO_{2(aq)} +H_2O}$$ occurs, than the original molecule is arguably destroyed.

An underlying assumption: I am considering a water molecule to never have been evaporated if it at some point is destroyed (i.e. the atoms no longer make up water).

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