From: shaw@s119.es.llnl.gov (Henry Shaw 510 423-4645)
Newsgroups: sci.chem
Subject: Re: Heavy water separation
Date: 10 Nov 92 01:01:28 GMT
Organization: Earth_Sciences_Dept.,_LLNL

In article <1992Nov5.160813.23748@engage.pko.dec.com>, moroney@ramblr.enet.dec.com writes:
|> In article <1992Nov5.095320.16450@neptune.inf.ethz.ch>, cohen@cumuli.vmsmail.ethz.ch (Mark Cohen) writes...
|> >I'm sure that you are right about the gravity thing but you have forgotten
|> >the evaporation aspect. At a guess what people (the Germans?) would have
|> >been after would be baths that were continuously filled with evaporation as
|> >the only source of water loss.
|> 
|> Out of curiosity, what is the difference in concentration of D in the
|> ocean, salt lakes etc. where water has been lost to evaporation for
|> eons, compared to fresh/rainwater?
|> 

Natural fractionation of the isotopes of hydrogen and oxygen form the basis of
stable isotope hydrology (and stable isotope geochemistry in general).  
There have been a number of books published on this topic;  I like the IAEA reports --
"Isotopes in Hydrology", 1967, IAEA Proceedings Series;
"Stable Isotope Hydrology:  Deuterium and Oxygen-18 in the Water Cycle", 1981, 
IAEA Technical Report Series #210;  and "Paleoclimates and Paleowaters:  A Collection 
of Environmental Isotope Studies", 1983, IAEA.

Two additional references that cover a wider range of 
stable isotope geochemistry topics are "Stable Isotopes in High Temperature 
Geological Processes", 1986, JW Valley et al., eds., Mineralogical Soc. 
Am. Reviews in Mineralogy v16.;  and "Stable Isotope Geochemistry:  A 
Tribute to Samuel Epstein", 1991, HP Taylor, et al., eds., Geochemical 
Soc. Spec. Pub. #3.


Your question seems to imply that there is some sort of accumulative effect 
on the isotopic composition of seawater, when, in fact, the effect is 
limited because nearly all the water that evaporates eventually finds its 
way back to the oceans.

On average, the D/H ratio of the water vapor in the air immediately above the 
surface of the oceans is 2.2% lower than that of the underlying water and the 
O-18/O-16 ratio is about 0.4% lower.  These fractionations are close to, 
but not quite what one would expect for equilibrium fractionation based 
on the relative vapor pressures of the species involved.  The discrepancy 
is because evaporation from the oceans, in general, is not an equilibrium 
process;  the air above the oceans is not at 100% relative humidity, and 
there is an additional "kinetic isotope effect".

In detail, the isotopic composition of the water in air immediately overlying the 
oceans varies as a function of relative humidity, salinity of the ocean at that 
location, and air and water temperatures.

Eventually, the water vapor that evaporates from the oceans rains out of the 
atmosphere as precipitation.  The isotopic composition of the precipitation 
will be "heavier" (i.e. higher D/H and O18/16 ratio) than that of the water 
vapor in the cloud where it condenses.  The exact composition is again a 
function of temperature and pressure at which the condensation occurs.  

As water is removed from a cloud as precipitation, the isotopic composition 
of the remaining water becomes progressively "lighter" in a process similar 
to Raleigh fractionation (though the fractionation factor is variable in 
this case because, in general, the temperature and pressure changes with 
time).  Precipitation from a given air mass thus changes with time, 
becoming progressively "lighter" as a more and more water is removed.  
This process has the effect of producing distinct geographic and 
orographic patterns in the average isotopic composition of precipitation 
around the world.  In general, the further from the oceans, the more depleted 
is the precipitation in the heavy isotopes of H and O.  Colder regions (at the 
poles and over mountain ranges) also have distinctly "lighter" precipitation.

Precipitation over the ocean has an isotopic composition nearly equal to 
that of ocean water because one is more or less reversing the process 
that put the water in the atmosphere in the first place.  Precipitation 
at the poles has the most extreme depletion in heavy isotopes, with D/H ratios 
as much as 30% lower than average ocean water and O18/16 ratios as much as 4% 
lower than seawater values.

Nearly all surface waters, groundwaters, and atmospheric water have H and O 
isotopic compositions that lie on a correlation line known as the 
"meteoric water line", the equation of which is given by:

	del-D = 8*del-18O + 10

where del-D is the isotopic composition of hydrogen expresses as the 
deviation in parts in 10^3 of the D/H ratio in a sample from the D/H 
ratio in a standard (which is normally taken as "Standard Mean 
Ocean Water, or SMOW):  del-D = [(D/H_sample)/(D/H_SMOW) - 1] * 1000

del-18O is similarly defined for the O18/16 ratio.

This correlation line is due to the combined effects of many processes that 
are averaged over space and time.
in 10^3 deviation)  

Gross deviations from this correlation occur under unusual circumstances,  
one not-so-unusual circumstance is evaporation from a closed water body 
under arid conditions.  Under these conditions, kinetic isotopic fractionation
effects can be dominant, and the surface waters in these regions lie on 
correlation lines with distinctly lower slopes (the exact slope depends 
on the specific conditions).


-- 
Henry Shaw, Earth Sciences Dept., LLNL |    All opinions are my own.
Internet: shaw4@llnl.gov               |  
CIS: 76347,2523                        |
Fidonet: Henry Shaw at 1:161/55        |
Newsgroups: sci.chem