[Geysers] Dating geyser water

[Ralph Taylor]

Lucille Reilly asked about the dating of geyser water in response to a post
noting the publication of a paper by Shaul Hurwitz on the relationship of
geyser periodicity and river levels.  I decided to ask Shaul for his input,
and he kindly responded with the following explanation.  The discussion is
technical but may answer some of the questions (and probably raise more
questions).

Ralph Taylor

Shaul's response:
-----------------
Ralph,

Here is something for the geyser mailing list:

Dating water is tricky, particularly in geysers where waters are boiling. 
One method scientists use to date water involves measuring the 
concentration of tritium (3H), which is a short-lived radioactive isotope 
of hydrogen with a half-life of 12.32 years.  Some tritium forms naturally 
as cosmic radiation interacts with the upper atmosphere, but during the 
1950s and early 1960s, testing of nuclear weapons raised atmospheric 
concentrations hundreds of times above the normal background 
concentration.  Tritium concentrations in the atmosphere have decreased 
following the signing of the Nuclear Test Ban Treaty in the early 1960s 
and have since approached natural levels.

By measuring tritium concentrations in spring (or geyser) waters, what is 
actually being dated is the last interaction between a water molecule and 
the atmosphere.   However, when precipitation (with a known tritium 
concentration) enters the subsurface (groundwater) system, mixing of the 
water molecules occurs, so that at any given location along the flow path 
from recharge to a geyser, a finite volume of water would represent  some 
integration of water molecules from many years.  It is not realistic to 
assume that the entire volume erupting from a geyser percolated into the 
subsurface at the same time.  More likely, the erupted volume is a mixture 
of water molecules that fell as precipitation over a rather long time 
period.  To illustrate this point of mixing my USGS colleague (emeritus) 
Bob Fournier reported in a 1969 paper in the journal Science that more 
than 24 consecutive eruptions were required to clear Old Faithful of a 
tracer (rhodamine B) in a 1963 experiment.

Nevertheless, some inferences can be made by using tritium.  For example, 
this year's precipitation in Yellowstone has a concentration of 10 Tritium 
Units.  (One tritium unit equals 1 tritium atom in 1018 hydrogen atoms). 
If for example the tritium concentration of a geyser is 5 TU, we calculate 
that the water could represent a 50:50 mixture of this year's 
precipitation and pre-bomb (more than 50-year-old) precipitation (0 TU). 
However, this is not a unique solution, both because of subsurface mixing 
and the time-variance of the tritium input.  (The most straightforward 
interpretation of tritium occurs when its concentration is 0 TU, and we 
can confidently infer that none of the water has interacted with the 
atmosphere for 50 years.)

 In the past year, our group at the USGS, in collaboration with the 
National Park Service, has sampled the waters of five geysers (Old 
Faithful, Daisy, Grand, Oblong, and Aurum) six times.  We are still in the 
process of analyzing the data, but so far we have found some detectable 
tritium in all five geysers (though always less than 1 TU).  This implies 
that some recent water molecules are part of the total volume erupted.

Other methods used by scientists for dating "young" water include 
measuring the concentrations of chloro-fluorocarbons (CFCs) and noble-gas 
(mainly argon) isotopes dissolved in water.  As with tritium, the data 
require considerable interpretation.  "Older" waters are often dated using 
radiocarbon methods, but this is problematic in volcanic areas such as 
Yellowstone because of the input of magmatic carbon.

The short answer to Lucille Reilly's question following what she heard 
during a geyser walk "OF's water supply is about 500 years old" - I don't 
know how that info came about....

I hope the discussion group finds this helpful and not confusing!
 
P.S.  There was some more press coverage of the geyser paper; here are two 
of them:

LiveScience.com - 
http://www.livescience.com/environment/080603-old-faithful.html

Casper Star Tribune - 
http://www.trib.com/articles/2008/06/04/news/wyoming/doc48469fcdaa2b65920643
77.txt


Cheers,

----------------------------------------------------
Shaul Hurwitz
U.S. Geological Survey MS #439
345 Middlefield Rd.
Menlo Park, CA 94025
Tel: (650) 329-4441
shaulh at usgs.gov
http://wwwrcamnl.wr.usgs.gov/hydrotherm/
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