Dear Rebecca,
I'm far from being an expert but I fund the following article quite
interesting on isotopic equilibration time.

Zeebe RE, Wolf-Gladrow DA, Jansen H. "On the time required to establish
chemical and isotopic equilibrium in the carbon dioxide system in seawater".
MARINE CHEMISTRY 65: (3-4) 135-153 JUN 1999

It refers only to seawater but I believe the calculations can give you an
idea of the order of magnitude for isotopic relaxation also for soils or
water films.

Regards,

Roberto Ventrella
Dept. Environmental Sciences & Energy Research
Weizmann Institute of Science
Rehovot, 76100 - Israel.
tel: +972 +8 934 4319 (2146)
fax: +972 +8 934 4124
e-mail     [log in to unmask]
internet  http://www.weizmann.ac.il/~roberto/

Rebecca Hood wrote:

> ...The key question is at
> higher enrichments would the exchange rates between H20 and CO2 preclude
> this as a useful technique????...