Diploma thesis in Physics submitted by Florian Freundt born in ...

Chapter 2
Theory
2.1 Noble gas temperatures
The concept of using dissolved noble gases in ground water as tracers for paleotemperature is
based on the observed temperature dependency of gas solubility in water, the mainly atmospherical
source of noble gases in ground water and the chemically inert behavior of noble gases.
Within the relevant temperature interval found at ground water recharge areas the solubility
decreases with increasing temperature, this effect is more distinctive for heavier noble gases.
The main source of all stable noble gases found in meteoric and ground water is the Earth’s
atmosphere. Radiogenic and terrigenic noble gas sources are of little influence, only 3 He, 4 He
and sometimes 40 Ar are influenced by non-atmospheric sources [Kipfer et al., 2002]. Due to this
influence and the small temperature dependance of its solubility, helium is not used for modeling
paleotemperatures.
Finally, the chemical inertness of the stable noble gases ensures the absence of chemical sinks,
making dissolved noble gases a conservative tracer and leading to a preservation of the temperature
information. This requires a confined aquifer however, since depressurization would cause
degassing and thereby loss of the temperature information.
When water from a confined aquifer is sampled and analyzed for its noble gas concentrations,
the resulting noble gas temperatures therefore represent mean annual soil temperatures at the
ground water table of the recharge area. The temporal resolution of the noble gas temperatures
depends on mixing, dispersion and transport processes within the aquifer and the accuracy of
the water sample dating.
The following description of gas solubility and noble gas temperature calculation is largely based
on Kipfer et al. [2002], Aeschbach-Hertig et al. [2008] and Schneider [2010].
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