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

1 Introduction
on the pressure gradient and a parameter describing the hydraulic conductivity of the given
soil. Flow rates and residence time of water within aquifers therefore vary depending on the
soil’s hydraulic properties an recharge rates. Water in confined aquifers can reach an age of up
to several 10 5 years [Sturchio et al., 2004]. Radiocarbon dating of dissolved inorganic carbon
in ground water is used to provide the age information for up to 50,000 year old water while
different isotopic dating methods have to be used for older ground waters.
Reconstructing paleotemperatures from ground water utilizes the dissolved noble gases neon,
argon, krypton and xenon and requires confined aquifers that limit gas exchange. The idea behind
this paleorecord is the temperature dependency of gas solubility: the infiltrating meteoric
water leading to ground water recharge is in contact with the soil atmosphere before entering
the aquifer. During this contact the gaseous and aqueous phases equilibrate. This equilibration
is dependent on soil temperature and the composition of the soil atmosphere. Due to temperature
damping within the soil, its temperature is closely related to annual mean atmospheric
temperatures [Hillel, 1980]. The standard assumption on soil atmosphere noble gas composition
is that it is close or equal to the atmospheric composition, showing only insignificant
fluctuations [Stute and Schlosser, 1993]. This however neglects the fact that soil atmospheres
greatly vary in O2 and CO2 composition spatially as well as temporally [Yamaguchi et al., 1967;
Dowdell and Smith, 1974; Amundson and Davidson, 1990; Magnusson, 1992]. This likely affects
the partial pressures of its remaining components. These changes in soil air composition are
mainly caused by microbiological activities and are influenced by a multitude of soil properties
like temperature, precipitation, soil hydraulic properties and others in a complex relationship
[Suarez and ˇ Sim˚unek, 1993; Welsch and Hornberger, 2004; Riveros-Iregui et al., 2011]. While
paleotemperature studies have largely neglected this possibility so far and have nonetheless successfully
employed modeling approaches to account for other factors affecting the dissolved noble
gases [Aeschbach-Hertig et al., 1999b; Kipfer et al., 2002; Peeters et al., 2003], studies on young
ground water in recharge areas [Stute and Sonntag, 1992; Ma et al., 2004; Castro et al., 2007]
have led to noble gas temperatures a few degrees below measured soil temperatures. Hall et al.
[2005] proposed that this shift could be caused by a deviation of noble gas partial pressures in
the soil atmosphere from atmospheric values. As the cause for this change they suggest the
process of oxygen depletion, meaning that the O2 removed from the soil atmosphere by microorganisms
is not replaced by an equimolar amount of CO2 produced by these organisms, leading
to a pressure deficit affecting the partial pressures of the remaining gases. While their modeling
approach including the proposed oxygen depletion effect successfully leads to matching noble
gas and mean atmospheric temperatures for their study area, an actual measurement of soil air
composition was not executed to prove oxygen depletion has the suggested effect on noble gas
partial pressures and could indeed be the main factor in causing the noble gas temperature shift.
Extensive research has been done on the composition of soil atmospheres for various reasons,
mainly owing to its importance in agricultural contexts. Modern climate research has also been
interested in soil atmospheres because of their part in the global atmospheric gas balance. Soils
provide both sinks and sources for CO2, CH4, N2O and various other gases relevant to the
Earth’s radiation balance. These studies mainly focused on O2, CO2 and nitrogenous gases and
usually their flux from the soil rather than in situ concentrations. Data on noble gases in the
soil atmospheres relevant to noble gas temperatures is sparse at best, as little research on soil
atmosphere composition focusing on the noble gas components has been done so far. While
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