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

Chapter 7
Summary
The results of this study’s measurements confirm that O2 and CO2 concentrations within the
soil atmosphere fluctuate strongly. The measured changes in noble gas concentrations should
not be neglected when calculating paleotemperatures from dissolved noble gas concentrations in
ground water. These changes are clearly correlated to the deviation of O2+CO2 concentration
from the atmospheric value. The combination of noble gas, O2 and CO2 readings implies that
the change in noble gas concentrations might be dependent on isotope mass, for reasons hitherto
unknown. It should be noted that the measurements of both the lightest isotope 3 He as well as
the two heaviest isotopes 84 Kr and 132 Xe suffer from low accuracy and significant scatter.
However, the observed increases in noble gas concentrations at this study’s sampling site were
not large enough to entirely account for deviations of noble gas temperatures and mean annual
air temperatures as observed by Ma et al. [2004] and Hall et al. [2005] in studies of recent ground
waters that prompted the proposition of the oxygen depletion model to better describe the excess
air component of dissolved noble gases. It is questionable whether it is reasonable to transfer and
compare these results directly to any ground water recharge area, as the processes controlling O2
consumption and CO2 production as well as removal are manifold and complicated. Variations of
diverse parameters on local as well as temporal scales have strong influences on the magnitude
of soil respiration and thereby on the noble gas concentrations of soil atmospheres. Due to
these mostly meteorological influences, soil respiration probably does not influence the absolute
calculated noble gas temperatures by introducing a constant offset but rather by individually
influencing every single data point depending on the local conditions at the recharge area.
This study could therefore confirm that the basic assumption of the OD model as proposed by
Hall et al. [2005] is valid, as noble gas enrichment indeed occurs in natural soil atmospheres and
is able to influence ground water noble gas concentrations significantly. In light of the measured
fluctuations of soil atmosphere compositions and the influence of several different parameters on
microbial activity in the soil, it is however likely that the OD model in its current implementation,
despite delivering statistically good results, does not capture the physical reality of most ground
water recharge sites very well.
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