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

6 Discussion
using a software solution developed by von Oehsen [2008], both the CE model and the OD model
were employed.
The OD model does reconstruct the initial temperature of 12 ◦ C well for all datasets but suffers
from low accuracy. While the CE model successfully reproduces the temperature of dataset UA0
at a much better accuracy compared to the OD model, it underestimates the temperatures of
datasets OD3 and OD6 by 0.8 ◦ C and 1.5 ◦ C respectively.
The observed elevated noble gas concentrations at this study’s sampling site were therefore
able to influence the temperatures reproduced by the CE model significantly. However, they
were not large enough to account entirely for deviations of noble gas temperatures from 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 OD model to better describe the
excess air component of dissolved noble gases. Yet it is not unlikely that stronger O2+CO2
depletion occurs naturally at increased depths closer to the capillary fringe. Additionally, the
magnitude of influence on noble gas temperatures is also dependent on local annual temperature
and precipitation patterns, determining whether ground water recharge occurs at the same time
as the maximum of oxygen depletion: If both temperature and precipitation variations were
in phase (or the temperature basically constant at a level ideal for microbial activity), soil
respiration would presumably be at a maximum (disregarding other parameters) parallel to
ground water recharge, leading to a strong increase in noble gas concentrations. The CE model
would then underestimate the noble gas temperatures. The other extreme would be a situation
where temperature and precipitation were out of phase, leading to minimal soil respiration during
the time of ground water recharge, limiting the influence of oxygen depletion on ground water
noble gas concentrations. In that case the CE model would likely deliver accurate results while
the OD model, if based on maximum or mean O2+CO2 deficits, would overestimate noble gas
temperatures. This consideration is a rather schematic estimation, as it ignores that parameters
like infiltration speed, depths of ground water tables, soil structure and diffusive transport
variations may introduce more complex relations between soil temperature, soil atmosphere
composition and ground water recharge.
Further complicating the situation is the fact that the OD model handles O2+CO2 deficits in a
single parameter POD to account for the additional fraction Ci,od introduced by oxygen depletion
into the total measured concentration Ci,m. This parameter is optimized for an entire dataset. It
is unlikely, though, that the local and global climate parameters that influenced soil respiration
would remain constant over time scales relevant to paleoclimatology, and therefore the use of the
same, single parameter for every sample collected from an aquifer appears unreasonable to properly
describe the influence of oxygen depletion. Noble gas paleotemperatures have been used to
measure the temperature differences between the last glacial maximum and today. These differences
were found to be within the range of 4 – 5 ◦ C [Stute et al., 1995; Aeschbach-Hertig et al.,
2000; Wieser, 2011]. Based on the Q10 factor provided by Winkler et al. [1996], assuming soil
respiration rates to be 25 % smaller during the glacial maximum just due to the temperature
change appears reasonable. Using the same parameter POD to characterize the effect of oxygen
depletion for the entire dataset unlikely reflects the physical conditions.
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