Noble gas thermometry
Basic principles
Processes in the recharge area
- solubility equilibrium (fig)
- excess air formation
- Xe vs Ne for two aquifers show fractionated and unfractionated
excess
air
relative to air (fig)
- processes near the water table (fig)
- dissolution of additional gas due to partial or total
dissolution of
bubbles
- possibility for secondary gas exchange
- solubility controlled (fig)
or
diffusion controlled (fig)
secondary
equilibration
- Cx/Cxo = (CNe/CNeo)(Dx/DNe)describes
the diffusion controlled process
- Cx/Cxo = 1/(1+bNe/bx*(CNeo/CNe -1))
describes solubility controlled process
- both processes result in an enrichment of the heavier noble
gases relative to atmospheric air (fig)
Evaluation models
- Models
- no excess air (e.g. Oana, 1957, Sugusaki, 1961, Mazor, 1972)
- ‘TD’: completely dissolved bubbles, unfractionated excess air
(Andrews
and Lee, 1979, Heaton and Vogel, 1981)
- ‘PR’: complete dissolution of bubbles, then partial diffusive
re-equilibration
(Stute et al., 1995)
- ‘CE’: closed system partial equilibrium dissolution of
entrapped
bubbles
(Aeschbach-Hertig et al., 2000)
- Techniques
- no correction necessary because only one process is considered
- graphical method (e.g. Heaton and Vogel, 1981) (fig)
- iterative schemes (e.g. Andrews and Lee, 1979, Rudolph et al,
1984,
Stute
et al., 1995) (fig)(fig)
- inverse modeling (Ballentine and Hall, 1999; Aeschbach-Hertig
et al.,
1999)
- the goal of the inverse technique is to minimize the
difference between
measured and modeled concentrations for the noble gases:
C 2 =S
(Ci-Cimod)2/si2
- contours of X2 in the parameter
space (fig),
the more circular the isolines, the smaller the error bars of the
derived
parameters
- method has the advantage to differentiate between models and
to yield
error
bars
- PR and CE models are not that different in the way how they
fractionate
(fig)
Noble gas and ground temperature
- noble gas temperatures reflect the soil (ground) temperature at
the
water
table, often the mean annual temperature (fig)
- temperature as a function of depth in soils is subject to daily
and
seasonal
fluctuations
- DT/Dz
is
the geothermal
temperature gradient (typically ~3oC/100m)
- z bar is the characteristic penetration depth for daily (~20cm)
or
seasonal
(~2.5m) temperature fluctuations
- the recharge rate can also be described by a sinusoidal
oszillation:
- both equations can be merged and differences between mean annual
ground
temperatures and obsreved noble gas temperature can be calculated
- deviations between ground and noble gas tempertares may be
significant
if the water table is very shallow or if the recharge occurs very fast
(fig)
- the idea that noble gas temperatures reflect mean annual ground
temperatures has recently been challenged by a study perfomed in Texas
(Castro et al., 2007), where large differences between ground
temperature and noble gas temperature were observed (fig).
The archive
- the archive: confined aquifers (fig)
- groundwater flow systems act as low-pass filters, can be
described by:
- Another way to express the function of a groundwater system as a
lowpass
filter is to calculate the characteristic
time constant after which the amplitude of an original sinusoidal
climate
oscillation is reduced to 1/e:
- Another way to look at this is to calculate the smoothing of a
high-frequency
climate signal, here borrowed from the GISP2 ice core (fig)
Noble gas thermometry and other methods of paleoclimate reconstruction
- disadvantages of the noble gas technique
- dating (s14C ages:
±
2000..3000
years)
- smoothing of climate signals
- accessibility of the archive
- dependence on extreme changes of the water table
- fractionated excess air
- advantages
- based on a physical principle
- no biology
- no calibration necessary
- yields mean annual temperatures
Case study: The Great Hungarian Plain, Hungary
- groundwater flow system in the great Hungarian Plain, map (fig)
and vertical section (fig)
- noble gas and stable istope data for the Great Hungarian Plain (fig)
- comparison between different correction models shows small
differences
(fig)
- noble gas temperature ~8oC lower than today for the
LGM
Case study: NE - Brazil
- noble gas temperatures from NE Brazil (fig)
- method has been applied at many locations all over the world (fig)
Other applications for atmospheric noble gases
- solubility of noble gases in organic phases (petroleum, for
example)
are
higher than in water
- characteristic concentration patterns can be used to identify
subsurafce
interactions between different phases (fig)
Resources
- Aeschbach-Hertig, W., F. Peeters, U. Beyerle, R. Kipfer, 1999.
Interpretation
of dissolved atmospheric noble gases in natural waters, Water Resour.
Res.
35, 2779-2792.
- Stute, M., Forster, M., Frischkorn, H., Serejo, A.,
Clark,
J.F. Schlosser, P., Broecker, W.S. and Bonani G.(1995) Cooling of
tropical
Brazil (5oC) during the last glacial maximum. Science, 269,
379-383.
- Kipfer, R., W. Aeschbach-Hertig, F. Peeters & M. Stute,
(2002) Noble gases in lakes and ground waters. In: Noble Gases in
Geochemistry and Cosmochemistry. pp. 615-700.
- Castro,
MC; Hall, CM; Patriarche, D; et al. (2007) A
new noble gas paleoclimate record in Texas - Basic assumptions
revisited EPSL, 257
(1-2): 170-187 MAY 15 2007