Methods to study old groundwaters
Groundwater in extended or deep aquifers can exhibit very high residence times in the subsurface, and thus constitute an archive of past precipitation. In particular, it is often possible to reconstruct the isotopic composition and the concentrations of conservative trace substances, such as the noble gases, back to the last ice age. These environmental tracers yield information on the environmental conditions (esp. temperature) at the time of infiltration, which itself is usually determined by 14C-dating. In this way the temperature history of the last 30 to 40 thousand years can be reconstructed. The following methods are applied in this field:
the noble gas thermometer, based on the temperature dependent solubility of the atmospheric noble gases (He, Ne, Ar, Kr, Xe);
the stable isotopes of water, based on isotope fractionation effects in the hydrological cycle (d2H, d18O);
the 14C-method, based on the radioactive decay of 14C in the dissolved inorganic carbon (DIC) in the water.
The reconstruction of recharge temperatures from dissolved noble gases and stable isotopes in 14C-dated groundwaters constitutes today an established method to determine the temperature change during the transition from the last glacial maximum to the present warm period.
Noble gases (He, Ne, Ar, Kr, Xe)
The solubilities of the heavy noble gases (Ar, Kr,
Xe) in water depend strongly on temperature. The measurement of noble gas
concentrations in groundwater allows therefore the determination of the
temperature at the time of infiltration, which has been successfully used in
many studies to reconstruct the temperature history back to the last ice age.
The advantage of the so-called noble gas thermometer compared to other
paleotemperature proxies such as the stable isotopes is that due to the exact
knowledge of the solubilities reliable absolute temperatures can be determined.
The noble gas paleothermometer is generally accepted as reliable indicator of
absolute temperatures, which is the basis of its importance for the calibration
of climate models. The majority of the noble gas paleoclimate studies comes from
temperate latitudes in Europe and North America. There, such studies reveal a
glacial cooling of 5 to 9 C, where the highest values have been explained by the
influence of the glacial ice shields (see reviews in Stute und Schlosser 1993,
2000, Kipfer et al. 2002). In recent time noble gas studies have increasingly
been conducted in (sub)tropical latitudes, which also yield coolings of about 5
C (Stute et al. 1995, Stute and Talma 1998, Edmunds et al. 1999, Weyhenmeyer et
al. 2000, Beyerle et al. 2003).
A difficulty of the noble gas method (as well as of the dating methods based on gaseous tracers) lies in the occurrence of the so-called "excess air" in groundwater, which has be be taken into account in the calculation of noble gas temperatures by inverse numerical methods (Ballentine and Hall 1999; Aeschbach-Hertig et al. 1999). Excess air in groundwater is however not just an annoyance, but can potentially yield useful indications about the recharge conditions. Especially in semi-arid regions a relationship between excess air and recharge or precipitation intensity has been suggested (Aeschbach-Hertig et al. 2002a, Beyerle et al. 2003, Kulongoski et al. 2004), which could render "excess air" a proxy for paleo-humidity.
Stable isotopes (d18O, d2H)
The stable isotopes of the water molecule (d18O, d2H) have been applied successfully for decades in hydrology and climatology (e.g. Clark and Fritz, 1997). The serve to identify recharge conditions, origin, and mixing of different waters. In regions with marked topography, the stable isotopes can be used to determine the recharge altitude (by the so-called altitude effect, Gonfinatini et al., 2001), which allows a delineation of the recharge areas and is of high importance for the interpretation of gas tracer data. Particularly important in paleoclimate studies on groundwater is the already mentioned possibility to use stable isotopes as paleotemperature proxies. Compared to the noble gases, the stable isotopes have a much wider field of application as temperature proxy, but their calibration as a thermometer is a difficult and much discussed problem (e.g. Fricke and O'Neil 1999). The combination of noble gas temperatures and stable isotopes in groundwater studies offers a unique possibility to locally calibrate the relationship between temperature and isotope ratios, which is needed as a basis for the interpretation of other continental climate archives (e.g. ice cores, lake sediments, speleothems).
The age dating of groundwaters for the
reconstruction of paleoclimate records is usually done by the 14C-method
on the dissolved inorganic carbon (DIC), despite the problmes of this method due
to carbon exchange processes in the subsurface. The basic idea of 14C-dating
of groundwater is that young (14C-active) in the form of soil air CO2
gets dissolved in groundwater during recharge and thereafter decreases only by
radioactive decay with a half-life of 5730 years. However, it has become evident
that already during infiltration also old (14C-dead) carbon enters
the groundwater by carbonate dissolution. The contributions of the different
carbon sources has to be estimated, which is usually done based on the stable
carbon isotopes (d13C). To this end, a
number of models exist, of which the most widely used is probably the one of
Fontes and Garnier (1979). Whereas these corrections can be performed reliably
in carbonate-poor aquifers, in some aquifers large difficulties occur due to
dissolution and precipitation of carbonates along the flow path. The 14C-method
is nevertheless still die most important and often the only quantitative method
to date groundwaters in the age range of a few thousand up to about 40'000
The accumulation of radiogenic He can be used at least as a semi-quantitative dating tool, which is so to speak built-in with the noble gas method. A quantitative dating with He is however often hindered by the uncertainty on the strength and origin of the He flux in aquifers (e.g. Torgersen and Clarke 1985, Solomon et al. 1996, Castro et al. 2000). There are however also successful examples of the use of radiogenic He for dating (Aeschbach-Hertig et al. 2002c, Beyerle et al. 2003).
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