Old groundwaters contain information on past environmental change. The temperature dependence of the air/water partition of atmospheric noble gases allows to calculate the annual mean temperature in the recharge area from the noble gas concentrations. Such noble gas recharge temperatures (NGRT) have been derived from aquifer systems with residence times of several 104 yr to reconstruct the continental temperature regime during the last ice age. On larger time scales, the tendency of dispersive mixing to smooth gradients might interfere with NGRT determination.
We report noble gas data from 4 locations in the western part of the GAB, Australia. All samples have large excesses of He relative to solubility equilibrium. Qualitative age estimates based on D 4He and other geochemical tracers indicate groundwater residence times up to several 105 yr. 36Cl and 81Kr dating will be applied to verify these ages. Except for a typical small excess air component, the heavier noble gases appear undisturbed, allowing reliable NGRT calculation. The sample with the lowest D 4He (~ 9000%) has a NGRT of (27±0.8)°C, whereas the samples with larger D 4He (up to 1.7.105 %) infiltrated at (22±0.8)°C.
Although caution has to be exercised in drawing
final conclusions, we offer the following hypothesis
The 'young' sample infiltrated in a warm climate similar to present day conditions (possibly in the Holocene), whereas the other samples recharged several 104 to some 105 yr ago during a colder climate state. NGRT indicate a cooling of at least a 5°C in Australia and show that the NGRT-method may be applied to reconstruct palaeoclimate conditions on much larger time scales than previously thought.