Noble Gases Dissolved In Groundwater

In recent years, a new approach that provides an independent paleotemperature proxy has evolved, i.e., the measurement of atmospheric noble gases dissolved in radiocarbon-dated groundwater. The principle of the "noble gas thermometer" is based on the temperature dependency of the solubility of noble gases (Ne, Ar, Kr, and Xe, and in certain cases also N2) in water (Mazor, 1972; Andrews and Lee, 1979; Stute and Schlosser, 1993). Groundwater percolating through the unsatu-rated zone continually equilibrates with soil air until it reaches the water table. While moving deeper into the subsurface, the groundwater becomes isolated from the atmosphere and carries the imprinted climate signal, recorded by its noble gas composition, along. Fluctuations of the water table typically result in the partial or complete dissolution of trapped air bubbles. In most cases, the individual processes can be separated by an iterative procedure (Stute and Schlosser, 1993; Stute et al., 1995). In suitable groundwater flow systems, the resulting noble gas temperature closely reflects the mean annual ground (soil) temperature at the water table in the recharge area. This climate signal is best preserved in confined aquifers. However, a groundwater flow system acts as a low-pass filter (Stute and Schlosser, 1993). At 18,000 14C B.P., for example, this smoothing effect is equivalent to a moving average of several thousand years. The weaknesses of this technique are the uncertainty in the excess air correction, the radiocarbon dating, and the smoothing of climate information; the advantage is that it is based on a simple physical principle and it is not sensitive to local or short-term climate fluctuations.

Two records have been obtained so far for the low-latitude Americas, i.e., for southern Texas (Stute et al., 1992) and northeastern Brazil (Stute et al., 1995). Both records indicate a 5°C cooler climate during the last glacial maximum.

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