Aeschbach-Hertig W, Peeters F, Beyerle U, Kipfer R
Department of Water Resources and Drinking Water, Swiss Federal Institute of Environmental Science and Technology (EAWAG), Dubendorf, Switzerland.
Nature. 2000 Jun 29;405(6790):1040-4. doi: 10.1038/35016542.
Noble-gas concentrations in ground water have been used as a proxy for past air temperatures, but the accuracy of this approach has been limited by the existence of a temperature-independent component of the noble gases in ground water, termed 'excess air' whose origin and composition is poorly understood. In particular, the evidence from noble gases in a Brazilian aquifer for a cooling of more than 5 C in tropical America during the Last Glacial Maximum has been called into question. Here we propose a model for dissolved gases in ground water, which describes the formation of excess air by equilibration of ground water with entrapped air in quasi-saturated soils. Our model predicts previously unexplained noble-gas data sets, including the concentration of atmospheric helium, and yields consistent results for the non-atmospheric helium isotopes that are used for dating ground water. Using this model of excess air, we re-evaluate the use of noble gases from ground water for reconstructing past temperatures. Our results corroborate the inferred cooling in Brazil during the Last Glacial Maximum, and indicate that even larger cooling took place at mid-latitudes.
地下水中的稀有气体浓度已被用作过去气温的替代指标，但这种方法的准确性受到地下水中存在与温度无关的稀有气体成分（称为“过剩空气”）的限制，其来源和组成尚不清楚。特别是，来自巴西含水层的稀有气体证据表明，末次盛冰期热带美洲的气温下降超过5摄氏度，这一点受到了质疑。在这里，我们提出了一个地下水中溶解气体的模型，该模型描述了地下水与准饱和土壤中截留的空气平衡形成过剩空气的过程。我们的模型预测了以前无法解释的稀有气体数据集，包括大气氦的浓度，并对用于地下水测年的非大气氦同位素产生了一致的结果。利用这个过剩空气模型，我们重新评估了利用地下水中的稀有气体重建过去温度的方法。我们的结果证实了末次盛冰期巴西推断的降温情况，并表明中纬度地区的降温幅度更大。
