Response of noble gas partial pressures in soil air to oxygen depletion

Soil air composition is known to fluctuate both temporally and spatially. These fluctuations are mainly caused by microbiological activity, leading to oxygen depletion. The exact composition of soil air is of importance to the reconstruction of paleotemperatures using dissolved noble gases in ground water, yet few data exist on the impact of soil air composition changes on the noble gas concentrations. This study documents the evolution of soil air composition, including noble gases, over a period of 17 months at a test site with clay dominated soil in Germany. It confirms that O2 and CO2 concentrations within the soil air vary strongly between a minimum of the sum of O2 and CO2 of 16.5% and a maximum of 24.5%. Soil air noble gas composition deviated from atmospheric composition as expected, i.e. noble gas concentrations increased when O2 + CO2 decreased, and vice versa. The highest observed increase in soil air argon mixing ratio was to 106.4% of the atmospheric air mixing ratio. This maximum increase of noble gas mixing ratios would cause an underestimation of paleotemperatures by 1.5 °C when employing the current models neglecting the response of noble gases to oxygen depletion. Due to the observed annual variation, however, the mean effect is much smaller.

► We present a 17-month time series of soil gas composition at a German test site. ► O2 + CO2 varied with temperature and water availability between 16.5% and 24.5%. ► Noble gas mixing ratios varied inversely, with a maximum of 106% of the air value. ► The maximum potential cold bias of noble gas temperatures (NGTs) is 1.5 °C. ► The annual mean enrichment of noble gases and bias of NGT is much smaller.