ATMOSPHERIC O2 CONCENTRATIONS FROM SOIL CARBONATE-HOSTED FLUID INCLUSIONS: A POTENTIAL NEW PROXY FOR PALEO-ATMOSPHERIC PO2

Proxies for the concentration of O₂ in the ancient atmosphere are scarce. We have developed a potential new proxy for ancient atmospheric O₂ composition based on soil carbonate-hosted fluid inclusions. Soils are in continuous atmospheric communication, and relatively static equilibration between soil gas and atmospheric gas during formation, such that a predictable amount of atmosphere infiltrates a soil. This atmosphere is trapped by inclusions during carbonate precipitation. Here we show that carbonate hosted fluid inclusions are faithful recorders of soil gas concentrations and isotope ratios, and specifically that soil O₂ partial pressures can be derived from the total gas contents of these inclusions. Using carbonate nodules from a span of depths in a modern vertisol near Dallas, TX, as a test case, we employ an online crushing technique to liberate gases from soil carbonates into a small custom-built quadrupole mass spectrometer where all gases are measured in real time. We quantify the total oxygen content of the gas using a matrix-matched calibration, and define each species as a partial pressure of the total gas released from the nodule. Atmospheric pO₂ is very simply derived from the soil-nodule partial pressures by accounting for the static productivity of the soil (using a small correction based on the CO₂ concentration). When corrected for aqueous solubility using Henry’s Law, these soil-carbonate hosted gas results reveal soil O₂ concentrations that are comparable to modern-day dry atmosphere. Armed with this achievement in modern soils, and as a test on the applicability of the approach to ancient samples, we successfully apply the new proxy to nodules from the Late Triassic. Analysis of soil O₂ from soil gas monitoring wells paired with measurements from contemporaneous soil carbonate nodules is needed to precisely calibrate the new proxy.