Microbial respiration and diffusive transport of O{sub}2, 16{sup left}O{sub}2, and 18{sup left}O{sup}16O in unsaturated soils and geologic sediments

摘要

Molecular oxygen (O{sub}2) in unsaturated geologic sediments plays an important role in soil respiration, biodegradation of organic contaminants, metal oxidation, and global oxygen and carbon cycling, yet little is known about oxygen isotope fractionation during the consumption and transport of O{sub}2 in unsaturated zones. We used a laboratory kinetic cell technique to quantify isotope fractionation due to respiration and a numerical model to quantify both consumptive and diffusive fractionation of O{sub}2 isotopes at a field site comprised of unsaturated lacustrine sandy materials. The combined use of laboratory-based kinetic cell experiments and field-based isotope transport modeling provided an effective tool to characterize microbial respiration in unsaturated media. Based on results from the closed-system kinetic cells, O{sub}2 consumption and isotope fractionation were attributed to the alternative cyanide-resistant respiration pathway. At the field site, the modeled depth profiles for O{sub}2 and δ{sup}(18)O matched the measured in situ data and confirmed that the consumption of O{sub}2 was via the alternative respiration pathway. If the cyanide-resistant respiration pathway is indeed widespread in soils, its high oxygen isotope enrichment factor could help to explain the discrepancy high between the predicted present-day Dole effect (+20.8 per thousand) and the observed Dole effect (+23.5 per thousand). Thus, further soil O{sub}2 isotope studies are needed to better characterize and model the fractionation of oxygen isotopes during subsurface respiration and the potential impact on the isotopic content of atmospheric O{sub}2.