18 O-O 2 isotopic signature over oxygen:argon ratio (O 2 /Ar). The composition of lab air-equilibrated water and lab air are represented by a gray and black circle, respectively. Error bars are 1 standard deviation (based on repeat analysis of lab air, n = 13) and are smaller than the symbols. The blue and black solid lines show different isotope effects that would be caused by fractionation (e) due to respiration (resp.). During consumption of O 2 by respiration (decreasing O 2 :Ar ratio), there is a preferential accumulation of 18 O in the remaining O 2 pool leading to higher 18 {{{mathrm{O}}}}_{{{{mathrm{O}}}}_{2}} values. As the degree of this isotope fractionation can vary the two solid lines represents two hypothetical but realistic scenarios for how the O 2 pool might change with microbial respiration. The black dashed line represents a mixing line between air-equilibrated water mixed with a hypothetical new source of O 2 that has a very low 18 {{{mathrm{O}}}}_{{{{mathrm{O}}}}_{2}} (-20 vs V-SMOW). Such isotopically light oxygen is consistent with the biological formation of O 2 . V-SMOW Vienna Standard Mean Ocean Water.
