Kinetic and equilibrium fractionation of O2 isotopologues during air-water gas transfer and implications for tracing oxygen cycling in the ocean

What Fractionates Oxygen Isotopes During Respiration? Insights from Multiple Isotopologues and Theory

“…For diffusive fractionation, we use 18 ε diffusion = -3.3‰, θ 17/18, diffusion = 0.5146, θ 35/34, diffusion = 1.462, and θ 36/34, diffusion = 1.900. The 18 ε diffusion value is derived by raising the measured 18 α value for kinetic fractionation of O 2 during gas exchange to 3/2 power; the mass dependence is for interdiffusion through 2 water molecules, which is consistent with experimental results in both 18 ε diffusion and θ values 29,118 . For enzymatic fractionation, we use θ 17/18, enzyme = 0.5233, θ 35/34, enzyme = 1.555, and θ 36/34, enzyme = 2.061, the simplified bond-breaking mass dependence, which has θ 35/34 and θ 36/34 values between that of heme-O 2 binding and one-electron reduction of O 2while keeping a similar θ 17/18 value.…”

“…28,63,67 In addition, bulk isotope compositions are corrected for pressure-baseline effects that yield a triple-oxygen difference between atmospheric O 2 and San Carlos Olivine that has been reproduced across three independent labs within 10 ppm. 27,29,30,36,68 This result implies that our measured λ 17/18 values are accurate to within 0.0006. Samples of atmospheric O 2 measured concurrently with respiration experiments (from June 2016-April 2017) had average values of δ 18 O air = 0.001 ± 0.032‰, Δ´1 7 O air, 0.518 = 0 ± 4 ppm, Δ 36 = 1.992 ± 0.042‰ and Δ 35 = 1.06 ± 0.08‰ (1σ, n = 67).…”

“…12,44 Diffusion in water has 18 ε ≈ -3.3‰ and θ 17/18 = 0.517 ± 0.002 (1σ), so diffusion control is unlikely in our experiments and cannot have driven θ 17/18,resp to higher values. 29,109 Light-dependent O 2 consumption mechanisms may have higher θ 17/18 values, but the absence of light in these experiments inhibited their expression. 3,4 In addition, O 2 isotope-exchange reactions on metal oxide surfaces are likely too slow at 25°C for This is a preprint submitted to Earth ArXiv.…”

“…19, [50][51][52] Yet as the triple-isotope field has matured, it has become clear that many factors can exert a significant influence on D ¢17 O-based estimates of productivity, e.g., uncertainties in gas exchange rates and isotopic perturbations associated with non-steady-state conditions; variable expression of respiration mechanisms and their fractionation factors; the source-water isotopic composition; and mathematical approximations used in calculations. 3,12,22,26,29,[53][54][55][56][57][58][59][60] To the best of our knowledge, however, the origin of the λ 17/18 value of 0.518 has not been studied. It is consistent with Rayleigh fractionation calculated using the accepted range of θ 17/18 This is a preprint submitted to Earth ArXiv.…”

“…107 Nevertheless, the resulting fractionation is small, contributing errors of <1‰ in 18 ε and negligible errors in θ values. 29,99 The best point of comparison for 16 O- 16 O stretching frequencies in the oxyhemes (given a lack of experimental data) is between Heme(+His)-O 2 and oxyhemoglobin, which have vibrational frequencies of ~1200 cm -1 and 1155 cm -1 respectively. 104 We consider these to be in good agreement considering that the observed vibrational frequency includes anharmonic effects.…”