Rationale: Oxygen isotope exchange between O 2 and CO 2 in the presence of heated platinum (Pt) is an established technique for determining the δ 17 O value of CO 2 . However, there is not yet a consensus on the associated fractionation factors at the steady state. Methods:We determined experimentally the steady-state α 17 and α 18 fractionation factors for Pt-catalyzed CO 2 -O 2 oxygen isotope exchange at temperatures ranging from 500 to 1200 C. For comparison, the theoretical α 18 equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO 2 and the corresponding α 17 values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO 2 and O 2 differing in δ 18 O value from À66 ‰ to +4 ‰. Results:The experimentally determined steady-state fractionation factors α 17 and α 18 are lower than those obtained from the updated theoretical calculations (of CO 2 -O 2 isotope exchange under equilibrium conditions) by 0.0024 ± 0.0001 and 0.0048 ± 0.0002, respectively. The offset is not due to scale incompatibilities between isotope measurements of O 2 and CO 2 nor to the neglect of non-Born-Oppenheimer effects in the calculations. There is a crossover temperature at which enrichment in the minor isotopes switches from CO 2 to O 2 . The direct sum evaluation yields a θ value of $0.54, i.e. higher than the canonical range maximum for a mass-dependent fractionation process.Conclusions: Updated theoretical values of α 18 for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO 2 yields θ values higher than the canonical range maximum for mass-dependent fractionation processes. This demonstrates the need to include anharmonic effects in the calculation and definition of mass-dependent fractionation processes for poly-atomic molecules. The discrepancy between the theory and the experimental α 17 and α 18 values may be due to thermal diffusion associated with the temperature gradient in the reactor.

<p>Atmospheric methane (CH<sub>4</sub>) is a potent greenhouse gas with natural and anthropogenic sources. Concentrations have been significantly increasing over the past few decades, which poses a problem for future climate change goals. The contribution of oceans to the global atmospheric CH<sub>4</sub> cycle is largely uncertain. It is accepted that oceans act as a small net source of atmospheric CH<sub>4</sub>. As the polar regions are warming faster than the global average, it is important that we can better quantify CH<sub>4</sub> emissions from the polar oceans. </p> <p><br />In this study, we combine various forms of shipborne data (ambient atmospheric methane concentrations, sea-air CH<sub>4</sub> fluxes and isotopic composition of atmospheric CH<sub>4</sub>) taken during cruises in the Arctic and Southern Oceans to present a more complete picture of atmospheric CH<sub>4</sub> above polar oceans, including addressing the question of how much the oceanic component is contributing towards the atmospheric budget in these regions. Measurements  were made around the Barents Sea and Greenland Sea in the Arctic, and in the Atlantic sector of the Southern Ocean, including the Scotia Sea.</p> <p><br />Sea-air CH<sub>4</sub> fluxes are measured using the eddy covariance method; indeed, this the first study to use this technique to directly measure how much CH<sub>4</sub> is released from the ocean into the atmosphere in both the Southern and Arctic Oceans. Atmospheric CH<sub>4</sub> measurements are then investigated in order to understand the impact that CH<sub>4</sub> released from the ocean has on the atmospheric burden. We also measure the isotopic composition of CH<sub>4</sub> (δ<sup>2</sup>H and δ<sup>13</sup>C) in air samples taken onboard polar cruises, to understand the sources of atmospheric CH<sub>4</sub> above these oceans. The isotope measurements can indicate if the CH<sub>4</sub> comes from a biogenic or thermogenic source, which can help determine if anthropogenic or natural processes are behind the production.</p> <p><br />We investigate the potential sources of CH<sub>4</sub> released by the polar ocean by looking at areas of known seabed CH<sub>4</sub> seepages, investigating phytoplankton abundance, and investigating the isotopic composition of atmospheric CH<sub>4</sub> in areas of elevated CH<sub>4</sub>. </p> <p><br />We find that the region of the Arctic Ocean investigated in this study is a slight atmospheric CH<sub>4</sub> source in boreal summer, while the region of the Southern Ocean investigated is a CH<sub>4</sub> source in areas of shallower water/continental shelves and a CH<sub>4</sub> sink in region of open ocean, in austral summer. This finding is consistent with previous studies that have detected seabed CH<sub>4</sub> emission. Seabed CH<sub>4</sub> seepage at shallower depths is more likely to penetrate the sea-air interface, while CH<sub>4</sub> produced at the seabed at deeper depths gets oxidised as it travels through the water column, making it less likely to reach the surface . We also find evidence of localised “hot spots” of methane emission which will be described. </p>

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