Isotopic composition, atmospheric methane

The interstitial air trapped during this process preserves a largely unaltered record of the composition of past atmospheres on time scales as short as decades and as long as several hundred thousand years. Such records have provided critical information about past variations in carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), and the isotopic composition of some of these trace species. In addition, studies of the major elements of air nitrogen, oxygen, and argon, and their isotopic composition, have contributed... 

Atmospheric methane has a mean 5 C-value of around —47%c (Stevens 1988). Quay et al. (1999) presented global time series records between 1988 and 1995 on the carbon and hydrogen isotope composition of atmospheric methane. They measured spatial and temporal variation in and D with a slight emichment observed for the southern hemisphere (—47.2%c) relative to the northern hemisphere (—47.4%o). The mean 5D was —86 3%c with a 10%o depletion in the northern relative to the southern hemisphere. 

Quay P. D., Stutsman J., Wilbur D., Snover A., Dlugokencky E., and Brown T. (1999) The isotopic composition of atmospheric methane. Global Biogeochem. Cycles 13, 445-461. 

D. R., and Dlugokencky E. J. (1999) Carbon isotope composition of atmospheric methane a comparison of surface level and upper tropospheric air. J. Geophys. Res. 104, 13895-13910. 

The correlation between surface temperature and the isotopic composition of snow forms the basis of ice core paleothermometry, a spectacular example of which is the record of the Vostok ice core (Fig. 12). This record has now been extended out to approximately 450,000 years. It is a record of oxygen and deuterium isotope concentrations in ice originally precipitated as snow and a record of methane and carbon dioxide concentrations in trapped gases as well as other trace elements recorded by the contamination of ice with atmospheric dust particles. The oxygen and deuterium isotope results corroborate the variations observed in the oxygen isotope record of deep sea cores and... 

Most of the geological evidence for this is abstract isotope ratios and rock compositions. Closer to normal experience, you can see fossilized raindrops, in a paper titled Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints. The size and splatter of raindrops back then is the same as today, showing that the atmosphere they fell through was about as dense as what you re breathing now, but with more methane and carbon dioxide, and less oxygen. 

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