Stable carbon isotopic composition of methane from ancient ice samples.

摘要

This study developed a method to extract gas from ice samples and measure the concentration and stable carbon isotope ratio of evolved methane. Ice samples were analyzed from 3 sites: (i) Agassiz ice cap (Ellesmere Island, Canada), (ii) Greenland Ice Sheet Project 2 (GISP2), and extensively from (iii) the western margin of the Greenland ice shield in Pakitsoq. Agassiz and GISP2 provided accuracy and precision tests of the analytical method. Pakitsoq ice yielded a record to reconstruct the atmospheric delta 13CCH4 history of the methane cycle across the cold Younger Dryas - warm Pre-Boreal transition (YD-PB) with a temporal resolution of decades.; delta13CCH4 values measured for YD-PB are relatively uniform from -46.0 ‰ to -45.8 ‰ (+/-0.4 ‰), i.e. that tropospheric methane in YD-PB is more enriched in 13C than previously expected or measured today.; delta13CCH4 measurements represent true atmospheric signals and are not affected by post-occlusion oxidation or production of methane in ice. Atmospheric mixing and isotope fractionation during diffusion of air in the unconsolidated snow/firn layer shift delta13C CH4 preserved in ice, but models developed in this study compensate for these effects.; Model work shows that variations in anthropogenic, climatic, and C 3, C4 vegetation changes affect delta13C CH4 of emissions and sinks.; The delta13CCH4 shift between the YD-PB and modern times may be explained by including emissions of thermogenic natural gas in the atmospheric methane budget and by revising delta13C CH4 for specific source types, in particular that of tropical wetlands. The impact of rice farming, before the start of the industrial period, may also be detectable in the delta13CCH4 record.; This YD-PB delta13CCH4 record does not support a catastrophic burst of methane from marine gas hydrates. Furthermore, a gradual emission of marine hydrate methane would require the release of the entire gas hydrate reserves in a globally synchronized, 200-year event.; The rapid increase in atmospheric methane concentration at YD-PB is likely caused by additional emissions from C4-dominated wetlands, having an isotope signature consistent with the YD-PB delta13C CH4 record. During the PB, this new source configuration persists, supported by the uniform delta13CCH4 values, despite the higher methane concentrations.