Atmospheric impacts and ice core imprints of a methane pulse from clathrates

In relation to Arctic warming, the possible occurrence of methane hydrate degassing events has attracted an increasing interest in recent years. We evaluate the atmospheric impact of rapid and massive emissions of methane and how they are imprinted in ice core records, by combining for the first time models of atmospheric chemistry and trace gas transport in firn. Different emission characteristics as well as climatic conditions (present, pre-industrial, glacial) are considered. The δδ isotopic signatures characterizing stable isotopologues of methane DCH3 and 13CH4 are also analysed.Our results suggest little effect of clathrate degassing on the main methane oxidant: OH radicals. Due to the relatively short atmospheric lifetime of methane, the simulated clathrate-induced perturbations last for less than a century. This time scale is comparable to or shorter than the duration of air bubble closure in polar ice sheets. As a consequence, rapid methane perturbations in the atmosphere are strongly smoothed in ice core records. This smoothing mostly depends on the snow accumulation rate at the site of ice core drilling. We propose a methodology to identify a potential clathrate degassing event in ice core records. Continuous CH4 records from high accumulation rate sites could allow to decipher short time scale events. δDδD of CH4 should reveal a typical “lying S” shape at high accumulation rate sites, reflecting the combined effects of the clathrate source signature (negative excursion) and subsequent OH fractionation in the atmosphere (positive excursion). The amplitude ratio of the negative and positive δDδD swings recorded in Greenland and Antarctica under similar accumulation rate conditions could also indicate the latitude of a clathrate degassing event.

► Impact of massive methane emissions from hydrates and their imprints in ice cores.
► Combined use of atmospheric chemistry and trace gas transport in firn models.
► A small decrease in the main methane sink (atmospheric OH radicals) is obtained.
► Snow accumulation rate largely determines how atmospheric signals are recorded in ice.
► Detailed temporal evolution of deuterium/hydrogen in CH4 constitutes the best tool to identify clathrate degassing events in ice cores.