A new record of the methane inter-polar difference across the Last Deglaciation

Abstract:

Ice core records of the difference in methane concentration between the Northern and Southern Hemispheres reflect the latitudinal distribution of atmospheric methane sources. The inter-polar difference (IPD), measured using discrete ice core samples from Greenland and Antarctica, is of particular interest during the abrupt, millennial-scale variability that is characteristic of the Last Glacial period and its termination. The recent discovery of non-atmospheric methane released during the discrete sample analysis of meteoric Greenland ice brings existing records of the IPD into question. Over 400 measurements of excess methane made on several Greenland ice cores of varying ice age reveal a robust positive correlation (R² = 0.78) between the quantity of excess methane and the concentration of Ca²⁺, a common proxy for dust content. This relationship was used to correct an existing discrete NEEM (Greenland) methane record (Rosen, J., 2014, PhD thesis, Oregon State University) that is highly synchronized to discrete WAIS Divide (Antarctic) samples covering the Last Glacial Termination (10 – 20 kya).

Here, we present a record of the methane IPD that uses the NEEM data corrected for excess methane contamination, which was found to approach 40ppb in the dustiest samples. Because dust content and, consequently, excess methane are both considerably enhanced in stadial Greenland ice, we find significant reductions of the IPD primarily during the Last Glacial Maximum (LGM) and Younger Dryas that are not captured in existing records. In other words, the larger change in our corrected IPD implies a larger change in Northern Hemisphere methane sources between stadial and interstadial periods, relative to uncorrected records. This suggests that the reduced methane concentrations observed during the LGM and Younger Dryas are primarily driven by reductions in Northern Hemisphere sources, such as emissions from boreal wetlands, rather than changes in tropical emissions.