We conducted airborne eddy-covariance (EC) measurements (cf. ref. 23) covering large areas of the Mackenzie Delta region during two extensive campaigns in July 2012 and 2013 (Fig. 1b).

The CH 4 flux map (cf. ref. 24) resulting from our measurements covers 10,000 km² at a spatial resolution of 100 m × 100 m (Fig. 2a). This CH 4 flux map enables to detect spatial patterns in CH 4 emission and to identify and localize CH 4 emission hotspots in our entire study area. The median of all 2012 and 2013 CH 4 flux data was 1.1 mg m−2 h−1, which corresponds to fluxes measured by the EC technique in similar ecosystems25,26,27,28,29. Most importantly, we found the largest emissions to be spatially stationary and temporally constant to <30% standard error, both within and between the two campaigns. A majority of high fluxes up to 14.7 mg CH 4 m−2 h−1 (with a standard error of <30%) occurred as clustered peaks in the northern part of the study area and only inside the delta (Fig. 2) where the permafrost is thin and discontinuous. Biogenic CH 4 emissions from arctic wetlands25,26,27,28,29 and lakes30 are typically lower and driven by changing meteorological and surface properties, making stationary, repeatedly observable emission peaks at this strength and spatial extent unlikely.

Figure 2 CH 4 flux map with location of wells. (a) CH 4 flux topography for both years containing data with a standard error <30%. Fluxes >5.0 mg m−2 h−1 are considered to be of geologic origin. Blue asterisks show the location of wells (oil or gas wells) derived from literature data. The locations of the towns of Inuvik and Aklavik are shown for orientation. (b) Magnification of the area with the highest CH 4 fluxes marked with the black square in Fig. 2a. Legend as in Fig. 2a. West of the black line the permafrost is discontinuous and thin, east of it continuous and thick. Data for background map from ref. 46. The map in Fig. 2 was created using ArcGIS software ArcMap 10.1 by Esri. Methods for deriving the CH 4 flux map are explained in chapters 4.2–4.5. Full size image

For lack of isotopic data, we used published biogenic CH 4 flux data from EC flux tower measurements in the Arctic north of 61 °N for a conservative, approximate separation between biogenic emissions and strong geologic CH 4 hotspots. The area of influence of tower based EC measurements, the so-called footprint, is of a comparable spatial scale as those of our airborne EC measurements, integrating over several hectares. Measurements with chambers and bubble traps, on the other hand, cover much smaller scales and do not readily compare with spatially integrated EC measurements. They were therefore not considered for the following threshold definition. The maximum daily biogenic CH 4 flux from permafrost landscapes found in the literature is roughly 5.0 mg m−2 h−1 (e.g. refs 25,26,27,28,29; max. 4.58 mg m−2 h−1, ref. 28). At thermokarst margins of lakes with strong biogenic point sources, CH 4 emission can reach 5.0 mg m−2 h−1 as well13. Individual hotspots of biogenic CH 4 emission in lakes in carbon rich Yedoma type permafrost areas, can exceed that threshold31, but have an area of a few square metres, and thus are much smaller than the areas with CH 4 emission peaks that we observed. These high emissions from lakes do occur on a seep scale and their signal would disappear among neighbouring areas with less CH 4 emission, when we consider spatially integrated fluxes resulting from the EC method.

Therefore, we defined a flux of 5.0 mg m−2 h−1 as upper threshold for biogenic CH 4 fluxes from arctic permafrost landscapes. We thus assumed that reoccurring emissions exceeding 5 mg m−2 h−1 independently of atmospheric or surface conditions, day, time or year of flight, as found in the northern Mackenzie Delta, were not of recent biogenic origin. Instead, we attributed them to deeper geologic sources that release CH 4 through seeps, which can be related to taliks, faults or artificial pathways such as oil and gas exploration wells (Fig. 2).

Combined, the footprints of our 2012 and 2013 measurements comprise 9,754 km² excluding areas with a standard error >30%. We find that about 1% of the mapped area (116 km2), releases CH 4 at rates exceeding 5 mg m−2 h−1. Single areas with these peak emissions are up to several square kilometres large. At this spatial extent, biogenic emissions from thermokarst lakes that exceeded this threshold have not been reported30. Single biogenic seeps in lakes would not result in an integrated signal that large.

We only found areas with high emissions within the northern part of the delta where the permafrost thickness is less than 100 m and the permafrost is discontinuous21 and therefore permeable for gas from the subsurface. In contrast, on the adjacent coastal plain and Richards Island, with their continuous and thick permafrost of up to 300 m and more than 500 m, respectively, such high emissions were not observed. We attribute that absence of emission peaks, despite established natural gas and oil deposits in the region (Fig. 2), to the thick impermeable permafrost in these areas. These CH 4 reserves might eventually be emitted into the atmosphere if the permafrost cap becomes permeable due to thawing.