The most probable gas source is biogenic methane from the microbial decomposition of wetland plant remains often found in the post-glacial river confluence of ancient rivers Eider and Elbe (Fig. 4d). Comparatively high bottom water temperatures in 2014 and 2015 (Fig. 3a) may have facilitated its ascent toward a shallow depth beneath the seafloor where it remained in an unstable state until it was released by a final trigger. The region is not affected by earthquakes, but man-made tremors were generated during the pile-driving works for the construction foundations of three offshore wind farms at the eastern end of the pockmark field between 2012 and 2014 (Fig. 1a). There are no records of magnitude of the vibrations on the seafloor but the energy of the blows is considered too low and presumably dampened too fast i.e., exponentially with increasing distance from the source33 to cause an ascent of gas as far as 30 km away from the wind farms.

Figure 3 Emergence of the Helgoland Reef pockmarks. (a) Time series of bottom water temperature records from three stations in the German Bight. (b) Time series of significant wave height at Helgoland Reef from model hindcast. The data were provided by the COSYNA system34 operated by Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung GmbH. (c,d) Zero-median bathymetries showing the emergence of the pockmarks. For the complete record of MBES bathymetries see Supplementary Fig. S3. The maps in this figure were generated using QGIS Version 2.14.1143. Full size image

The proposed trigger for the final outbreak of the gas from the shallow subsurface is a series of storms in November 2015 (Fig. 3b). Wave model hindcasts indicate significant wave heights exceeding 7 m in the pockmarked field34. Typical wave peak periods measured during the winter storms in this area are in the order of 8 s to 12 s. Following linear wave theory, this results in a wave length L of 96 m to 177 m for an average water depth d of 30 m and transitional conditions (0.05 < d/L < 0.5). The effect of wave orbital motion and pressure oscillations reaches depths of 48 m to 89 m. The surface pressure oscillations and the horizontal component of the orbital velocity are reduced to around 48% by wave attenuation at a depth of 30 m. Assuming a Raleigh distribution for the wave height spectrum, the highest 1% of the waves reach 11.7 m and a maximum wave height of 14 m is possible. This allows a penetration depth of the wave-induced effective stress of up to 3.5 m for the significant and 7 m for the maximum wave21.

From the evidence described above, the following formation mechanism for the characteristic pockmark craters and mounds can be deduced. Triggered by a relief of pressure under a passing wave trough, the stored gas erupts and ejects sediment into the water column. The suspended material then settles in the lee of wave or current direction (whichever is dominant at that time) in a distance from the eruption point as a function of grain size. While the coarser shell debris settles back into the eruption crater, the sandy fraction is settled in a well-defined mound and the fine fraction is transported further away. An alternative mechanism for the generation of the characteristic trough-mound structures is the generation of subsidence depressions after the gas is released and a generation of the mounds as secondary sorted bedforms from the initial defects35. Measured near-bottom current velocities measured reach 0.3 ms−1 during ebb and 0.4 ms−1 during flood. The resulting shear stresses are capable of moving the seafloor sediment and of generating small scale bedforms with centimeter height and decimeter length scales. The dimensions and morphology of the pockmarks however are different from the typical triangular bedform cross-sections. Furthermore, a number of scour holes of around 0.5 m depth found in the area throughout all MBES surveys do not indicate any lateral mounds (Fig. 3c,d). Therefore, this formation mechanism is unlikely.

The storm events in fall 2015 that must have triggered the eruption of the pockmark field were not exceptionally extreme events. Wave heights of equal amplitude also occurred in the winters of 2013–2016 (Fig. 3b), but no pockmarks were found in the respective subsequent MBES surveys (see Supplementary Fig. S3). Although pockmarks have been observed for the first time in November 2015, it can be assumed that their emergence is a reappearing phenomenon. Following the release of the potential energy stored by the gas beneath the seafloor due to a storm event, a certain recovery period may be required to accumulate enough gas to create a new instable state. In the meantime, the shallow pockmarks as morphological symptoms of the gas release are leveled by wave and current action on the mobile sands.

As the exact date of the eruption cannot be determined, the recent morphology of the pockmarks depicts the combined effect of gas expulsion and successive scouring of the initial defects in the seafloor. While individual features can be traced throughout the calm period between February and August 2016, there is no overlap of pockmark morphologies from surveys at the beginning (HE455, Nov. 2015) and at the end (HE456, Feb. 2016) of the stormy season. The latest observed extent and the distribution of pockmarks within the field may be controlled by (a) the extent and local source strength of the presumed methane reservoir, (b) the thickness of the overlying Holocene layer and porosity and permeability of the sediment, (c) the absolute water depth as lower limit for wave impact and (d) the local variation of water depth and slope of the bathymetry providing exposure toward or shelter from wave and current action.

As this is the first description of pockmark emergence in the Helgoland Reef area, future surveys will have to shed light on the fate of the pockmarks after seepage has ceased and possible recurrence cycles of this phenomenon.