Abrupt emergence of a large pockmark field in the German Bight, southeastern North Sea

Abstract

A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. Covering an area of around 915 km2, up to 1,200 pockmarks per square kilometer have been identified. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. The gas source and the trigger for the simultaneous outbreak remain speculative. Subseafloor structures and high methane concentrations of up to 30 μmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of postglacial deposits in a paleo river valley. Storm waves are suggested as the final trigger for the eruption of the gas. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping.

Pockmarks in the Witch Ground Basin, Central North Sea

“…The density of pockmarks per tier is decreasing with increasing burial depth (Figures a and a, Long, ), which suggests that the late stages of pockmark formation had greater source strength and overpressure than the earlier ones. This is similar to what was proposed for the southern North Sea (Krämer et al, ).…”

“…In the Witch Ground Basin, temperature conditions may have changed due to warm water inflow from the North Atlantic (e.g., Becker et al, 2018). Pressure conditions may have changed due to rapid sedimentation (Hustoft et al, 2009;Reinardy et al, 2017), disequilibrium compaction (Flemings et al, 2008;Talukder, 2012), tidal currents (Chen & Slater, 2016), large storms (Krämer et al, 2017), or earthquakes (Field & Jennings, 1987;Hasiotis et al, 1996), which may have ultimately led to episodic fluid escape from the shallow marine sediments in the Witch Ground Basin.…”

“…Methane from both onshore and offshore microseep and macroseep contributes to the atmospheric methane budget and can be supplied by geothermal, volcanic, or sedimentary sources (Dean et al, 2018;Etiope et al, 2008;Saunois et al, 2016). In the marine environment, methane may accumulate in the subsurface, when gas pressure exceeds the ambient hydrostatic pressure and the methane forms gas bubbles, which may be released by diffusion or episodic ebullition (Boudreau et al, 2005;Krämer et al, 2017;Maeck et al, 2013). However, the global significance of marine methane sources and their impact on the global methane budget remains poorly constrained (Ciais et al, 2013;Etiope et al, 2008;Petrenko et al, 2017).…”

Distribution and classification of pockmarks on the seabed around western Scotland

“…It is widely held that the presence of pockmarks at the seabed reflects the presence or former presence of gas-rich sub-surface sediments (King & Maclean, 1970). Determining the age and activity status of pockmarks can be challenging, however there is a growing agreement that their distribution, density and morphology could be useful indicators of the gas-storage potential of the sediments beneath (Hovland, Heggland, De Vries, & Tjelta, 2010;Krämer et al, 2017). Furthermore the presence of gas release features are a significant geo-hazard to offshore developments (Best et al, 2006).…”

“…Pockmarks have been discovered in many locations on ocean floors worldwide and at a range of depths from >1000 m in the abyssal ocean (Panieri et al, 2017;Pilcher & Argent, 2007) to much shallower settings on the continental shelf (<100 m), providing evidence of their wide bathymetric range. In European waters, gas/fluidescape related pockmarks have been identified from high-resolution bathymetry and geophysical data in all the shelf seas: in the Mediterranean (Marinaro et al, 2006), Black Sea (Çifi¸, Dondurur, & Ergün, 2003;Papatheodorou, Hasiotis, & Ferentinos, 1993), Baltic (Whiticar & Werner, 1981), Barents Sea (Hovland & Judd, 1988;Solheim & Elverhøi, 1993) and North Sea Basins (Gafeira & Long, 2015a;Judd & Hovland, 2007;Krämer et al, 2017). So far, no inventory or detailed studies have been conducted of pockmarks in Scottish fjords or the adjacent shelf seas west of the UK.…”