Detailed acoustic imaging above the Troll gas field in the northern North Sea off western Norway has revealed tens of thousands of elliptical pits on the seabed. At around 10 to 20 per square kilometre over an area of about 15,000 km2 there are probably between 150 to 300 thousand of them. They range between 10 to 100 m across and are about 6 m deep on average, although some are as deep as 20 m. They are pretty much randomly distributed but show alignment roughly parallel to regional N-S sea-floor currents. Many of the world’s continental shelves display such pockmark fields, but the Troll example is among the most extensive. Almost certainly the pockmarks formed by seepage of gas or water to the surface. However, detailed observations suggest they are inactive structures with no sign of bubbles or fluid seepage. Yet the pits cut though glacial diamictites deposited by the most recent Norwegian Channel Ice Stream through which icebergs once ploughed and which is overlain by thin Holocene marine sediments. One possibility is that they record gas loss from the Troll field, another being destabilisation of shallow gas hydrate deposits.
Norwegian geoscientists have studied part of the field in considerable detail, analysing carbonate-rich blocks and foraminifera in the pits (Mazzini, A. and 8 others 2017. A climatic trigger for the giant Troll pockmark field in the northern North Sea. Earth and Planetary Science Letters, v. 464, p. 24-34; http://dx.doi.org/10.1016/j.epsl.2017.02.014). The carbonates show very negative δ13C values that suggest the carbon in them came from methane, which could indicate either of the two possible means of formation. However, U-Th dating of the carbonates and radiocarbon ages of forams in the marine sediment infill suggest that they formed at around 10 ka ago; 1500 years after the end of the Younger Dryas cold episode and the beginning of the Holocene interglacial. Most likely they represent destabilisation of a once-extensive, shallow layer of methane hydrates in the underlying sediments, conditions during the Younger Dryas having been well within the stability field of gas hydrates. Sporadic leaks from the deeper Troll gas field hosted by Jurassic sandstones is unlikely to have created such a uniform distribution of gas-release pockmarks. Adriano Mazzini and colleagues conclude that rapid early Holocene warming led to sea-floor temperatures and pressures outside the stability field of gas hydrates. There are few signs that hydrates linger in the area, explaining the present inactivity of the pockmarks – all the methane and CO2 escaped before 10 ka.
Gas hydrates are thought to be present beneath shallow seas today, wherever sea-floor sediments have a significant organic carbon content and within the pressure-temperature window of stability of these strange ice-like materials. Mazzini et al.’s analysis of the Troll pockmark field clearly has profound implications for the possible behaviour of gas hydrates at a time of global climatic warming. As well as their destabilisation adding to methane release from onshore peat deposits currently locked by permafrost and a surge in global warming, there is an even more catastrophic possibility. The whole of the seaboard of the southern North Sea was swept by a huge tsunami about 8000 years ago, which possibly wiped out Mesolithic human occupancy of lowland Britain, the former land mass of Doggerland, and the ‘Low Countries’ of western Europe. This was created by a massive submarine landslide – the Storegga Slide just to the north of the Troll field – which may have been triggered by destabilisation of submarine gas hydrates during early Holocene warming of the oceans.