Doggerland and the Storegga tsunami

Britain is only an island when sea level stands high; i.e. during interglacial conditions. Since the last ice age global sea level have risen by about 130 m as the great northern ice sheets slowly melted. That Britain could oscillate between being part of Europe and a large archipelago as a result of major climatic cycles dates back only to between 450 and 240 ka ago. Previously it was a permanent part of what is now Europe, as befits its geological identity, joined to it by a low ridge buttressed by Chalk across the Dover Strait/Pas de Calais. All that remains of that are the white cliffs on either side. The drainage of what became the Thames, Seine and Rhine passed to the Atlantic in a much larger rive system that flowed down the axis of the Channel. Each time an ice age ended the ridge acted as a dam for glacial meltwater to form a large lake in what is now the southern North Sea. While continuous glaciers across the northern North Sea persisted the lake remained, but erosion during interglacials steadily wore down the ridge. About 450 ka ago it was low enough for this pro-glacial lake to spill across it in a catastrophic flood that began the separation. Several repeats occurred until the ridge was finally breached (See: When Britain first left Europe; September 2007). Yet sufficient remained that the link reappeared when sea level fell. What remains at present is a system of shallows and sandbanks, the largest of which is the Dogger Bank roughly halfway between Newcastle and Denmark. Consequently the swamps and river systems that immediately followed the last ice age have become known collectively as Doggerland.

The shrinkage of Doggerland since 16,000 BCE (Credit: Europe’s Lost Frontiers Project, University of Bradford)

Dredging of the southern North Sea for sand and gravel frequently brings both the bones of land mammals and the tools of Stone Age hunters to light – one fossil was a skull fragment of a Neanderthal. At the end of the Younger Dryas (~11.7 ka) Doggerland was populated and became a route for Mesolithic hunter-gatherers to cross from Europe to Britain and become transient and then permanent inhabitants. Melting of the northern ice sheets was slow and so was the pace of sea-level rise. A continuous passage across Dogger Land  remained even as it shrank. Only when the sea surface reached about 20 m below its current level was the land corridor breached bay what is now the Dover Strait, although low islands, including the Dogger Bank, littered the growing seaway. A new study examines the fate of Doggerland and its people during its final stage (Walker, J. et al. 2020. A great wave: the Storegga tsunami and the end of Doggerland? Antiquity, v. 94, p. 1409-1425; DOI: 10.15184/aqy.2020.49).

James Walker and colleagues at the University of Bradford, UK, and co-workers from the universities of Tartu, Estonia, Wales Trinity Saint David and St Andrews, UK, focus on one devastating event during Doggerland’s slow shrinkage and inundation. This took place around 8.2 ka ago, during the collapse of a section of the Norwegian continental edge. Known as the Storegga Slides (storegga means great edge in Norse), three submarine debris flows shifted 3500 km3 of sediment to blanket 80 thousand km2 of the Norwegian Sea floor, reaching more than half way to Iceland.  Tsunami deposits related to these events occur along the coast western Norway, on the Shetlands and the shoreline of eastern Scotland. They lie between 3 and 20 m above modern sea level, but allowing for the lower sea level at the time the ‘run-up’ probably reached as high as 35 m: more than the maximum of both the 26 December 2004 Indian Ocean tsunami and that in NW Japan on 11 March 2011. Two Mesolithic archaeological sites definitely lie beneath the tsunami deposit, one close to the source of the slid, another near Inverness, Scotland. At the time part of the Dogger Bank still lay above the sea, as did a wide coastal plain and offshore islands along England’s east coast. This catastrophic event was a little later than a sudden cooling event in the Northern Hemisphere. Any Mesolithic people living on what was left of Doggerland would not have survived. But quite possibly they may already have left as the climate cooled substantially

A seabed drilling programme financed by the EU targeted what lies beneath more recent sediments on the Dogger Bank and off the embayment known as The Wash of Eastern England. Some of the cores contain tsunamis deposits, one having been analysed in detail in a separate paper (Gaffney, V. and 24 others 2020. Multi-Proxy Characterisation of the Storegga Tsunami and Its Impact on the Early Holocene Landscapes of the Southern North Sea. Geosciences, v. 10, online; DOI: 10.3390/geosciences10070270). The tsunami washed across an estuarine mudflat into an area of meadowland with oak and hazel woodland, which may have absorbed much of its energy. Environmental DNA analysis suggests that this relic of Doggerland was roamed by bear, wild boar and ruminants. The authors also found evidence that the tsunamis had been guided by pre-existing topography, such as the river channel of what is now the River Great Ouse. Yet they found no evidence of human occupation. Together with other researchers, the University of Bradford’s Lost Frontiers Project have produced sufficient detail about Doggerland to contemplate looking for Mesolithic sites in the excavations for offshore wind farms.

See also: Addley, E. 2020.  Study finds indications of life on Doggerland after devastating tsunamis. (The Guardian, 1 December 2020); Europe’s Lost Frontiers website

A tsunami and NW European Mesolithic settlements

About 8.2 ka ago sediments on the steep continental edge of the North and Norwegian Seas slid onto the abyssal plain of the North Atlantic. This huge mass displacement triggered a tsunami whose effects manifest themselves in sand inundations at the heads of inlets and fjords along the Norwegian and eastern Scottish coasts that reach up to 10 m above current sea level. At that time actual sea level was probably 10 m lower than at present as active melting of the last glacial ice sheets was still underway: the waves may have reached 20-30 m above the 8.2 ka sea level. So powerful were the tsunami waves in the constricted North Sea that they may have separated the British Isles from the European mainland by inundating Doggerland, the low-lying riverine plain that joined them before global sea level rose above their elevation at around the same time. Fishing vessels plying the sandbanks of the southern North Sea often trawl-up well preserved remains of land mammals and even human tools: almost certainly Doggerland was prime hunting territory during the Mesolithic, as well as an easily traversed link to the then British Peninsula. Mesolithic settlements close by tsunami deposits are known from Inverness in Scotland and Dysvikja north of Bergen in Norway and individual Mesolithic dwellings occur on the Northumberland coast. The tsunami must have had some effect on Mesolithic hunter gatherers who had migrated into a game-rich habitat. The question is: How devastating was it.

English: Maelmin - reconstruction of Mesolithi...
Reconstruction of Mesolithic hut based on evidence from two archaeological sites in Northumberland, UK. (credit: Lisa Jarvis; see )

Hunter gatherers move seasonally with favoured game species, often returning to semi-permanent settlements for the least fruitful late-autumn to early spring season. The dominant prey animals, red deer and reindeer also tend to migrate to the hills in summer, partly to escape blood-feeding insects, returning to warmer, lower elevations for the winter. If that movement pattern dominated Mesolithic populations then the effects of the tsunami would have been most destructive in late-autumn to early spring. During warmer seasons, people may not even have noticed its effects although coastal habitations and boats may have been destroyed.

Splendid Feather Moss, Step Moss, Stair Step Moss
Stair-step moss (credit: Wikipedia)

Norwegian scientists Knut Rydgren and Stein Bondevik from Sogn og Fjordane University College, Sognda devised a clever means of working out the tsunami’s timing from mosses preserved in the sand inundations that added to near-shore marine sediments. (Rydgren, K. & Bondevik, S. 2015. Most growth patterns and timing of human exposure to a Mesolithic tsunami in the North Atlantic. Geology, v. 43, p. 111-114). Well-preserved stems of stair-step moss Hylocomium splendens still containing green chlorophyll occur, along with ripped up fragments of peat and soil, near the top of the tsunami deposit which has been uplifted by post-glacial isostatic uplift to form a bog. This moss grows shoots annually, the main growth spurt being at the end of the summer-early autumn growing season. Nineteen preserved samples preserved such new shoots that were as long as or longer than the preceding year’s shoots. This suggests that they were torn up by the tsunami while still alive towards the end of the growing season, around late-October. All around the North Sea Mesolithic people could have been returning from warm season hunting trips to sea-shore winter camps, only to have their dwellings, boats and food stores devastated, if indeed they survived such a terrifying event.

Short fuse on clathrate bomb?

Structure of a gas hydrate (methane clathrate)...
Gas hydrate (methane clathrate) block embedded in seabed sediment (Photo credit: Wikipedia)

The biggest tsunami to affect inhabitants of Britain, mentioned in the earlier post Landslides and multiple dangers, emanated from the Storegga Slide in the northern North Sea west of Norway. That submarine debris flow was probably launched by gas hydrates beneath the sea bed breaking down to release methane thereby destabilising soft sediments on the continental slope. Similar slides were implicated in breaking Europe-America communications in the 20th century, such as the Grand Banks Slide of 1929 that severed submarine cables up to 600 km from the source of the slide. Even now, much Internet traffic is carried across oceans along optic-fibre cables, breakages disrupting and slowing services. A more mysterious facet of clathrate breakdown is its possible implication in unexplained and sudden losses of ships. When gas escapes to the surface, the net density of seawater decreases, the more so as the proportion of bubbles increases. Ship design and cargo loading rests on an assumed water density range from fresh to salt water and for different temperatures at high and low latitudes.

Gulf stream map
Gulf stream map (credit: Wikipedia)

The Atlantic seaboard of the USA hosts some of the best-studied accumulations of clathrates in the top 100-300 m of seabed sediments. Since their discovery these ‘cage complexes’ of mainly methane and carbon dioxide trapped within molecules of water ice have been studied in detail. Importantly, the temperatures at which they form and the range over which they remain stable depend on pressure and therefore depth below the sea surface. At atmospheric pressure solid methane hydrate is unstable at any likely temperature and requires -20°C to form at a pressure equivalent to 200 m water depth. Yet is stable at temperatures up to 10°C 500 m down and 20°C at a depth of 2 km. Modern sea water cools to around 0°C at depths greater than 1.5 km, so gas hydrates can form virtually anywhere that there is a source of methane or CO2 in seafloor sediment. In the sediments temperature increases sharply with depth beneath the seabed due to geothermal heat flow thereby limiting the clathrate stability zone to the top few hundred metres.

Two factors may lead to clathrate instability: falling sea level and sea-floor pressure or rising sea-floor temperature. Many gas-hydrate deposits, especially on the continental shelf and continental edge are likely to be close to their stability limits, hence the worries about destabilisation should global warming penetrate through the water column. The western North Atlantic is an area of especial concern because the Gulf Stream flows northward from the Caribbean to pass close to the US seaboard off the Carolinas: that massive flow of tropical warm water has been increasing during the last 5 thousand years so that its thermal effects are shifting westwards.

Geophysicists Benjamin Phrampus and Matthew Hornbach of the Southern Methodist University in Dallas, Texas have used thermal modelling to predict that gas-hydrate instability is imminent across 10 thousand square kilometres of the Caroline Rise (Phrampus, B.J. & Hornbach, M.J. 2012. Recent changes to the Gulf Stream causing widespread gas hydrate destabilization. Nature, v. 490, p. 527-530). As a test they analysed two seismic reflection profiles across the Carolina Rise, seeking anomalies known as bottom-simulating reflectors that signify free gas in the sediments. These are expected at the base of the gas-hydrate zone and their presence helps assess sediment temperature. At depths less than 1 km the base of the gas-hydrates modelled from the present temperature profile through the overlying seawater lies significantly above the base’s signature on seismic lines. The deeper levels probably formed under cooler conditions than now – probably eight degrees cooler – and may be unstable. If that is correct, the Caroline Rise area seems set to release around 2.5 Gt of methane to add to atmospheric greenhouse warming. The Storegga Slide also lies close to the northern track of the Gulf-Stream – North Atlantic Drift…