Fukushima Daiichi nuclear disaster on the 12 March 2011 was a direct result of the Tōhoku earthquake and tsunamis the day before. The reactors automatically shut down when the earthquake hit but the tsunami led to flooding of the emergency cooling generators causing three nuclear meltdowns and the release of radioactive material. The Fukushima disaster is the largest nuclear disaster since the 1986 Chernobyl incident and the second disaster to be given the Level 7 event classification of the International Nuclear Event Scale. It was a sobering event for the international radiological community but what if any are the implications for a country such as the UK? Most of the UK’s nuclear plants are coastal; the availability of cheap land and abundant water for cooling being the key. Unlike Japan the UK is not located on an active tectonic boundary, but on what is called a ‘passive margin’, but a UK tsunami is not beyond the bounds of probability.
Tsunami records in the UK
In the late 1980s and early 1990s tsunami records were recognised first from the Norwegian coast and then from Scotland. They consist of an anomalous layer of sand containing shell fragments and rip-up clasts often set within terrestrial peat. These layers evidence wave run-up way beyond the norm often between 1 and 100 m (Smith et al., 2004). They have been widely studied and have been variously dated to around 7.1 radiocarbon years BP (7.9 Calibrated years BP). Particularly impressive is the dating of chlorophyll from mosses washed out to sea and preserved in marine sediment by the tsunami. These give calibrated ages of 8.12 K and 8.175 K years BP (Bondevik et al., 2012).
Tsunamis can be caused by earthquakes, meteorite impacts, volcanic cone collapse and by submarine landslides. The smoking gun for the UK tsunami was the Storrega Slide on the Norwegian continental slope. This complex multi-phase slide appears to have been last active around 7.9-8.1 K years BP when a failure of some 4000 cubic km occurred. The antimony of this slide is of particular interest. Ocean basins have a distinct marginal geometry; the continental shelf, the continental slope, and abyssal plain. As move away from the shoreline the rate of sedimentation declines, also sedimentary flows are not always straight forward – shallow to deep. As dense turbid water moves basin ward it may in time encounter denser bottom water (colder) at this point it may cease to move downhill and will also be deflected by Coriolis Force. These flows may now contour the edge of the basin eroding and depositing sediment as contourites. If we now enter a glacial phase ice sheets will build up over the continent and slowly advance to the edge of the continental shelf. Where ice flow is concentrated in ice streams big fans of sediment will extend the edge of the continental shelf. These are called trough mouth fans. The steepening of the continental slope is potentially a source of instability and therefore of failure. The Sorrega Slide is no exception being at the mouth of the Norwegian Ice Stream which at the height of the last glacial cycle drained out of the Baltic.
To this sediment pile we need to add methane clathyate in which methane is trapped in a crystal structure of water forming a solid similar to ice – but one that can burn! It occurs in various places around the Earth, one of which is at depth in ocean sediments. It instability, and therefore release, can triggered by changes in ocean temperature and pressure (i.e. water depth via sea level changes). It has been implemented as potential cause for failure of the Storrega Slide.
There are two issues worth exploring further here: the first is the implications of this tsunami record for future events in the North Atlantic, and secondly some potential climatic implications given the global 8.2k BP climate event.
Tsunamis in the future?
The Storegga Slide tsunami demonstrates the potential for such events to occur on passive continental margins. It is not the only tsunami to impact on Britain. There are some other more restricted sand layers that have been documented and dated some of which are just 1.5 years BP. These are not as extensive as those of 7.9kBP and the cause is uncertain. There is the ever present need to separate out what is an actually tsunamis from other coastal flood waves such as tidal surges and storm waves. In historical times the Lisbon earthquake of 1755 was associated with a modest tsunami which impacted on the UK’s southern coast.
Tsunamis can be modelled relatively easily and a number of studies have attempted to marry up models with observed historical details. The following You Tube link provides results from modelling the Storegga Slide.
So what are the chances of a tsunami similar to that of Storegga happening again. In the case of the slide itself it quite stable, but in truth it is just one location around the North Atlantic where large trough mouth fans accumulated during the last glacial cycle.
The Grand Banks Earthquake and tsunami of 1929 which has been modelled and reconstructed by Fine et al. (2005) involved the failure of the Laurentian Fan off Newfoundland and killed 28 people. Failure of other trough mouth fans is a real possibility.
Off greater impact might be a volcanic collapse in the Canary Islands (Ward, 2001). In particular attention has focused on the potential for flank collapse of the Cumbre Vieja Volcano. Such a failure would drop between 150 and 500 km cubed of rock into the sea, setting off a tsunami that would have the potential to inundate the eastern seaboard of the USA. The role of submarine slides around volcanic islands is reviewed by Whelan and Kelletat (2001).
While most authorities rank tsunami risk in countries like the UK to be small it is not insignificant and the impact on an unprotected coast without any warning systems or disaster plans in place could be quite catastrophic especially to wind farms and other off shore infrastructure.
Storegga Slide and Climate?
When is a correlation between two event causative, or just coincidence? This is a particular problem in dealing what a colleague of mine calls ‘wiggly line science’. Take a couple of climate proxies with slightly different fidelity and one of more events; if you want them to line up then they tend to, if you don’t they don’t. More to the point if they do show a degree of correlation how do you know they are linked?
The Storegga Slide provides an example. Beget and Addison (2007) proposed tentatively that the age of the Storegga Slide’s most recent episode corresponded to a small spike in the methane within the Greenland GRIP ice core. They suggested that release of methane clathrate during the slide may have been the cause of this spike. In fact they sugest that it might have helped the Earth recover from the global cooling event of 8.2ka. It might provide an example of something known as the ‘clathrate gun hypothesis’. A rise in sea temperature and/or a fall in sea level causes the release of methane which as a greenhouse gas causes further warming and further methane releases. Such a cycle has been implicated in the Paleocene–Eocene Thermal Maximum 56 million years ago, and perhaps the Permian–Triassic extinction event, when up to 96% of marine species became extinct.
The 8.2k year cooling event was both severe and appears to have been global. It has been attributed to the partial shutdown of the thermohaline conveyor caused by the drainage of the Lake Agassiz via a mega-flood (jökulhaup).
Figure 1: Reconstruction of Doggerland. (Source: http://nationalgeographic.org/maps/doggerland/)
More recent work has challenged this idea Dawson et al. (2011). First, analysis of Storegga Slide sediments suggest that little methane may have actually been released during the slide. Secondly, revised dating suggests that the slide occurred during the cold spell and was not associated with the subsequent warming event.
Spare a thought for your ancestors living on Doggerland (Fig. 1). You may have heard of Dogger Bank if nothing else from the Shipping Forcast; it is a low lying body of submerged land in the North Sea. It was once home to a flourishing community. The Storegga Tsunami would have had a catastrophic impact on both the human and animal communities living there sweeping across the low lying terrain.
References
Bondevik, S., Løvholt, F., Harbitz, C., Mangerud, J., Dawson, A. and Svendsen, J.I., 2005. The Storegga Slide tsunami—comparing field observations with numerical simulations. Marine and Petroleum Geology, 22(1), pp.195-208.
Bondevik, S., Stormo, S.K. and Skjerdal, G., 2012. Green mosses date the Storegga tsunami to the chilliest decades of the 8.2 ka cold event. Quaternary Science Reviews, 45, pp.1-6.
Dawson, A.G., Long, D. and Smith, D.E., 1988. The Storegga slides: evidence from eastern Scotland for a possible tsunami. Marine geology, 82(3-4), pp.271-276.
Dawson, A., Bondevik, S. and Teller, J.T., 2011. Relative timing of the Storegga submarine slide, methane release, and climate change during the 8.2 ka cold event. The Holocene, 21(7), pp.1167-1171.
Haflidason, H., Lien, R., Sejrup, H.P., Forsberg, C.F. and Bryn, P., 2005. The dating and morphometry of the Storegga Slide. Marine and Petroleum Geology, 22(1), pp.123-136.
Haflidason, H., Sejrup, H.P., Nygård, A., Mienert, J., Bryn, P., Lien, R., Forsberg, C.F., Berg, K. and Masson, D., 2004. The Storegga Slide: architecture, geometry and slide development. Marine geology, 213(1), pp.201-234.
Haflidason, H., Sejrup, H.P., Nygård, A., Mienert, J., Bryn, P., Lien, R., Forsberg, C.F., Berg, K. and Masson, D., 2004. The Storegga Slide: architecture, geometry and slide development. Marine geology, 213(1), pp.201-234.
Smith, D.E., Shi, S., Cullingford, R.A., Dawson, A.G., Dawson, S., Firth, C.R., Foster, I.D., Fretwell, P.T., Haggart, B.A., Holloway, L.K. and Long, D., 2004. The holocene storegga slide tsunami in the United Kingdom. Quaternary Science Reviews, 23(23), pp.2291-2321.
Whelan, F. and Kelletat, D., 2003. Submarine slides on volcanic islands-a source for mega-tsunamis in the Quaternary. Progress in Physical Geography, 27(2), pp.198-216.
Day, S., Day, 2001. Cumbre Vieja Volcano–Potential collapse and tsunami at La Palma, Canary Islands. Geophysical Research Letters, 28(17), pp.3397-3400.
Other Sources:
http://www.bbc.com/earth/story/20160323-the-terrifying-tsunami-that-devastated-britain
http://www.bbc.co.uk/news/business-24604218
Professor Sandcastle 