Are huge landslide-tsunami more frequent, larger and less predictably linked to glacial cycles?

Submarine landslides can be far larger than any terrestrial landslide and generate exceptionally dangerous and far-travelling tsunami[1-4]. For example, the Storegga Slide that occurred 8,150 years ago offshore Norway involved ~3,000 km3 of material, and is larger than Scotland[5,6] (Fig. 1). The largest terrestrial landslide in the last 350,000 years is ~30 km3[7]. Storegga Slide generated a tsunami reaching heights of up to 20m around many northern European coastlines[3,4] (Fig. 1). These coasts now host densely-populated cities and critical infrastructure. A repeat of the Storegga landslide-tsunami is probably the most-damaging natural hazard that can potentially impact northern Europe[8], and there is currently no way to warn against landslide-tsunami, unlike earthquake-tsunami. There is a compelling need to understand the frequency and maximum-credible-size of landslide-tsunami from this Norwegian Margin. On average mega-landslides are infrequent, but oceans are warming rapidly at high-latitudes, which may trigger landslides, such as via hydrate dissociation[8,9]. There is a remarkable coincidence between Storegga Slide’s age and the last period of abrupt climate change (‘8.2ka event’) before now[10]. We urgently need to understand if mega-landslides are more common during rapid ocean-warming, and their relationship to glacial-climate cycles[8]. Previously it was assumed a repeat of Storegga Slide needed another ice-stream-advance to deposit more sediment on the shelf-edge, which could fail as another mega-landslide[11]. The final trigger was assumed to be a large earthquake caused by isostatic rebound during deglaciation[11]. This view implies there is just one mega-landslide per ~100,000-year glacial cycle, and another mega-landslide cannot occur in the foreseeable future. However, recent geophysical-mapping of Storegga Slide’s headwall suggested it might be not one but two mega-landslides, with a second mega-landslide occurring ~20,000 years ago[6], well-before deglaciation (‘Nyegga Slide’). Moreover, a newly proposed landslide headwall (‘Solsikke Slide’) suggests another mega-landslide potentially occurred in the last glacial cycle[12], which is ~5 times larger than Storegga[12]. However, ages of Nyegga and Solsikke Slides are poorly known, as neither is penetrated by cores. But if correct, this implies multiple landslides occur in a glacial cycle, and tsunami magnitudes can far exceed that from Storegga. A 20,000-year mega-landslide cannot be triggered during deglaciation, so mega-landslides may not be predictably linked to glacial cycles, casting doubt on conclusions underpinning UK and other national tsunami-hazard assessments. We therefore need to accurately date a longer-record of mega-landslides. Most mega-landslides (e.g. Solsikke Slide) are too deeply buried beneath younger deposits to core and date[12]. But fast-moving mega-landslides that produce tsunami disintegrate to create longer-runout sediment flows called turbidity currents, which produce thick sediment-layers (‘mega-turbidites’) in deep-sea basins[1,8]. Mega-turbidites are an excellent record of mega-landslides as they are less-deeply buried, and easier to date. Storegga Slide produced a 30-m-thick mega-turbidite in the Aegir Basin[5], but there is a thicker mega-turbidite dated to 55-to-60,000-years underneath, and a 10-times-thicker mega-turbidite below that is undated[13]. This project will establish an exact and far longer chronology of mega-landslides over the last ~1.5 million years, to determine their frequency, maximum-credible-size and relationship to glacial-climate cycles. The project is feasible and timely because NERC recently purchased a Giant Piston Corer costing ~£1 million, which can produce 40-m-long cores[14]. This new corer can reach mega-turbidites and some mega-landslides (e.g. Nyegga Slide) that were previously too deeply-buried to core. The project will provide the basis for appropriate responses to mega-landslide tsunami by UK and European coastal flood-risk managers.