A ‘seiche’ wave can outpace a tsunami, and both can be triggered by meteorites and earthquakes

File 20190403 177184 r6mkdz.jpg?ixlib=rb 1.1
Waves can be generated in lakes and other bodies of water when seismic energy travels through land.
Leo Roomets / Unsplash, CC BY

Craig O’Neill, Macquarie University

A catastrophic event occurred on Earth 66 million years ago. A huge meteorite struck our planet in what is now Mexico, triggering mass extinctions of the dinosaurs and most other living creatures.

A new paper shows the first recorded victims of this impact were fish and other marine animals, stranded by a wave that left them high and dry in an ancient river in North Dakota, at a site called Tanis.

For scientists unpacking the evidence around the event, a full picture of the cataclysm has involved looking into the details of planetary surface physics during giant impacts.

But beyond the first layer of fascinating results – little glass impact beads stuck in the gills of fish, for example – one really interesting aspect of this work is around how water behaves when it’s exposed to extreme forces.

If you’ve never heard of a form of wave called a seiche, this is your chance to catch up.

This is a seiche – a standing wave – in a swimming pool, during a large earthquake in Nepal.

Waves of damage

The Chicxulub meteorite crater in coastal Mexico is strongly associated with the mass extinction of the dinosaurs (and 75% of all species), 66 million years ago.

The first victims were right at the site. Any marine creatures close to the point of impact would have been instantly vaporised (sadly leaving no fossil record), along with much of the surrounding rock.

Around the periphery, the energy of the impact melted and ejected tonnes of molten rock, which together with condensing rock vapour, formed little glass beads (“impact spherules”) that can be found in a layer around the world at this time.

The shock wave itself pulverised the adjacent rock enough to metamorphise it, forming features like “shocked quartz” – fractured quartz indicative of enormous pressures. It carried the energy equivalent of a magnitude 11 earthquake – 1,000 times more energy than the 2004 Boxing Day quake which killed almost 230,000 people.

Vast inland sea now gone

North Dakota is more than 3,000km away from the Chicxulub crater, and was a similar distance at the time of the meteorite impact event.

Separating them back then, however, was a vast inland sea that covered much of midwest USA, from Texas up to the Dakotas. Feeding into that inland sea was a river system upon which the Tanis site in North Dakota was formed. This site has preserved the earliest recorded deaths of the Chicxulub impact.

Different views of the Tanis site. A: Tanis (starred) within a regional context (large map) and on a national map (inset). B: Photo and interpretive overlay of an oblique cross-section through Tanis. C: Simplified schematic depicting the general deposits at the site (not to scale). Most fish carcasses were found at point 3.
Robert A DePalma and colleagues

The site itself is unusual. The deposition of sediments can tell us about the flow of water in the river.

Most ripples (or flame structures) indicate a southerly flow of the river before and after the Tanis deposit. However, these flow indicators point the wrong way during the time the Tanis unit formed. Water was flowing upstream, fast.

At the site are also found the fossilised remains of species, like sharks and rays, that occupied brackish water, rather than the freshwater of the stream. These had to be brought inland from the sea by something, and left to die, smothered in sediment, on a riverbank.

Stranded in Dakota

The obvious candidate is an impact tsunami. Perhaps the impact of the meteorite hitting the ocean generated a huge wave that carried fish from the inland sea, and against the flow of fresh water, to leave the creatures stranded in Dakota?

But there are problems with this hypothesis. The tiny impact spherules that formed in Chicxulub can be found throughout the deposit (many clogging the gills of fish), and pockmarks in the sedimentary layers means rocks were still raining down. This means the surge of water occurred within around 15 minutes to two hours of the impact itself.

For a tsunami to travel the 3,000km from the point of impact, to the Tanis site across the inland sea, would have taken almost 18 hours. Something else killed these creatures.

The seismic waves from the impact would have travelled through the Earth much faster than a tsunami travelled across water – and arrived near Tanis between 6-13 minutes later. The authors of the Tanis study suggest these seismic waves may have triggered an unusual type of wave in the inland sea, called a seiche.

Standing waves

Seiches are standing waves in bodies of water, and are often found in large lake systems during strong winds. The winds themselves cause waves and water displacement, which can have a harmonic effect, causing the water to slosh side to side like an overfull bathtub.

However, earthquakes are also known to cause seiches. Particularly dramatic seiches are often seen in swimming pools during large quakes. The interaction of the seismic wave’s period (the time between two waves) with the timescale of waves sloshing in a pool can amplify their effect.

But seiches can affect larger bodies of water too.

During the 2011 Tohuku earthquake in Japan, seiches over 1m high were observed in Norwegian fjords more than 8,000km away. With an energy more than 1,000 times greater, the Chicxulub event could quite conceivably have generated bigger than 10 metre swells in the North American inland sea – the scale implied by the deposition of the Tanis site.

These waves in Norwegian fjords were created by seismic waves from the 2011 Tohoku earthquake in Japan.

Given a seiche can be driven by seismic waves, it’s conceivable that one drove the surge that stranded marine creatures at Tanis, resulting in the short time between the impact debris and the surge deposit.

Still lots of questions

But a lot remains unclear regarding exactly what did happen 66 million years ago.

Could the fish stranding have been driven by the first seismic activity to appear at Tanis (the P and S waves in science parlance, which travel through the interior of the Earth, arriving at Tanis 6 and 10 minutes after impact, respectively), or the more destructive but slower surface waves at the top of the Earth’s crust, which arrived 13 minutes after impact?

How might seiche waves have interacted with global hurricane-strength wind storms caused by the impact?

Would the period of sloshing of a seiche be consistent with the scale of the inland sea? (The inland sea was much larger than most lakes seiches are traditionally observed in – and may or may not have been open to the ocean). Given so little is really known about the dimensions of the inland sea, this is hard to constrain.

The Tanis site has given us an incredible window into the first few hours of a mass-extinction. But it has also highlighted how little we have probed into the fatal surface physics of these extreme events.The Conversation

Craig O’Neill, Director of the Macquarie Planetary Research Centre/Associate Professor in Geodynamics, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Catching the waves: it’s time for Australia to embrace ocean renewable energy

Mark Hemer, CSIRO; Irene Penesis, University of Tasmania; Kathleen McInnes, CSIRO; Richard Manasseh, Swinburne University of Technology, and Tracey Pitman, CSIRO

Wind and solar may be currently leading the way in Australia’s renewable energy race, but there’s another contender lurking in the nation’s oceans.

Australia arguably possesses the world’s largest wave energy resource, around 1,800 terawatt hours. Most of this is concentrated in the southern half of the continent, between Geraldton and Brisbane. To put this in context, Australia used 248 terawatt hours of electricity in 2013-14.

Waves aren’t the only renewable power source in our oceans. The daily movements of the tides shift vast amounts of water around the Australian coast, and technology for conversion of tidal energy to electricity is more mature than any wave converters.

Ocean renewable energy also spans ocean thermal energy conversion, and energy captured from our large ocean currents (such as the East Australian Current). These represent less mature technologies with less opportunity in Australia.

Australia has abundant energy resources – both renewables and fossil fuels. So what will it take to get ocean energy out of the water, and into our homes?

The task at hand

The Paris Agreement, to which Australia is a signatory, aims to limit global warming to well-below 2℃. This will require almost complete decarbonisation of global electricity systems by 2050.

Of the 248 terawatt hours of electricity used in Australia, around 17 terawatt hours of this came from large scale renewable energy technology, equivalent to about half of Australia’s Renewable Energy Target of 33 terawatt hours by 2020.

To keep us on track to meet our international commitments, members of Australia’s Climate Change Authority recently proposed a target of 65% by 2030. This would require a rapid, large scale transition to alternative emission-free energy systems.

Wind and solar are currently leading the way, but we’ll need other technologies. This is not only to boost low emissions energy supply, but also to overcome the problem of intermittency due to the natural variability of the energy sources (when the sun doesn’t shine, or when the wind doesn’t blow).

Out to sea

Ocean renewable energy technologies (including wave and tidal) are emerging as a future contributor to Australia’s energy mix, and have a number of advantages over other sources.

Both wave and tidal energy devices are deployed offshore (not taking up limited land space) and are typically out of sight (deployed under the surface, or sufficiently offshore and low profile to not be obvious to the casual observer).

Although ocean energy resources also vary day-to-day like wind and solar, wave power has only a third of the variability of wind power. It can also be forecast three-times further ahead than wind. Tidal energy is predictable over very long time-frames.

These attributes provide an advantage in a portfolio of clean energy technologies and have led to notable government and other investments in ocean renewable energy technologies in Australia.

Ocean energy in Australia

The Australian Renewable Energy Agency (ARENA) has contributed more than A$44.3 million to at least nine ocean renewable energy projects to date (two closed before completion owing to technical and financial challenges). With other funds, more than A$122 million has been invested in ocean energy in Australia.

These funds have supported demonstration projects, including notable international successes (Carnegie Wave Energy Ltd, and BioPower Systems), and other research. Several other demonstration projects have also been undertaken in recent years by start-up companies with self-funded support, and unique technologies.

The expected installed capacity from approved ocean projects in Australia is around 3.5 megawatts. So far total global installed capacity of wave energy projects is less than 5 megawatts. The EU has also been a major investor in wave energy projects, with approximately €185 million (around A$275 million) invested to date, for a total expected installed capacity of 26 megawatts by 2018.

Although tidal energy converters are the most ready of ocean renewables, a high-quality assessment of Australia’s national tidal energy resource is yet to be done.

Nevertheless several prospective sites in northern Australia and near Tasmania are attracting national and international attention for potential development owing to their attractive resource. Significant projects are in development, particularly in Europe, where tidal installed capacity is set to increase to about 57 megawatts by 2018.

Falling costs

At the moment, the lifetime costs of ocean energy technologies are high. Until there are more than 10 megawatts of wave energy installed globally, costs will remain around A$500-900 per megawatt hour.

By comparison, in 1981, when there were less than 10 megawatts of installed wind energy capacity, wind turbines cost around A$720 per megawatt hour. In 1990 there were 2 gigawatts, and costs fell to around A$190 per megawatt hour. Now there are around 500 gigawatts of installed wind energy, and the cost of onshore wind is around A$110 per megawatt hour, similar to coal.

This experience suggests that costs for wave energy will decrease to A$170-340 per megawatt hour when installed capacity reaches 2 gigawatts. But costs should not be the only performance indicator for ocean renewables.

Options are being explored to combine and integrate design of other infrastructure (such as wave energy capture as a coastal protection mechanism, powering offshore aquaculture, or recreational amenities) which will reduce relative costs.

Support for an emerging industry

To put ocean energy generators in our seas, planners, operators and financiers will increasingly require more knowledge of how much energy is available and where.

These decision-makers also need to understand barriers or constraints to ocean energy (in particular areas such as access to transmission infrastructure, or other uses of the sea such as fishing, aquaculture, tourism, shipping, ports, marine-protected areas).

To help answer these questions, ARENA and CSIRO have developed the Australian Wave Energy Atlas. The atlas provides wave energy resource information together with details of available electricity infrastructure and spatial constraints for deployment. This allows users to identify the most viable sites for future wave energy projects, and ultimately ease the process of attracting capital and negotiating the consenting process.

While ocean renewable energy has many attractive features, there are still many challenges. The advantages of consistency and predictability of ocean energy become diminished if costs don’t fall below those of wind or solar supplemented with storage, which will offer the same advantages.

Other challenges include the technological advances needed to make generation devices ready and reduce costs; policy and regulatory barriers to project development; lack of awareness of ocean renewables and the potential they provide; limited body of knowledge on the environmental effects of large scale deployments; and the finance mechanisms to support the growing industry.

To overcome these challenges we’ll need to bring decision-makers, researchers, manufacturers, and businesses together to unlock the potential of our oceans.

The Australian Ocean Renewable Energy Symposium, running from today until October 20.

The Conversation

Mark Hemer, Senior Research Scientist, Oceans and Atmosphere, CSIRO; Irene Penesis, Associate professor, Mathematics, University of Tasmania; Kathleen McInnes, Senior research scientist, CSIRO; Richard Manasseh, Associate professor, Centre for Ocean Engineering, Swinburne University of Technology, and Tracey Pitman, Project Manager, Data61, CSIRO

This article was originally published on The Conversation. Read the original article.