Ancient sea-levels give new clues on ice ages

International researchers, led by the Australian National University (ANU), have developed a new way to determine sea-level changes and deep-sea temperature variability over the past 5.3 million years.

The findings will help scientists better understand the climate surrounding ice ages over the past two million years, and could help determine the relationship between carbon dioxide levels, global temperatures and sea levels.

The team from ANU, the University of Southampton (UoS) and the National Oceanography Centre (NOC) in the United Kingdom, examined oxygen isotope levels in fossils of microscopic plankton recovered from the Mediterranean Sea, dating back as far as 5.3 million years.

“This is the first step for reconstructions from the Mediterranean records,” says lead researcher Eelco Rohling from the ANU Research School of Earth Sciences.

Professor Rohling said the team focused on the flow of water through Strait of Gibraltar, which was particularly sensitive to sea-level changes.

“As continental ice sheets grew during the ice ages, flow through the Strait of Gibraltar was reduced, causing measurable changes in oxygen isotope ratios in Mediterranean waters, which became preserved in the shells of the ancient plankton,” he said.

Co-author Gavin Foster from UoS said the research for the first time found long-term trends in cooling and continental ice-volume build-up cycles over the past 5.3 Million years were not the same.

“In fact, for temperature the major step toward the ice ages of the past two million years was a cooling event at 2.7 million years ago,” he said.

“But for ice-volume, the crucial step was the development of the first intense ice age at around 2.15 million years ago. Before our results, these were thought to have occurred together at about 2.5 million years ago.”
Professor Rohling said the findings will help scientists better understand the nature of ice ages and development of coastal sediment.

“The observed decoupling of temperature and ice-volume changes provides crucial new information for our understanding of how the ice ages came about,” he said.

“However, there are wider implications. For example, a more refined sea-level record over millions of years is commercially interesting because it allows a better understanding of coastal sediment sequences that are relevant to the petroleum industry.

“Our record is also of interest to climate policy developments, because it opens the door to detailed comparisons between past atmospheric carbon dioxide concentrations, global temperatures, and sea levels, which has enormous value to long-term future climate projections.”

The findings have been published in the latest on-line edition of Nature.

Study provides crucial new information about how the ice ages came about

An international team of scientists has discovered new relationships between deep-sea temperature and ice-volume changes to provide crucial new information about how the ice ages came about.

Researchers from the University of Southampton, the National Oceanography Centre and the Australian National University developed a new method for determining sea-level and deep-sea temperature variability over the past 5.3 million years. It provides new insight into the climatic relationships that caused the development of major ice-age cycles during the past two million years.

The researchers found, for the first time, that the long-term trends in cooling and continental ice-volume cycles over the past 5.3 million years were not the same. In fact, for temperature the major step toward the ice ages that have characterised the past two to three million years was a cooling event at 2.7 million years ago, but for ice-volume the crucial step was the development of the first intense ice age at around 2.15 million years ago. Before these results, these were thought to have occurred together at about 2.5 million years ago.

The results are published in the scientific journal Nature.

Co-author Dr Gavin Foster, from Ocean and Earth Science at the University of Southampton, says: “Our work focused on the discovery of new relationships within the natural Earth system. In that sense, the observed decoupling of temperature and ice-volume changes provides crucial new information for our understanding of how the ice ages developed.

“However, there are wider implications too. For example, a more refined sea-level record over millions of years is commercially interesting because it allows a better understanding of coastal sediment sequences that are relevant to the petroleum industry. Our record is also of interest to climate policy developments, because it opens the door to detailed comparisons between past atmospheric CO2 concentrations, global temperatures, and sea levels, which has enormous value to long-term future climate projections.”

The team used records of oxygen isotope ratios (which provide a record of ancient water temperature) from microscopic plankton fossils recovered from the Mediterranean Sea, spanning the last 5.3 million years. This is a particularly useful region because the oxygen isotopic composition of the seawater is largely determined by the flow of water through the Strait of Gibraltar, which in turn is sensitive to changes in global sea level – in a way like the pinching of a hosepipe.

As continental ice sheets grew during the ice ages, flow through the Strait of Gibraltar was reduced, causing measurable increases in the oxygen isotope O-18 (8 protons and 10 neutrons) relative to O-16 (8 protons and 8 neutrons) in Mediterranean waters, which became preserved in the shells of the ancient plankton. Using long drill cores and uplifted sections of sea-floor sediments, previous work had analysed such microfossil-based oxygen isotope records from carefully dated sequences.

The current study added a numerical model for calculating water exchange through the Strait of Gibraltar as a function of sea-level change, which allowed the microfossil records to be used as a sensitive recorder of global sea-level changes. The new sea-level record was then used in combination with existing deep-sea oxygen isotope records from the open ocean, to work out deep-sea temperature changes.

Lead author, Professor Eelco Rohling of Australian National University, says: “This is the first step for reconstructions from the Mediterranean records. Our previous work has developed and refined this technique for Red Sea records, but in that location it is restricted to the last half a million years because there are no longer drill cores. In the Mediterranean, we could take it down all the way to 5.3 million years ago. There are uncertainties involved, so we included wide-ranging assessments of these, as well as pointers to the most promising avenues for improvement. This work lays the foundation for a concentrated effort toward refining and improving the new sea-level record.”

Noting the importance of the Strait of Gibraltar to the analysis, co-author Dr Mark Tamisiea from the National Oceanography Centre, Southampton adds: “Flow through the Strait will depend not only on the ocean’s volume, but also on how the land in the region moves up and down in response to the changing water levels. We use a global model of changes in the ocean and the ice sheets to estimate the deformation and gravity changes in the region, and how that will affect our estimate of global sea-level change.”

Study uncovers new evidence for assessing tsunami risk from very large volcanic island landslides

A core is extracted from the seabed. -  Russell Wynn
A core is extracted from the seabed. – Russell Wynn

The risk posed by tsunami waves generated by Canary Island landslides may need to be re-evaluated, according to researchers at the National Oceanography Centre. Their findings suggest that these landslides result in smaller tsunami waves than previously thought by some authors, because of the processes involved.

The researchers used the geological record from deep marine sediment cores to build a history of regional landslide activity over the last 1.5 million years. They found that each large-scale landslide event released material into the ocean in stages, rather than simultaneously as previously thought.

The findings – reported recently in the scientific journal Geochemistry Geophysics Geosystems – can be used to inform risk assessment from landslide-generated tsunamis in the area, as well as mitigation strategies to defend human populations and infrastructure against these natural hazards. The study also concluded that volcanic activity could be a pre-condition to major landslide events in the region.

The main factor influencing the amplitude of a landslide-generated tsunami is the volume of material sliding into the ocean. Previous efforts, which have assessed landslide volumes, have assumed that the entire landslide volume breaks away and enters the ocean as a single block. Such studies – and subsequent media coverage – have suggested an event could generate a ‘megatsunami’ so big that it would travel across the Atlantic Ocean and devastate the east coast of the US, as well as parts of southern England.

The recent findings shed doubt on this theory. Instead of a single block failure, the landslides in the past have occurred in multiple stages, reducing the volumes entering the sea, and thereby producing smaller tsunami waves. Lead author Dr James Hunt explains: “If you drop a block of soap into a bath full of water, it makes a relatively big splash. But if you break it up into smaller pieces and drop it in bit by bit, the ripples in the bath water are smaller.”

The scientists were able to identify this mechanism from the deposits of underwater sediment flows called turbidity currents, which form as the landslide mixes with surrounding seawater. Their deposits, known as ‘turbidites’, were collected from an area of the seafloor hundreds of miles away from the islands. Turbidites provide a record of landslide history because they form from the material that slides down the island slopes into the ocean, breaks up and eventually settles on this flatter, deeper part of the seafloor.

However, the scientists could not assume that multistage failure necessarily results in less devastating tsunamis – the stages need to occur with enough time in between so that the resulting waves do not compound each other. “If you drop the smaller pieces of soap in one by one but in very quick succession, you can still generate a large wave,” says Dr Hunt.

Between the layers of sand deposited by the landslides, the team found mud, providing evidence that the stages of failure occurred some time apart. This is because mud particles are so fine that they most likely take weeks to settle out in the ocean, and even longer to form a layer that would be resistant enough to withstand a layer of sand moving over the top of it.

While the authors suggest that the tsunamis were not as big as originally thought, they state that tsunamis are a threat that the UK should be taking seriously. The Natural Environment Research Council (NERC) is investing in a major programme looking at the risk of tsunamis from Arctic landslides as part of the Arctic Research Programme, of which NOC is the lead collaborator. The EU have also just funded a £6 million FP7 project called ASTARTE, looking at tsunami risk and resilience on the European North Atlantic and Mediterranean coasts, of which NOC is a partner.

The current study was funded by NERC, through a NOC studentship.

Study finds earlier peak for Spain’s glaciers

Jane Willenbring (upper right) takes samples to date a boulder in Spain's Bejar mountain range. Her findings helped show that ancient glaciers in the region reached their maximum size several thousands of years earlier than once believed. -  University of Pennsylvania
Jane Willenbring (upper right) takes samples to date a boulder in Spain’s Bejar mountain range. Her findings helped show that ancient glaciers in the region reached their maximum size several thousands of years earlier than once believed. – University of Pennsylvania

The last glacial maximum was a time when Earth’s far northern and far southern latitudes were largely covered in ice sheets and sea levels were low. Over much of the planet, glaciers were at their greatest extent roughly 20,000 years ago. But according to a study headed by University of Pennsylvania geologist Jane Willenbring, that wasn’t true in at least one part of southern Europe. Due to local effects of temperature and precipitation, the local glacial maximum occurred considerably earlier, around 26,000 years ago.

The finding sheds new light on how regional climate has varied over time, providing information that could lead to more-accurate global climate models, which predict what changes Earth will experience in the future.

Willenbring, an assistant professor in Penn’s Department of Earth and Environmental Science in the School of Arts and Sciences, teamed with researchers from Spain, the United Kingdom, China and the United States to pursue this study of the ancient glaciers of southern Europe.

“We wanted to unravel why and when glaciers grow and shrink,” Willenbring said.

In the study site in central Spain, it is relatively straightforward to discern the size of ancient glaciers, because the ice carried and dropped boulders at the margin. Thus a ring of boulders marks the edge of the old glacier.

It is not as easy to determine what caused the glacier to grow, however. Glaciers need both moisture and cold temperatures to expand. Studying the boulders that rim the ancient glaciers alone cannot distinguish these contributions. Caves, however, provide a way to differentiate the two factors. Stalagmites and stalactites – the stony projections that grow from the cave floor and ceiling, respectively – carry a record of precipitation because they grow as a result of dripping water.

“If you add the cave data to the data from the glaciers, it gives you a neat way of figuring out whether it was cold temperatures or higher precipitation that drove the glacier growth at the time,” Willenbring said.

The researchers conducted the study in three of Spain’s mountain ranges: the Bejár, Gredos and Guadarrama. The nearby Eagle Cave allowed them to obtain indirect precipitation data.

To ascertain the age of the boulders strewn by the glaciers and thus come up with a date when glaciers were at their greatest extent, Willenbring and colleagues used a technique known as cosmogenic nuclide exposure dating, which measures the chemical residue of supernova explosions. They also used standard radiometric techniques to date stalagmites from Eagle Cave, which gave them information about fluxes in precipitation during the time the glaciers covered the land.

“Previously, people believe the last glacial maximum was somewhere in the range of 19-23,000 years ago,” Willenbring said. “Our chronology indicates that’s more in the range of 25-29,000 years ago in Spain.”

The geologists found that, although temperatures were cool in the range of 19,000-23,000 years ago, conditions were also relatively dry, so the glaciers did not regain the size they had obtained several thousand years earlier, when rain and snowfall totals were higher. They reported their findings in the journal Scientific Reports.

Given the revised timeline in this region, Willenbring and colleagues determined that the increased precipitation resulted from changes in the intensity of the sun’s radiation on the Earth, which is based on the planet’s tilt in orbit. Such changes can impact patterns of wind, temperature and storms.

“That probably means there was a southward shift of the North Atlantic Polar Front, which caused storm tracks to move south, too,” Willenbring said. “Also, at this time there was a nice warm source of precipitation, unlike before and after when the ocean was colder.”

Willenbring noted that the new date for the glacier maximum in the Mediterranean region, which is several thousands of years earlier than the date the maximum was reached in central Europe, will help provide more context for creating accurate global climate models.

“It’s important for global climate models to be able to test under what conditions precipitation changes and when sources for that precipitation change,” she said. “That’s particularly true in some of these arid regions, like the American Southwest and the Mediterranean.”

When glaciers were peaking in the Mediterranean around 26,000 years ago, the American Southwest was experiencing similar conditions. Areas that are now desert were moist. Large lakes abounded, including Lake Bonneville, which covered much of modern-day Utah. The state’s Great Salt Lake is what remains.

“Lakes in this area were really high for 5,000-10,000 years, and the cause for that has always been a mystery,” Willenbring said. “By looking at what was happening in the Mediterranean, we might eventually be able to say something about the conditions that led to these lakes in the Southwest, too.”

Mediterranean ‘due a tsunami’ research suggests





Calculation of the sea wave (tsunami) caused by the AD 365 earthquake at 30 minutes after the earthquake. Red and blue shades correspond to peaks and troughs of about 50 cm.
Calculation of the sea wave (tsunami) caused by the AD 365 earthquake at 30 minutes after the earthquake. Red and blue shades correspond to peaks and troughs of about 50 cm.

Studying an ancient earthquake has enabled Oxford University researchers to quantify the likelihood of a tsunami in the Eastern Mediterranean.



They estimate that a ring of faults around the south of Greece and the Aegean Sea generates tsunami earthquakes approximately once every 800 years and, because the last such earthquake took place in 1303, the probability of a tsunami affecting the region is much higher than had been thought.



The Oxford researchers – working with colleagues from the Universities of Cambridge, Nice and Imperial College London – identified the cause of an earthquake that generated a tsunami that destroyed Alexandria on 21 July AD 365. Reporting in Nature Geoscience, the group describe how they tracked down the origin of this ancient quake to a fault beneath western Crete. Very precise radiocarbon dates of uplifted shorelines show that western Crete was lifted by about ten metres within a few decades of AD 365, and the shape of the uplifted shorelines is diagnostic of distortion of the land surface by an earthquake.



The researchers then used GPS stations to take very precise measurements of how the Earth’s surface is being slowly compressed all around the southern Aegean Sea today. From these measurements they predict that the energy being built up will be released in tsunami-earthquakes somewhere along this fault approximately every 800 years.


Professor Philip England, Head of Oxford’s Department of Earth Sciences and a co-author of the paper, said: ‘The AD 365 event is important because it is the only earthquake in the Mediterranean where the evidence can be studied on land, rather than being hidden under the ocean. It was one of the most devastating events in the ancient world: destroying cities and drowning thousands of people in coastal regions from the Nile Delta to modern day Dubrovnik.’



Calculations suggest that, as it crossed the open ocean, the wave height of the AD 365 tsunami was similar to that of the 2004 Sumatra tsunami – around one metre high. This leads the researchers to believe that, when it hit the shore, this sea wave would have been highly destructive.



‘This is our first real stab at quantifying the risk of a tsunami event in the Eastern Mediterranean,’ said Professor England. ‘What we can say for sure is that if the AD 365 earthquake were to be repeated it would have a devastating impact on the Mediterranean region.’



This work was supported by the UK’s Natural Environment Research Council (NERC) and by Oxford University.