North Atlantic signalled Ice Age thaw 1,000 years before it happened, reveals new research

The Atlantic Ocean at mid-depths may have given out early warning signals – 1,000 years in advance – that the last Ice Age was going to end, scientists report today in the journal Paleoceanography.

Scientists had previously known that at the end of the last Ice Age, around 14,700 years ago, major changes occurred to the Atlantic Ocean in a period known as the Bolling-Allerod interval. During this period, as glaciers melted and the Earth warmed, the currents of the Atlantic Ocean at its deepest levels changed direction.

The researchers have analysed the chemistry of 24 ancient coral fossils from the North Atlantic Ocean to learn more about the circulation of its waters during the last Ice Age. They found that the corals recorded a high variability in the currents of the Atlantic Ocean at mid-depths, around 2km below the surface, up to 1,000 years prior to the Bolling-Allerod interval. The team suggests that these changes may have been an early warning signal that the world was poised to switch from its glacial state to the warmer world we know today, and that the changes happened first at mid-depths.

The study was carried out by researchers from Imperial College London in conjunction with academics from the Scottish Marine Institute, the University of Bristol and Caltech Division of Geology and Planetary Sciences.

Dr David Wilson, from the Department of Earth Science and Engineering at Imperial College London, said: “The world’s oceans have always been an important barometer when it comes to changes in our planet. Excitingly, the coral fossils we’ve studied are showing us that the North Atlantic Ocean at mid-depths was undergoing changes up to 1,000 years earlier than we had expected. The tantalising prospect is that this high variability may have been a signal that the last Ice Age was about to end.”

The fossil corals analysed by the team come from a species called Desmophyllum dianthus, which are often around 5cm in diameter and look like budding flowers. They typically only live for 100 years, giving the team a rare insight into what was happening to the ocean’s currents during this relatively brief time. Thousands of years ago they grew on the New England Seamounts, which are a chain of undersea mountains approximately 1000km off the east coast of the US, located at mid-depths 2km beneath the surface. This underwater area is important for understanding the North Atlantic’s currents.

While some of the corals analysed by the team come from historical collections, most have been collected by researchers from previous expeditions in 2003 and 2005 to the New England Seamounts. The researchers used deep sea robotic submergence vehicles called Hercules and Alvin to collect the ancient coral fossils.

These ancient coral fossils accumulated rare earth elements from seawater, including neodymium, which leached from rocks on land into the Atlantic Ocean and circulated in its currents, eventually ending up in the coral skeletons. Neodymium isotopes in different regions of the world have specific signatures, created by radioactive decay over billions of years. The scientists studied the chemistry of the coral fossils to determine where the neodymium isotopes had come from, giving them a glimpse into the circulation of the Atlantic Ocean at the end of the Ice Age.

Since the world’s oceans are connected by currents, the next step will see the team integrating the evidence they gathered from the North Atlantic Ocean into a picture of global changes in the mid-depths of oceans around the world. In particular, the team is interested in exploring how the Southern Ocean around Antarctica changed around the same time by analysing neodymium isotopes in a collection of Southern Ocean corals.

Ancient ocean currents may have changed pace and intensity of ice ages

About 950,000 years ago, North Atlantic currents and northern hemisphere ice sheets underwent changes. -  NASA
About 950,000 years ago, North Atlantic currents and northern hemisphere ice sheets underwent changes. – NASA

Climate scientists have long tried to explain why ice-age cycles became longer and more intense some 900,000 years ago, switching from 41,000-year cycles to 100,000-year cycles.

In a paper published this week in the journal Science, researchers report that the deep ocean currents that move heat around the globe stalled or may have stopped at that time, possibly due to expanding ice cover in the Northern Hemisphere.

“The research is a breakthrough in understanding a major change in the rhythm of Earth’s climate, and shows that the ocean played a central role,” says Candace Major, program director in the National Science Foundation (NSF)’s Division of Ocean Sciences, which funded the research.

The slowing currents increased carbon dioxide (CO2) storage in the oceans, leaving less CO2 in the atmosphere. That kept temperatures cold and kicked the climate system into a new phase of colder, but less frequent, ice ages, the scientists believe.

“The oceans started storing more carbon dioxide for a longer period of time,” says Leopoldo Pena, the paper’s lead author and a paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory (LDEO). “Our evidence shows that the oceans played a major role in slowing the pace of the ice ages and making them more severe.”

The researchers reconstructed the past strength of Earth’s system of ocean currents by sampling deep-sea sediments off the coast of South Africa, where powerful currents originating in the North Atlantic Ocean pass on their way to Antarctica.

How vigorously those currents moved can be inferred by how much North Atlantic water made it that far, as measured by isotope ratios of the element neodymium bearing the signature of North Atlantic seawater.

Like tape recorders, the shells of ancient plankton incorporate these seawater signals through time, allowing scientists to approximate when currents grew stronger and when weaker.

Over the last 1.2 million years, the conveyor-like currents strengthened during warm periods and lessened during ice ages, as previously thought.

But at about 950,000 years ago, ocean circulation slowed significantly and stayed weak for 100,000 years.

During that period the planet skipped an interglacial–the warm interval between ice ages. When the system recovered, it entered a new phase of longer, 100,000-year ice age cycles.

After this turning point, deep ocean currents remained weak during ice ages, and ice ages themselves became colder.

“Our discovery of such a major breakdown in the ocean circulation system was a big surprise,” said paper co-author Steven Goldstein, a geochemist at LDEO. “It allowed the ice sheets to grow when they should have melted, triggering the first 100,000-year cycle.”

Ice ages come and go at predictable intervals based on the changing amount of sunlight that falls on the planet, due to variations in Earth’s orbit around the sun.

Orbital changes alone, however, are not enough to explain the sudden switch to longer ice age intervals.

According to one earlier hypothesis for the transition, advancing glaciers in North America stripped away soils in Canada, causing thicker, longer-lasting ice to build up on the remaining bedrock.

Building on that idea, the researchers believe that the advancing ice might have triggered the slowdown in deep ocean currents, leading the oceans to vent less carbon dioxide, which suppressed the interglacial that should have followed.

“The ice sheets must have reached a critical state that switched the ocean circulation system into a weaker mode,” said Goldstein.

Neodymium, a key component of cellphones, headphones, computers and wind turbines, also offers a good way of measuring the vigor of ancient ocean currents.

Goldstein and colleagues had used neodymium ratios in deep-sea sediment samples to show that ocean circulation slowed during past ice ages.

They used the same method to show that changes in climate preceded changes in ocean circulation.

A trace element in Earth’s crust, neodymium washes into the oceans through erosion from the continents, where natural radioactive decay leaves a signature unique to the land mass from which it originated.

When Goldstein and Lamont colleague Sidney Hemming pioneered this method in the late 1990s, they rarely worried about surrounding neodymium contaminating their samples.

The rise of consumer electronics has changed that.

“I used to say you could do sample processing for neodymium analysis in a parking lot,” said Goldstein. “Not anymore.”

New hi-tech approach to studying sedimentary basins

A radical new approach to analysing sedimentary basins also harnesses technology in a completely novel way. An international research group, led by the University of Sydney, will use big data sets and exponentially increased computing power to model the interaction between processes on the earth’s surface and deep below it in ‘five dimensions’.

As announced by the Federal Minister for Education today, the University’s School of Geosciences will lead the Basin GENESIS Hub that has received $5.4 million over five years from the Australian Research Council (ARC) and industry partners.

The multitude of resources found in sedimentary basins includes groundwater and energy resources. The space between grains of sand in these basins can also be used to store carbon dioxide.

“This research will be of fundamental importance to both the geo-software industry, used by exploration and mining companies, and to other areas of the energy industry,” said Professor Dietmar Müller, Director of the Hub, from the School of Geosciences.

“The outcomes will be especially important for identifying exploration targets in deep basins in remote regions of Australia. It will create a new ‘exploration geodynamics’ toolbox for industry to improve estimates of what resources might be found in individual basins.”

Sedimentary basins form when sediments eroded from highly elevated regions are transported through river systems and deposited into lowland regions and continental margins. The Sydney Basin is a massive basin filled mostly with river sediments that form Hawkesbury sandstone. It is invisible to the Sydney population living above it but has provided building material for many decades.

“Previously the approach to analysing these basins has been based on interpreting geological data and two-dimensional models. We apply infinitely more computing power to enhance our understanding of sedimentary basins as the product of the complex interplay between surface and deep Earth processes,” said Professor Müller.

Associate Professor Rey, a researcher at the School of Geosciences and member of the Hub said, “Our new approach is to understand the formation of sedimentary basins and the changes they undergo, both recently and over millions to hundreds of millions of years, using computer simulations to incorporate information such as the evolution of erosion, sedimentary processes and the deformation of the earth’s crust.”

The researchers will incorporate data from multiple sources to create ‘five-dimensional’ models, combining three-dimensional space with the extra dimensions of time and estimates of uncertainty.

The modelling will span scales from entire basins hundreds of kilometres wide to individual sediment grains.

Key geographical areas the research will focus on are the North-West shelf of Australia, Papua New Guinea and the Atlantic Ocean continental margins.

The Hub’s technology builds upon the exponential increase in computational power and the increasing amount of available big data (massive data sets of information). The Hub will harness the capacity of Australia’s most powerful computer, launched in 2013.

The Atlantic Ocean dances with the sun and volcanoes

Imagine a ballroom in which two dancers apparently keep in time to their own individual rhythm. The two partners suddenly find themselves moving to the same rhythm and, after a closer look, it is clear to see which one is leading.

It was an image like this that researchers at Aarhus University were able to see when they compared studies of solar energy release and volcanic activity during the last 450 years, with reconstructions of ocean temperature fluctuations during the same period.

The results actually showed that during the last approximately 250 years – since the period known as the Little Ice Age – a clear correlation can be seen where the external forces, i.e. the Sun’s energy cycle and the impact of volcanic eruptions, are accompanied by a corresponding temperature fluctuation with a time lag of about five years.

In the previous two centuries, i.e. during the Little Ice Age, the link was not as strong, and the temperature of the Atlantic Ocean appears to have followed its own rhythm to a greater extent.

The results were recently published in the scientific journal Nature Communications.

In addition to filling in yet another piece of the puzzle associated with understanding the complex interaction of the natural forces that control the climate, the Danish researchers paved the way for linking the two competing interpretations of the origin of the oscillation phenomenon.

Temperature fluctuations discovered around the turn of the millennium

The climate is defined on the basis of data including mean temperature values recorded over a period of thirty years. Northern Europe thus has a warm and humid climate compared with other regions on the same latitudes. This is due to the North Atlantic Drift (often referred to as the Gulf Stream), an ocean current that transports relatively warm water from the south-west part of the North Atlantic to the sea off the coast of Northern Europe.

Around the turn of the millennium, however, climate researchers became aware that the average temperature of the Atlantic Ocean was not entirely stable, but actually fluctuated at the same rate throughout the North Atlantic. This phenomenon is called the Atlantic Multidecadal Oscillation (AMO), which consists of relatively warm periods lasting thirty to forty years being replaced by cool periods of the same duration.

The researchers were able to read small systematic variations in the water temperature in the North Atlantic in measurements taken by ships during the last 140 years.

Although the temperature fluctuations are small – less than 1°C – there is a general consensus among climate researchers that the AMO phenomenon has had a major impact on the climate in the area around the North Atlantic for thousands of years, but until now there has been doubt about what could cause this slow rhythm in the temperature of the Atlantic Ocean. One model explains the phenomenon as internal variability in the ocean circulation – somewhat like a bathtub sloshing water around in its own rhythm. Another model explains the AMO as being driven by fluctuations in the amount of solar energy received by the Earth, and as being affected by small changes in the energy radiated by the Sun itself and the after-effects of volcanic eruptions. Both these factors are also known as ‘external forces’ that have an impact on the Earth’s radiation balance.

However, there has been considerable scepticism towards the idea that a phenomenon such as an AMO could be driven by external forces at all – a scepticism that the Aarhus researchers now demonstrate as unfounded.

“Our new investigations clearly show that, since the Little Ice Age, there has been a correlation between the known external forces and the temperature fluctuations in the ocean that help control our climate. At the same time, however, the results also show that this can’t be the only driving force behind the AMO, and the explanation must therefore be found in a complex interaction between a number of mechanisms. It should also be pointed out that these fluctuations occur on the basis of evenly increasing ocean temperatures during the last approximately fifty years – an increase connected with global warming,” says Associate Professor Mads Faurschou Knudsen, Department of Geoscience, Aarhus University, who is the main author of the article.

Convincing data from the Earth’s own archives

Researchers have attempted to make computer simulations of the phenomenon ever since the discovery of the AMO, partly to enable a better understanding of the underlying mechanism. However, it is difficult for the computer models to reproduce the actual AMO signal that can be read in the temperature data from the last 140 years.

Associate Professor Knudsen and his colleagues instead combined all available data from the Earth’s own archives, i.e. previous studies of items such as radioactive isotopes and volcanic ash in ice cores. This provides information about solar energy release and volcanic activity during the last 450 years, and the researchers compared the data with reconstructions of the AMO’s temperature rhythm during the same period.

“We’ve only got direct measurements of the Atlantic Ocean temperature for the last 140 years, where it was measured by ships. But how do you measure the water temperature further back in time? Studies of growth rings in trees from the entire North Atlantic region come into the picture here, where ‘good’ and ‘bad’ growth conditions are calibrated to the actual measurements, and the growth rings from trees along the coasts further back in time can therefore act as reserve thermometers,” explains Associate Professor Knudsen.

The results provide a new and very important perspective on the AMO phenomenon because they are based on data and not computer models, which are inherently incomplete. The problem is that the models do not completely describe all the physical correlations and feedbacks in the system, partly because these are not fully understood. And when the models are thus unable to reproduce the actual AMO signal, it is hard to know whether they have captured the essence of the AMO phenomenon.

Impact of the sun and volcanoes

An attempt to simply explain how external forces such as the Sun and volcanoes can control the climate could sound like this: a stronger Sun heats up the ocean, while the ash from volcanic eruptions shields the Sun and cools down the ocean. However, it is hardly as simple as that.

“Fluctuations in ocean temperature have a time lag of about five years in relation to the peaks we can read in the external forces. However, the direct effect of major volcanic eruptions is clearly seen as early as the same year in the mean global atmospheric temperature, i.e. a much shorter delay. The effect we studied is more complex, and it takes time for this effect to spread to the ocean currents,” explains Associate Professor Knudsen.

“An interesting new theory among solar researchers and meteorologists is that the Sun can control climate variations via the very large variations in UV radiation, which are partly seen in connection with changes in sunspot activity during the Sun’s eleven-year cycle. UV radiation heats the stratosphere in particular via increased production of ozone, which can have an impact on wind systems and thereby indirectly on the global ocean currents as well,” says Associate Professor Knudsen. However, he emphasises that researchers have not yet completely understood how a development in the stratosphere can affect the ocean currents on Earth.

Towards a better understanding of the climate

“In our previous study of the climate in the North Atlantic region during the last 8,000 years, we were able to show that the temperature of the Atlantic Ocean was presumably not controlled by the Sun’s activity. Here the temperature fluctuated in its own rhythm for long intervals, with warm and cold periods lasting 25 years. The prevailing pattern was that this climate fluctuation in the ocean was approximately 30󈞔% faster than the fluctuation we’d previously observed in solar activity, which lasted about ninety years. What we can now see is that the Atlantic Ocean would like to – or possibly even prefer to – dance alone. However, under certain circumstances, the external forces interrupt the ocean’s own rhythm and take over the lead, which has been the case during the last 250 years,” says Associate Professor Bo Holm Jacobsen, Department of Geoscience, Aarhus University, who is the co-author of the article.

“It’ll be interesting to see how long the Atlantic Ocean allows itself to be led in this dance. The scientific challenge partly lies in understanding the overall conditions under which the AMO phenomenon is sensitive to fluctuations in solar activity and volcanic eruptions,” he continues.

“During the last century, the AMO has had a strong bearing on significant weather phenomena such as hurricane frequency and droughts – with considerable economic and human consequences. A better understanding of this phenomenon is therefore an important step for efforts to deal with and mitigate the impact of climate variations,” Associate Professor Knudsen concludes.

What sculpted Africa’s margin?

Break-up of the supercontinent Gondwana about 130 Million years ago could have lead to a completely different shape of the African and South American continent with an ocean south of today’s Sahara desert, as geoscientists from the University of Sydney and the GFZ German Research Centre for Geosciences have shown through the use of sophisticated plate tectonic and three-dimensional numerical modelling. The study highlights the importance of rift orientation relative to extension direction as key factor deciding whether an ocean basin opens or an aborted rift basin forms in the continental interior.

For hundreds of millions of years, the southern continents of South America, Africa, Antarctica, Australia, and India were united in the supercontinent Gondwana. While the causes for Gondwana’s fragmentation are still debated, it is clear that the supercontinent first split along along the East African coast in a western and eastern part before separation of South America from Africa took place. Today’s continental margins along the South Atlantic ocean and the subsurface graben structure of the West African Rift system in the African continent, extending from Nigeria northwards to Libya, provide key insights on the processes that shaped present-day Africa and South America. Christian Heine (University of Sydney) and Sascha Brune (GFZ) investigated why the South Atlantic part of this giant rift system evolved into an ocean basin, whereas its northern part along the West African Rift became stuck.

“Extension along the so-called South Atlantic and West African rift systems was about to split the African-South American part of Gondwana North-South into nearly equal halves, generating a South Atlantic and a Saharan Atlantic Ocean”, geoscientist Sascha Brune explains. “In a dramatic plate tectonic twist, however, a competing rift along the present-day Equatorial Atlantic margins, won over the West African rift, causing it to become extinct, avoiding the break-up of the African continent and the formation of a Saharan Atlantic ocean.” The complex numerical models provide a strikingly simple explanation: the larger the angle between rift trend and extensional direction, the more force is required to maintain a rift system. The West African rift featured a nearly orthogonal orientation with respect to westward extension which required distinctly more force than its ultimately successful Equatorial Atlantic opponent.

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.

New ‘embryonic’ subduction zone found

A new subduction zone forming off the coast of Portugal heralds the beginning of a cycle that will see the Atlantic Ocean close as continental Europe moves closer to America.

Published in Geology, new research led by Monash University geologists has detected the first evidence that a passive margin in the Atlantic ocean is becoming active. Subduction zones, such as the one beginning near Iberia, are areas where one of the tectonic plates that cover the Earth’s surface dives beneath another plate into the mantle – the layer just below the crust.

Lead author Dr João Duarte, from the School of Geosciences said the team mapped the ocean floor and found it was beginning to fracture, indicating tectonic activity around the apparently passive South West Iberia plate margin.

“What we have detected is the very beginnings of an active margin – it’s like an embryonic subduction zone,” Dr Duarte said.

“Significant earthquake activity, including the 1755 quake which devastated Lisbon, indicated that there might be convergent tectonic movement in the area. For the first time, we have been able to provide not only evidences that this is indeed the case, but also a consistent driving mechanism.”

The incipient subduction in the Iberian zone could signal the start of a new phase of the Wilson Cycle – where plate movements break up supercontinents, like Pangaea, and open oceans, stabilise and then form new subduction zones which close the oceans and bring the scattered continents back together.

This break-up and reformation of supercontinents has happened at least three times, over more than four billion years, on Earth. The Iberian subduction will gradually pull Iberia towards the United States over approximately 220 million years.

The findings provide a unique opportunity to observe a passive margin becoming active – a process that will take around 20 million years. Even at this early phase the site will yield data that is crucial to refining the geodynamic models.

“Understanding these processes will certainly provide new insights on how subduction zones may have initiated in the past and how oceans start to close,” Dr Duarte said.

Arctic current flowed under deep freeze of last ice age, study says

Arctic sea ice formation feeds global ocean circulation. New evidence suggests that this dynamic process persisted through the last ice age. -  National Snow & Ice Data Center
Arctic sea ice formation feeds global ocean circulation. New evidence suggests that this dynamic process persisted through the last ice age. – National Snow & Ice Data Center

During the last ice age, when thick ice covered the Arctic, many scientists assumed that the deep currents below that feed the North Atlantic Ocean and help drive global ocean currents slowed or even stopped. But in a new study in Nature, researchers show that the deep Arctic Ocean has been churning briskly for the last 35,000 years, through the chill of the last ice age and warmth of modern times, suggesting that at least one arm of the system of global ocean currents that move heat around the planet has behaved similarly under vastly different climates.

“The Arctic Ocean must have been flushed at approximately the same rate it is today regardless of how different things were at the surface,” said study co-author Jerry McManus, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory.

Researchers reconstructed Arctic circulation through deep time by measuring radioactive trace elements buried in sediments on the Arctic seafloor. Uranium eroded from the continents and delivered to the ocean by rivers, decays into sister elements thorium and protactinium. Thorium and protactinium eventually attach to particles falling through the water and wind up in mud at the bottom. By comparing expected ratios of thorium and protactinium in those ocean sediments to observed amounts, the authors showed that protactinium was being swept out of the Arctic before it could settle to the ocean bottom.From the amount of missing protactinium, scientists can infer how quickly the overlying water must have been flushed at the time the sediments were accumulating.

“The water couldn’t have been stagnant, because we see the export of protactinium,” said the study’s lead author, Sharon Hoffmann, a geochemist at Lamont-Doherty.

The upper part of the modern Arctic Ocean is flushed by North Atlantic currents while the Arctic’s deep basins are flushed by salty currents formed during sea ice formation at the surface. “The study shows that both mechanisms must have been active from the height of glaciation until now,” said Robert Newton, an oceanographer at Lamont-Doherty who was not involved in the research. “There must have been significant melt-back of sea ice each summer even at the height of the last ice age to have sea ice formation on the shelves each year. This will be a surprise to many Arctic researchers who believe deep water formation shuts down during glaciations.”

The researchers analyzed sediment cores collected during the U.S.-Canada Arctic Ocean Section cruise in 1994, a major Arctic research expedition that involved several Lamont-Doherty scientists. In each location, the cores showed that protactinium has been lower than expected for at least the past 35,000 years. By sampling cores from a range of depths, including the bottom of the Arctic deep basins, the researchers show that even the deepest waters were being flushed out at about the same rate as in the modern Arctic.

The only deep exit from the Arctic is through Fram Strait, which divides Greenland and Norway’s Svalbard islands. The deep waters of the modern Arctic flow into the North Atlantic via the Nordic seas, contributing up to 40 percent of the water that becomes North Atlantic Deep Water-known as the “ocean’s lungs” for delivering oxygen and salt to the rest of world’s oceans.

One direction for future research is to find out where the missing Arctic protactinium of the past ended up. “It’s somewhere,” said McManus. “All the protactinium in the ocean is buried in ocean sediments. If it’s not buried in one place, it’s buried in another. Our evidence suggests it’s leaving the Arctic but we think it’s unlikely to get very far before being removed.”

The key role of the oceans’ subpolar regions in the climate control of the tropics is confirmed

An international team of researchers, led by the members of the Institut de Ciència i Tecnologia Ambientals (ICTA) at the Universitat Autònoma de Barcelona (UAB), has published the first registers of the evolution of Northern Pacific and Southern Atlantic sea-surface temperatures, dating from the Pliocene Era -some 3.65 million years ago- to the present. The data obtained in the reconstruction indicate that the regions closer to the poles of both oceans have played a fundamental role in climate evolution in the tropics.

This research solves another piece of the jigsaw puzzle that is the study of oceanic behavior and its influence on climate. The results are based on the doctoral thesis presented by Dr Alfredo Martínez-García (currently, a researcher with both the Swiss Federal Institute of Technology, ETH Zurich, and with the DFG-Leibniz Centre at the University of Postdam, Germany). The thesis was undertaken at the UAB and directed by Dr Antoni Rosell Melé, an ICTA ICREA researcher and adjunct professor with the Department of Geography. This work was carried out in collaboration with Dr Gerald H. Haug, of ETH and DFG-Leibniz Centre; Dr Erin L. McClymont of Newcastle University (UK); and Dr Rainer Gersonde, of the Alfred Wegener Institute (Germany).

The study of Pliocene climate has now been the object of intense research for several years, as this era represents-in the Earth’s history-the most recent climatic period in which, over a sustained period of time, average temperatures on the planet were significantly higher than those of the present. As a result, the Pliocene is thought of as a climatic period that might be representative of the Earth’s climate in future conditions of global warming.

In this study, the researchers analyzed marine sediment collected by the Integrated Ocean Drilling Program (an international initiative), and measured its composition of organic compounds termed alkenones.

Reconstruction of the surface temperature in the Northern Pacific and Southern Atlantic has enabled a simultaneous sea-surface cooling to be identified in the subpolar regions of the two hemispheres in the period between 1.8 and 1.2 million years ago. This finding coincides in time with the formation of the equatorial Pacific cold tongue-which currently almost disappears during the “El Niño” phenomenon.

Previous studies have shown that, during the warm conditions of the Pliocene, this cold tongue was not present; thus, conditions in the equatorial Pacific were similar to those of a permanent “El Niño” episode. Data obtained in this study indicate that the cooling and expansion of polar waters towards the tropics intensified atmospheric circulation. And this fact played a fundamental role in the equatorial Pacific, leading to the reduction in depth of the thermocline-the layer of ocean water in which the temperature fall rapidly-and therefore to the appearance of the equatorial cold tongue that we can currently observe.

The research undertaken provides empirical evidence, previously suggested by studies using climatic models, that the oceans in high latitudes may play a key role in the control of tropical climate and, most especially, in the thermocline depth in the equatorial Pacific.

The study contributes to the debate on which regions on the planet are those that, when their local climates change, give rise to processes of global change. It is often indicated that these regions are found in the tropics, since, when phenomena such as “El Niño” occur, they have global repercussions. This study provides evidence for the key role that may be played by the polar regions of the planet.

Currently, high latitudes are the ones that appear to be responding in the clearest way to global warming. Given the direct relationship established in this study between high-latitude climate variation and thermocline depth in the equatorial Pacific, it appears possible that the equatorial Pacific cold tongue will eventually respond to the current warming, giving rise to a climatic scenario similar to that of the Pliocene.

Sea level is rising along US Atlantic coast, say environmental scientists

Professor Ben Horton (University of Pennsylvania)
collects salt marsh sediment in North Carolina. -  University of Pennsylvania
Professor Ben Horton (University of Pennsylvania)
collects salt marsh sediment in North Carolina. – University of Pennsylvania

An international team of environmental scientists led by the University of Pennsylvania has shown that sea-level rise along the Atlantic Coast of the United States was 2 millimeters faster in the 20th century than at any time in the past 4,000 years.

Sea-level rise prior to the 20th century is attributed to coastal subsidence. Put simply, land is being lost to subsidence as the earth continues to rise in response to the removal of the huge weight of ice sheets during the last glacial period. Using sediment cores from the U.S. Atlantic coast, researchers found significant spatial variations in land movement, with the mid-Atlantic coastlines of New Jersey, Delaware and Maryland subsiding twice as much as areas to the north and south. Coastal subsidence enhances sea-level rise, which leads to shoreline erosion and loss of wetlands and threatens coastal populations.

Researchers corrected relative sea-level data from tide gauges using the coastal-subsidence values. Results clearly show that the 20th-century rate of sea-level rise is 2 millimeters higher than the background rate of the past 4,000 years. Furthermore, the magnitude of the sea-level rise increases in a southerly direction from Maine to South Carolina. This is the first demonstrated evidence of this phenomenon from observational data alone. Researchers believe this may be related to the melting of the Greenland Ice Sheet and ocean thermal expansion.

“There is universal agreement that sea level will rise as a result of global warming but by how much, when and where it will have the most effect is unclear,” said Ben Horton, assistant professor in the Department of Earth and Environmental Science at Penn. “Such information is vital to governments, commerce and the general public. An essential prerequisite for accurate prediction is understanding how sea level has responded to past climate changes and how these were influenced by geological events such as land movements.”

The study provides the first accurate dataset for sea-level rise for the U.S. Atlantic coast, identifying regional differences that arise from variations in subsidence and demonstrate the possible effects of ice-sheet melting and thermal expansion for sea level rise.