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.”

Study links Greenland ice sheet collapse, sea level rise 400,000 years ago

A research team is hiking to sample the Greenland ice-sheet margin in south Greenland. -  (Photo by Kelsey Winsor, courtesy Oregon State University)
A research team is hiking to sample the Greenland ice-sheet margin in south Greenland. – (Photo by Kelsey Winsor, courtesy Oregon State University)

A new study suggests that a warming period more than 400,000 years ago pushed the Greenland ice sheet past its stability threshold, resulting in a nearly complete deglaciation of southern Greenland and raising global sea levels some 4-6 meters.

The study is one of the first to zero in on how the vast Greenland ice sheet responded to warmer temperatures during that period, which were caused by changes in the Earth’s orbit around the sun.

Results of the study, which was funded by the National Science Foundation, are being published this week in the journal Nature.

“The climate 400,000 years ago was not that much different than what we see today, or at least what is predicted for the end of the century,” said Anders Carlson, an associate professor at Oregon State University and co-author on the study. “The forcing was different, but what is important is that the region crossed the threshold allowing the southern portion of the ice sheet to all but disappear.

“This may give us a better sense of what may happen in the future as temperatures continue rising,” Carlson added.

Few reliable models and little proxy data exist to document the extent of the Greenland ice sheet loss during a period known as the Marine Isotope Stage 11. This was an exceptionally long warm period between ice ages that resulted in a global sea level rise of about 6-13 meters above present. However, scientists have been unsure of how much sea level rise could be attributed to Greenland, and how much may have resulted from the melting of Antarctic ice sheets or other causes.

To find the answer, the researchers examined sediment cores collected off the coast of Greenland from what is called the Eirik Drift. During several years of research, they sampled the chemistry of the glacial stream sediment on the island and discovered that different parts of Greenland have unique chemical features. During the presence of ice sheets, the sediments are scraped off and carried into the water where they are deposited in the Eirik Drift.

“Each terrain has a distinct fingerprint,” Carlson noted. “They also have different tectonic histories and so changes between the terrains allow us to predict how old the sediments are, as well as where they came from. The sediments are only deposited when there is significant ice to erode the terrain. The absence of terrestrial deposits in the sediment suggests the absence of ice.

“Not only can we estimate how much ice there was,” he added, “but the isotopic signature can tell us where ice was present, or from where it was missing.”

This first “ice sheet tracer” utilizes strontium, lead and neodymium isotopes to track the terrestrial chemistry.

The researchers’ analysis of the scope of the ice loss suggests that deglaciation in southern Greenland 400,000 years ago would have accounted for at least four meters – and possibly up to six meters – of global sea level rise. Other studies have shown, however, that sea levels during that period were at least six meters above present, and may have been as much as 13 meters higher.

Carlson said the ice sheet loss likely went beyond the southern edges of Greenland, though not all the way to the center, which has not been ice-free for at least one million years.

In their Nature article, the researchers contrasted the events of Marine Isotope Stage 11 with another warming period that occurred about 125,000 years ago and resulted in a sea level rise of 5-10 meters. Their analysis of the sediment record suggests that not as much of the Greenland ice sheet was lost – in fact, only enough to contribute to a sea level rise of less than 2.5 meters.

“However, other studies have shown that Antarctica may have been unstable at the time and melting there may have made up the difference,” Carlson pointed out.

The researchers say the discovery of an ice sheet tracer that can be documented through sediment core analysis is a major step to understanding the history of ice sheets in Greenland – and their impact on global climate and sea level changes. They acknowledge the need for more widespread coring data and temperature reconstructions.

“This is the first step toward more complete knowledge of the ice history,” Carlson said, “but it is an important one.”

New study shows: Large landmasses existed 2.7 billion years ago

A Cologne working group involving Prof. Carsten Münker and Dr. Elis Hoffmann and their student Sebastian Viehmann (working with Prof. Michael Bau from the Jacobs University Bremen) have managed for the first time to determine the isotope composition of the rare trace elements Hafnium and Neodymium in 2,700 million year-old seawater by using high purity chemical sediments from Temagami Banded Iron Formation (Canada) as an archive.

Earlier work has shown that these rocks from Canada only contain chemical elements that directly precipitated from ocean water. The Temagami Banded Iron Formation, which was formed 2,700 million years ago during the Neoarchean period, can be used as an archive because the isotopic composition of many chemical elements such as Hafnium and Neodymium directly mirrors the composition of Neoarchean seawater. These two very rare elements allow many valuable conclusions about weathering processes to be drawn.

During their investigations, the research team came to the surprising result that has been published in the renowned journal Geology: 2,700 million years ago, seawater contained an unusually high abundance of the radioactive isotope Hafnium 176 but a comparably low abundance of the radioactive isotope Neodymium 143, similar to what can be observed in present day seawater.

“In present day seawater, this can be explained by weathering and the erosion of the Earth’s exposed surface,” explains Prof. Münker. “If in the Neoarchean period 97% of the Earth’s surface had been, as estimated from computer models, covered by water, these geochemical signals would not have been found for Neoarchean seawater,” adds Dr. Hoffmann.

According to the scientific team, the new findings show that 2,700 million years ago relatively large landmasses emerged from the oceans that were exposed to weathering and erosion by the sun, wind and rain. Dr. Hoffmann: “The isotope Hafnium 176 in contrast to its counterpart Neodymium 143 was transported by means of weathering into the oceans and became part of iron-rich sediments on the sea floor 2,700 million years ago.”

The examinations were carried out in the joint clean laboratory of the Universities of Cologne and Bonn. Prof. Münker: “We are able to carry out these isotope measurements for very rare elements, the concentrations of which are in the ppb range, i.e. only a few parts per billion.”

Scientists solve a 14,000-year-old ocean mystery

At the end of the last Ice Age, as the world began to warm, a swath of the North Pacific Ocean came to life. During a brief pulse of biological productivity 14,000 years ago, this stretch of the sea teemed with phytoplankton, amoeba-like foraminifera and other tiny creatures, who thrived in large numbers until the productivity ended-as mysteriously as it began-just a few hundred years later.

Researchers have hypothesized that iron sparked this surge of ocean life, but a new study led by Woods Hole Oceanographic Institution (WHOI) scientists and colleagues at the University of Bristol (UK), the University of Bergen (Norway), Williams College and the Lamont Doherty Earth Observatory of Columbia University suggests iron may not have played an important role after all, at least in some settings. The study, published in the journal Nature Geoscience, determines that a different mechanism-a transient “perfect storm” of nutrients and light-spurred life in the post-Ice Age Pacific. Its findings resolve conflicting ideas about the relationship between iron and biological productivity during this time period in the North Pacific-with potential implications for geo-engineering efforts to curb climate change by seeding the ocean with iron.

“A lot of people have put a lot of faith into iron-and, in fact, as a modern ocean chemist, I’ve built my career on the importance of iron-but it may not always have been as important as we think,” says WHOI Associate Scientist Phoebe Lam, a co-author of the study.

Because iron is known to cause blooms of biological activity in today’s North Pacific Ocean, researchers have assumed it played a key role in the past as well. They have hypothesized that as Ice Age glaciers began to melt and sea levels rose, they submerged the surrounding continental shelf, washing iron into the rising sea and setting off a burst of life.

Past studies using sediment cores-long cylinders drilled into the ocean floor that offer scientists a look back through time at what has accumulated there-have repeatedly found evidence of this burst, in the form of a layer of increased opal and calcium carbonate, the materials that made up phytoplankton and foraminifera shells. But no one had searched the fossil record specifically for signs that iron from the continental shelf played a part in the bloom.

Lam and an international team of colleagues revisited the sediment core data to directly test this hypothesis. They sampled GGC-37, a core taken from a site near Russia’s Kamchatka Peninsula, about every 5 centimeters, moving back through time to before the biological bloom began. Then they analyzed the chemical composition of their samples, measuring the relative abundance of the isotopes of the elements neodymium and strontium in the sample, which indicates which variant of iron was present. The isotope abundance ratios were a particularly important clue, because they could reveal where the iron came from-one variant pointed to iron from the ancient Loess Plateau of northern China, a frequent source of iron-rich dust in the northwest Pacific, while another suggested the younger, more volcanic continental shelf was the iron source.

What the researchers found surprised them.

“We saw the flux of iron was really high during glacial times, and that it dropped during deglaciation,” Lam says. “We didn’t see any evidence of a pulse of iron right before this productivity peak.”

The iron the researchers did find during glacial times appeared to be supplemented by a third source, possibly in the Bering Sea area, but it didn’t have a significant effect on the productivity peak. Instead, the data suggest that iron levels were declining when the peak began.

Based on the sediment record, the researchers propose a different cause for the peak: a chain of events that created ideal conditions for sea life to briefly flourish. The changing climate triggered deep mixing in the North Pacific ocean, which stirred nutrients that the tiny plankton depend on up into the sea’s surface layers, but in doing so also mixed the plankton into deep, dark waters, where light for photosynthesis was too scarce for them to thrive. Then a pulse of freshwater from melting glaciers-evidenced by a change in the amount of a certain oxygen isotope in the foraminifera shells found in the core-stopped the mixing, trapping the phytoplankton and other small creatures in a thin, bright, nutrient-rich top layer of ocean. With greater exposure to light and nutrients, and iron levels that were still relatively high, the creatures flourished.

“We think that ultimately this is what caused the productivity peak-that all these things happened all at once,” Lam says. “And it was a transient thing, because the iron continued to drop and eventually the nutrients ran out.”

The study’s findings disprove that iron caused this ancient bloom, but they also raise questions about a very modern idea. Some scientists have proposed seeding the world’s oceans with iron to trigger phytoplankton blooms that could trap some of the atmosphere’s carbon dioxide and help stall climate change. This idea, sometimes referred to as the “Iron Hypothesis,” has met with considerable controversy, but scientific evidence of its potential effectiveness to sequester carbon and its impact on ocean life has been mixed.

“This study shows how there are multiple controls on ocean phytoplankton blooms, not just iron,” says Ken Buesseler, a WHOI marine chemist who led a workshop in 2007 to discuss modern iron fertilization. “Certainly before we think about adding iron to the ocean to sequester carbon as a geoengineering tool, we should encourage studies like this of natural systems where the conditions of adding iron, or not, on longer and larger time scales have already been done for us and we can study the consequences.”

Moon and Earth may be younger than originally thought

New research using a technique that measures the isotopes of lead and neodymium in lunar crustal rocks shows that the moon and Earth may be millions of years younger than originally thought.

The common estimate of the moon’s age is as old as 4.5 billion years old (roughly the same age as the solar system) as determined by mineralogy and chemical analysis of moon rocks gathered during the Apollo missions. However, Lawrence Livermore National Laboratory scientist Lars Borg and international collaborators have analyzed three isotopic systems, including the elements lead, samarium and neodymium found in ancient lunar rocks, and determined that the moon could be much younger than originally estimated. In fact, its age may be 4.36 billion years old.

The new research has implications for the age of Earth as well. Common belief is that the moon formed from a giant impact into the Earth and then solidified from an ocean of molten rock (magma).

“If our analysis represents the age of the moon, then the Earth must be fairly young as well,” said chemist Borg. “This is in stark contrast to a planet like Mars, which is argued to have formed around 4.53 billion years ago. If the age we report is from one of the first formed lunar rocks, then the moon is about 165 million years younger than Mars and about 200 million years younger than large asteroids.”

The isotopic measurements were made by taking samples of ferroan anorthosite (FAN), a type of moon crustal rock, which is considered to represent the oldest lunar crustal rock type.

Borg said that these analyses showed that the moon likely solidified significantly later than most previous estimates or that the long-held belief that FANs are flotation cumulates of a primordial magma ocean is incorrect.

Chemical evolution of planetary bodies ranging from asteroids to large rocky planets is thought to begin with differentiation through solidification of magma oceans hundreds of kilometers in depth. The Earth’s moon is the typical example of this type of differentiation. However, one interpretation of Borg’s findings is that this may not have occurred on the moon.

“The moon is supposed to be old and have a lunar magma ocean, but our new measurements show the moon is young and did not have a magma ocean,” Borg said.

“The isotopic measurements showed that a specific FAN yields consistent ages from multiple isotopic dating techniques and strongly suggest that the ages record the time at which the rock crystallized,” Borg said. “Other studies have not been able to do this.”