Fly away home? Ice Age may have clipped bird migration

The onset of the last ice age may have forced some bird species to abandon their northerly migrations for thousands of years, says new research led by a University of Nebraska-Lincoln ornithologist.

A study led by Nebraska's Robert Zink proposes that many bird species, such as the Canada warbler, may have 
completely stopped migrating during the last ice age [Credit: University of Nebraska-Lincoln]

Published in the journal Science Advances, the study challenges a long-held presumption that birds merely shortened their migratory flights when glaciers advanced south to cover much of North America and northern Europe about 21,000 years ago.

The study concluded that the emergence of glaciers in those regions instead acted as an "adaptive switch" that turned off migratory behavior, transforming the tropics from a cold-weather resort into a long-term residence for certain bird species.

Of the 29 long-distance migrant species examined in the study, 20 likely saw their northern breeding grounds become uninhabitable, according to models developed by the researchers. When the climate again warmed and glaciers retreated back to the Arctic, those species presumably resumed their seasonal migrations.

Lead author Robert Zink said the conclusions could alter how scientists reconstruct the history of bird migration.

"It fundamentally changes the way we study the evolution of migration and think about the migratory behavior of birds," said Zink, professor of natural resources and biological sciences at Nebraska.

Putting Migration On Ice

Researchers generally agree that, millions of years ago, many birds did not migrate from the tropics. But as the global climate began to warm, some species ventured beyond their native habitats to capitalize on better breeding and feeding opportunities afforded by the longer days and insect-rich environments of northern latitudes.

Those species eventually ventured farther and farther from their habitats, finally stopping when they reached environments that could not sustain them during the autumn and winter. They continued to migrate south when seasonal temperatures dropped and food sources waned.

In that context, Zink said his hypothesis suggests that the origin story of bird migration simply underwent multiple reboots, with the "migratory machinery" of birds halting for each of the 20 or so ice ages that have glazed Earth during the past 2.5 million years.

The University of Nebraska-Lincoln's Robert Zink has authored a new study suggesting that the last ice age 
completely halted the northerly migrations of some bird species from about 21,000 to 12,000 years ago 
[Credit: Craig Chandler, University of Nebraska-Lincoln]

"Migrations are costly and risky," said Zink, curator of zoology at the University of Nebraska State Museum. "They're costly in terms of safety, energy -- anything you can think of."

Rather than paying those costs to reach breeding grounds that the encroaching glaciers had shrunk to tiny fractions of their former size, he said, birds instead resorted to their ancestral state: tropical homebodies.

"Some of them were forced so far south that it was no longer a fitness advantage to migrate, because the extra young they could produce south of the glacier wasn't enough to compensate for the cost of migration," Zink said, "and then coming back to the tropics and re-establishing their territory.

"To some people, that's so completely off the wall that they may have trouble wrapping their heads around it -- except that it's the way they would explain to their classes the evolution of migration in the first place. So, in a sense, what I'm proposing is nothing novel. What's novel about it is that (the advent of migration) probably occurred many times."

Redrawing the Map

Zink and his co-author, the University of Minnesota's Aubrey Gardner, conducted their study using a computer model that linked the modern-day distribution of bird species with climate variables -- temperature, precipitation, seasonality -- that characterize their habitats. By comparing those climates with conditions that existed during the last ice age, the model mapped the regions that likely could have supported each of those species from about 21,000 to 12,000 years ago.

In many cases, Zink said, the model either found no habitable regions beyond the tropics or located habitats so miniscule that they would have struggled to support sizable populations of the species.

"Some species were probably just forced (slightly) south of the glaciers, and their habitats were extensive enough that they would maybe maintain some migratory ability," he said. "But for others, I think there was so little predicted habitat that they just ceased migration all together.

"This evolution of migration is a very (variable) thing. Normally, when we think of evolution, we think of singular, unique events in evolutionary history. But in this case, the ability to migrate is entrenched in birds. They have the ability to navigate using the sun, the stars, the (Earth's) magnetic field. They have the ability to put on large amounts of fat and sustain trans-gulf migrations. Birds are (adaptive) enough in their behavior and physiology that this wasn't a reinvention of some incredible phenomenon."

And if some species did transition back and forth from sedentary to migratory states, researchers should consider pruning certain evolutionary trees accordingly, Zink said. Many evolutionary trees currently treat migration as an irreversible trait rather than a variable behavior, he said, and that assumption could be misinforming discussions of when and where it evolved.

"I wanted to point out that this was a real danger and fallacy that's being committed: mapping something onto an evolutionary tree where the feature -- migration or sedentariness -- changes faster than new species evolved," he said. "You would have constructed the history of migration totally differently."

Author: Scott Schrage | Source: University of Nebraska-Lincoln [September 20, 2017]

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Mathematics predicts a sixth mass extinction

In the past 540 million years, the Earth has endured five mass extinction events, each involving processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world.

Scientists have analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass 
extinction events. They have identified 'thresholds of catastrophe' in the carbon cycle that, if exceeded, would lead
 to an unstable environment, and ultimately, mass extinction [Credit: MIT]

The question for many scientists is whether the carbon cycle is now experiencing a significant jolt that could tip the planet toward a sixth mass extinction. In the modern era, carbon dioxide emissions have risen steadily since the 19th century, but deciphering whether this recent spike in carbon could lead to mass extinction has been challenging. That's mainly because it's difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today's disruptions, which have taken place over just a little more than a century.

Now Daniel Rothman, professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences and co-director of MIT's Lorenz Center, has analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events. He has identified "thresholds of catastrophe" in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

In a paper published in Science Advances, he proposes that mass extinction occurs if one of two thresholds are crossed: For changes in the carbon cycle that occur over long timescales, extinctions will follow if those changes occur at rates faster than global ecosystems can adapt. For carbon perturbations that take place over shorter timescales, the pace of carbon-cycle changes will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event.

Taking this reasoning forward in time, Rothman predicts that, given the recent rise in carbon dioxide emissions over a relatively short timescale, a sixth extinction will depend on whether a critical amount of carbon is added to the oceans. That amount, he calculates, is about 310 gigatons, which he estimates to be roughly equivalent to the amount of carbon that human activities will have added to the world's oceans by the year 2100.

Does this mean that mass extinction will soon follow at the turn of the century? Rothman says it would take some time -- about 10,000 years -- for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into "unknown territory."

"This is not saying that disaster occurs the next day," Rothman says. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction."

History follows theory

Rothman had previously done work on the end-Permian extinction, the most severe extinction in Earth's history, in which a massive pulse of carbon through the Earth's system was involved in wiping out more than 95 percent of marine species worldwide. Since then, conversations with colleagues spurred him to consider the likelihood of a sixth extinction, raising an essential question:

"How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?" Rothman says. "So I sat down one summer day and tried to think about how one might go about this systematically."

He eventually derived a simple mathematical formula based on basic physical principles that relates the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change. He hypothesized that this formula should predict whether mass extinction, or some other sort of global catastrophe, should occur.

Rothman then asked whether history followed his hypothesis. By searching through hundreds of published geochemistry papers, he identified 31 events in the last 542 million years in which a significant change occurred in Earth's carbon cycle. For each event, including the five mass extinctions, Rothman noted the change in carbon, expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon-12 and carbon-13. He also noted the duration of time over which the changes occurred.

He then devised a mathematical transformation to convert these quantities into the total mass of carbon that was added to the oceans during each event. Finally, he plotted both the mass and timescale of each event.

"It became evident that there was a characteristic rate of change that the system basically didn't like to go past," Rothman says.

In other words, he observed a common threshold that most of the 31 events appeared to stay under. While these events involved significant changes in carbon, they were relatively benign -- not enough to destabilize the system toward catastrophe. In contrast, four of the five mass extinction events lay over the threshold, with the most severe end-Permian extinction being the farthest over the line.

"Then it became a question of figuring out what it meant," Rothman says.

A hidden leak

Upon further analysis, Rothman found that the critical rate for catastrophe is related to a hidden process within the Earth's natural carbon cycle. The cycle is essentially a loop between photosynthesis and respiration. Normally, there is a "leak" in the cycle, in which a small amount of organic carbon sinks to the ocean bottom and, over time, is buried as sediment and sequestered from the rest of the carbon cycle.

Rothman found that the critical rate was equivalent to the rate of excess production of carbon dioxide that would result from plugging the leak. Any additional carbon dioxide injected into the cycle could not be described by the loop itself. One or more other processes would instead have taken the carbon cycle into unstable territory.

He then determined that the critical rate applies only beyond the timescale at which the marine carbon cycle can re-establish its equilibrium after it is disturbed. Today, this timescale is about 10,000 years. For much shorter events, the critical threshold is no longer tied to the rate at which carbon is added to the oceans but instead to the carbon's total mass. Both scenarios would leave an excess of carbon circulating through the oceans and atmosphere, likely resulting in global warming and ocean acidification.

The century's the limit

From the critical rate and the equilibrium timescale, Rothman calculated the critical mass of carbon for the modern day to be about 310 gigatons.

He then compared his prediction to the total amount of carbon added to the Earth's oceans by the year 2100, as projected in the most recent report of the Intergovernmental Panel on Climate Change. The IPCC projections consider four possible pathways for carbon dioxide emissions, ranging from one associated with stringent policies to limit carbon dioxide emissions, to another related to the high range of scenarios with no limitations.

The best-case scenario projects that humans will add 300 gigatons of carbon to the oceans by 2100, while more than 500 gigatons will be added under the worst-case scenario, far exceeding the critical threshold. In all scenarios, Rothman shows that by 2100, the carbon cycle will either be close to or well beyond the threshold for catastrophe.

"There should be ways of pulling back [emissions of carbon dioxide]," Rothman says. "But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present."

Author: Jennifer Chu | Source: Massachusetts Institute of Technology [September 20, 2017]

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Changes in Earth's crust caused oxygen to fill the atmosphere

Scientists have long wondered how Earth's atmosphere filled with oxygen. UBC geologist Matthijs Smit and research partner Klaus Mezger may have found the answer in continental rocks that are billions of years old.

Matthijs Smit of the University of British Columbia examines ancient rocks from the 
deep crust in Norway during the summer of 2017 [Credit: Matthijs Smit]

"Oxygenation was waiting to happen," said Smit. "All it may have needed was for the continents to mature."

Earth's early atmosphere and oceans were devoid of free oxygen, even though tiny cyanobacteria were producing the gas as a byproduct of photosynthesis. Free oxygen is oxygen that isn't combined with other elements such as carbon or nitrogen, and aerobic organisms need it to live. A change occurred about three billion years ago, when small regions containing free oxygen began to appear in the oceans. Then, about 2.4 billion years ago, oxygen in the atmosphere suddenly increased by about 10,000 times in just 200 million years. This period, known as the Great Oxidation Event, changed chemical reactions on the surface of the Earth completely.

Smit, a professor in UBC's department of earth, ocean & atmospheric sciences, and colleague, professor Klaus Mezger of the University of Bern, were aware that the composition of continents also changed during this period. They set out to find a link, looking closely at records detailing the geochemistry of shales and igneous rock types from around the world -- more than 48,000 rocks dating back billions of years.

"It turned out that a staggering change occurred in the composition of continents at the same time free oxygen was starting to accumulate in the oceans," Smit said.

Before oxygenation, continents were composed of rocks rich in magnesium and low in silica -- similar to what can be found today in places like Iceland and the Faroe Islands. But more importantly, those rocks contained a mineral called olivine. When olivine comes into contact with water, it initiates chemical reactions that consume oxygen and lock it up. That is likely what happened to the oxygen produced by cyanobacteria early in Earth's history.

However, as the continental crust evolved to a composition more like today's, olivine virtually disappeared. Without that mineral to react with water and consume oxygen, the gas was finally allowed to accumulate. Oceans eventually became saturated, and oxygen crossed into the atmosphere.

"It really appears to have been the starting point for life diversification as we know it," Smit said. "After that change, the Earth became much more habitable and suitable for the evolution of complex life, but that needed some trigger mechanism, and that's what we may have found."

As for what caused the composition of continents to change, that is the subject of ongoing study. Smit notes that modern plate tectonics began at around the same time, and many scientists theorize that there is a connection.

Smit and Mezger published their findings today in the journal Nature Geoscience. The research was funded by the Natural Sciences and Engineering Research Council.

Source: University of British Columbia [September 18, 2017]

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Rapid climate changes across northern hemisphere in the earliest Middle Pleistocene

By studying climate changes that took place thousands of years ago, we can better understand the global climate system and predict Earth's future climate. A multi-organization research team led by Professor HYODO Masayuki (Research Center for Inland Seas, Kobe University) has discovered evidence of rapid climate changes on a millennial-to-centennial scale that occurred 780 to 760 thousand years ago. The findings were published in Scientific Reports.

Figure 1: Sample locations (a) Chiba section, Osaka Bay, North Pacific mid-latitude point (U1313). (b)(c) show
 the Chiba section location. (d) Location of core TB2 near the Chiba section along the Yoro River
[Credit: Kobe University]

During the 2.6 million year Quaternary Period, the climate repeated a glacial and interglacial cycle, caused by changes in the geographical distribution of solar radiation due to orbital changes including those of Earth's orbit and the tilt of its axis. These changes are regarded as "Milankovitch cycles," over 20,000 years in period. But in the Holocene and last glacial periods, a number of millennial-to-centennial scale climate changes have been observed. Such rapid climate changes have scarcely been reported before the last glacial period.

In the interglacial period between 780 and 760 thousand years ago, Earth's orbital patterns were quite similar to the current (Holocene) era, so this interglacial climate could be useful in predicting Earth's future climate.

Figure 2: Records of climate and environment between 790 and 750 thousand years ago in three areas 
[Credit: Kobe University]

The research team focused on the Kazusa Group (Chiba prefecture, Japan), which has the fastest sedimentation rate in the world for strata of that era, and obtained high resolution paleoceanic environmental records every 10 years. When combined with records from Osaka Bay and the North Atlantic, they found evidence of multiple instances of rapid warming and cooling across all three regions at the same time. The data includes the unusual phenomenon of a rapid temperature rise with cyclicity suddenly finishing with a cold event. The cold events occurred at the same time as the great iceberg flow reached mid-latitudes in the North Atlantic, so they are thought to be caused by meltwater that covered the North Atlantic Ocean.

This cyclic warming and rapid cooling repeated twice just after a geomagnetic reversal, a key event for the Early/Middle Pleistocene boundary, and a third time about 10 thousand years later. All occurred after Earth had recovered its geomagnetic strength. This shows that the second half of this interglacial period, namely the earliest stage of the Middle Pleistocene, was a time of extreme climate change when ice sheets expanded and shrunk causing changes of several meters in sea levels, repeating every 500 to 2000 years.

Figure 3: Close-up of events A,B and G,H [Credit: Kobe University]

The phenomenon of rapid temperature rises modulated by bi-centennial cycles ending with a sudden freeze only occurred during a very brief portion of this interglacial period, during the two warmest periods. There is a high possibility that this 200 year period marks the de Vries Cycle (205 years), when the climate was particularly sensitive to solar activity.

Researchers will now verify whether the same phenomenon can be observed in other regions. Evidence from the southern hemisphere will be the key to showing whether it was a global phenomenon. This discovery is very unusual among the climate warming that occurred in the past, as well as being an important key to learning about the diversity of temperature rises and understanding current global warming.

Additionally, this discovery was made in the Chiba Section (Japan), a candidate section for the Early/Middle Pleistocene era global boundary stratotype sections and points (GSSP), currently under review by the International Union of Geological Sciences. These findings provide further evidence for the academic value of the Chiba Section.

Source: Kobe University [September 12, 2017]

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Ancient tree reveals cause of spike in Arctic temperature

A kauri tree preserved in a New Zealand peat swamp for 30,000 years has revealed a new mechanism that may explain how temperatures in the Northern Hemisphere spiked several degrees centigrade in just a few decades during the last global ice age.

It's only when a human stands beside the tree stump of an ancient kauri, that you can get 
a clear sense of the size of these ancient trees [Credit: www.ancientkauriproject.com]

Unexpectedly, according to new research led by scientists from UNSW Sydney and published in Nature Communications, it looks like the origin of this warming may lie half-a-world away, in Antarctica.

Rapid warming spikes of this kind during glacial periods, called Dansgaard-Oeschger events, are well known to climate researchers. They are linked to a phenomenon known as the "bipolar seesaw," where increasing temperatures in the Arctic happen at the same time as cooling over the Antarctic, and vice versa.

Until now, these divergences in temperature at the opposite ends of Earth were believed to have been driven by changes in the North Atlantic, causing deep ocean currents, often referred to as the ocean "conveyor belt," to shut down. This led to warming in the Northern Hemisphere and cooling in the south.

But the study, which examines a specific Dansgaard-Oeschger event that occurred around 30,000 years ago, suggests Antarctica plays a role too.

The paper describes how the researchers used a detailed sequence of radiocarbon dates from an ancient New Zealand kauri tree to precisely align ice, marine and sediment records across a period of greatly changing climate.

"Intriguingly, we found that the spike in temperature preserved in the Greenland ice core corresponded with a 400-year-long surface cooling period in the Southern Ocean and a major retreat of Antarctic ice," said lead author and UNSW scientist Professor Chris Turney.

Summary of CSIRO Mk3L ensemble simulations showing the impact of a 338-year duration freshwater flux 
of 0.54 Sv into the Weddell and Ross Seas [Credit: Keele University]

"As we looked more closely for the cause of this opposite response we found that there were no changes to the global ocean circulation during the Antarctic cooling event that could explain the warming in the North Atlantic. There had to be another cause."

A clue to what might be going on if the oceans weren't involved appeared in lake sediments from the Atherton Tableland, Queensland. The sediments showed a simultaneous collapse of rain-bearing trade winds over tropical northeast Australia.

It was a curious change, so the researchers turned to climate models to see if these climate events might somehow be linked.

They started by modelling the release of large volumes of freshwater into the Southern Ocean, exactly as would happen with rapid ice retreat around the Antarctic.

Consistent with the data, they found that there was cooling in the Southern Ocean but no change in the global ocean circulation.

They also found that the freshwater pulse caused rapid warming in the tropical Pacific. This in turn led to changes to the atmospheric circulation that went on to trigger sharply higher temperatures over the North Atlantic and the collapse of rain-bearing winds over tropical Australia.

Essentially, the model showed the formation of a 20,000 km long "atmospheric bridge" that linked melting ice in Antarctica to rapid atmospheric warming in the North Atlantic.

"Our study shows just how important Antarctica's ice is to the climate of the rest of the world and reveals how rapid melting of the ice here can affect us all. This is something we need to be acutely aware of in a warming world," Professor Turney said.

It also showed how deeply the climate was linked across great distances said fellow author and climate modeller from the University of Tasmania, Dr Steven Phipps.

"Our research has revealed yet another remarkable example of the interconnections that are so much a part of our climate system," Dr Phipps said.

"By combining past records of past events with climate modelling, we see how a change in one region can have major climatic impacts at the opposite ends of Earth."

Source: University of New South Wales [September 12, 2017]

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Photosynthesis under light conditions different from the Earth

Researchers at the Astrobiology Center (ABC) of National Institutes of Natural Science (NINS) in Japan and their colleagues have proposed that Earth-like red-edge reflection patterns could be observed on exoplanets around M-dwarfs. They point out that the first oxygenic phototrophs are most likely to have evolved underwater to utilize visible light, as occurred in the primordial ocean on Earth.

Artists impressions of a habitable planet around M-dwarfs (left) and primordial Earth (right). The surface of M-dwarf 
planet is illuminated by visible light. On the other hand, similar light conditions are expected underwater, since only 
blue-green light can penetrate meters of water [Credit: Astrobiology Center]

M-dwarfs or red dwarfs are small (0.5-0.1 solar-masses) and cool ( ~3000 Kelvin) stars, and are abundant in universe. The sun-like stars are considered plausible targets for searching habitable exoplanets. However, nearby M-dwarfs are becoming the most extensive targets for habitable planet searches because they are the most abundant nearby stars and thus could be the first candidate for detecting biosignatures on exoplanets via transit or direct imaging observations in near future.

One of the most important exoplanetary biosignatures is a specific reflection pattern on the land surface called "red-edge," which is caused by vegetation such as forests and grasslands. On the Earth, red-edge appears between red and infrared (IR) wavelengths, since red light is absorbed for photosynthesis while IR radiation is reflected. In previous studies, it was predicted that red-edge position on exoplanets should be decided by the radiation spectrum of nearby stars. Around M-dwarfs, red-edge was expected to be shifted to a longer wavelength, since planets on the exoplanets use abundant IR radiation for photosynthesis.

Researchers at the Astrobiology Center (ABC) of National Institutes of Natural Science (NINS) and their colleagues have proposed an alternative prediction that red-edge could be observed as on the Earth even on exoplanets around M-dwarfs in the online journal Scientific Reports. They point out that the first oxygenic phototrophs are most likely to have evolved underwater to utilize visible light just as in the primordial ocean on the Earth. They examined light adaptation mechanisms of visible and IR radiation-using phototrophs required for adapting to land habitats and found that IR-using phototrophs struggle to adapt to changing light condition at the boundary of water and land. Kenji Takizawa, read author of the study, said "It is too risky to utilize IR-radiation during water-to-land evolution."

Source: National Astronomical Observatory of Japan [September 12, 2017]

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