Stalagmites store paleoclimate data

The North Atlantic Oscillation (NAO) is the dominant atmospheric pressure mode over the North Atlantic that plays a significant role in determining the winter climate in Europe.

Zoolithen Cave in Burggaillenreuth with flowstones, stalactites, and stalagmites
[Credit: Jasper Wassenburg]

Depending on the prevailing state of the NAO, Europe experiences mild or very cold winters and even strong storms. Geoscientists based at Johannes Gutenberg University Mainz (JGU) in Germany are currently reconstructing the fluctuations of the NAO over the last 10,000 years with the aim of being able to predict future developments.

For this purpose, they use stalagmites obtained from subterranean caves as natural climate archives and are examining new indicators of climate change to retrieve climate information that is as accurate as possible. Initial results indicate that it is likely that the NAO will respond to the melting of the Arctic ice cap in the future, with consequences for our climate, environment, and society as a whole.

Dr. Jasper Wassenburg works with stalagmites from caves in the Middle Atlas, a mountain range in the northwest of Morocco. Stalagmites are calcium carbonate deposits that grow from the floor of a cave upwards due to precipitation of calcium carbonate minerals deposited from the dripwater. Calcite is the most common form of calcium carbonate although in some cases it can also be aragonite. "Aragonite, if well preserved, can be dated with remarkable precision. So we prefer aragonite stalagmites over calcitic ones," explained Wassenburg, who is a member of the research team headed by Professor Denis Scholz at the Institute of Geosciences at Mainz University.

The incorporation of chemical elements in speleothems, which is the term scientists use for the secondary mineral deposits in caves, is often depending on changes in the environment. These elements are known as climate proxies because they provide indirect evidence of climatic history. Wassenburg's study of seven speleothem samples obtained from Morocco, India, France, Spain, and a cave known as the Hüttenbläserschachthöhle in Germany's Sauerland region is the first attempt to identify in detail the concentrations at which trace elements tend to be incorporated in aragonite. "We have been able to demonstrate that the concentration of uranium in aragonite stalagmites is a very precise indicator of prehistoric rainfall patterns," he added. This means that stalagmites can tell us qualitatively how much it rained 200,000 years ago.

The Bab Mafraque cave in the Middle Atlas in Morocco with flowstones and stalactites 
[Credit: Jasper Wassenburg]

Reconstruction of the North Atlantic Oscillation as far back as the onset of the current interglacial period

Uranium and strontium concentrations and the relative ratios of oxygen isotopes were also analyzed in order to obtain information on past rainfall for a recent study of past NAO variability. The NAO index reflects the difference in atmospheric pressure between the Icelandic Low to the north and the Azores High to the south. One particular phenomenon of interest is that if the NAO brings dry weather to Europe, it rains in Morocco—and vice versa. The weather of the northwestern region of Morocco seems to react particularly sensitive to changes in the NAO. In this case, the samples used by Dr. Jasper Wassenburg came from a fairly small cave in which the host rock is dolomite. The Grotte de Piste is located in the Atlas Mountains at an elevation of some 1,250 meters above sea level. It is 70 to 80 meters in extent and 15 to 20 meters from floor to ceiling.

The results of analysis of the speleothems from the north-west of Morocco were compared with a rainfall reconstruction obtained from other cave deposits from the Bunkerhöhle or Bunker cave in western Germany. This enabled the climate researchers to trace back the fluctuations of the NAO over the past 11,000 years to the end of the last Ice Age. The best reconstruction previously available went back only 5,200 years. "We were surprised to discover that the situation during the early Holocene 11,000 years ago was quite different to that of today. The weather regimes in Europe and Morocco seem to have behaved similarly so that wet weather in Europe also meant more rain in Morocco," explained Wassenburg. This positive correlation disappeared at some point during the transition from the early Holocene to the mid-Holocene.

The researchers postulate that this was attributable to a major reduction in the melt water contribution from the Laurentide Ice Sheet that still covered large areas of North America at the end of the Ice Age. "The pattern of the North Atlantic Oscillation is not as stable as we thought," stated Professor Dennis Scholz and added that the NAO will probably also be influenced by today's melting of the Greenland Ice Sheet, with potential effects on the atmosphere, the oceans, and other biological phenomena, including farming and fishing. The team plans to conduct further research in order to reconstruct the changes of the NAO over the last 10,000 years.

The findings are published in Geochimica et Cosmochimica Acta and Nature Geoscience.

Source: University of Mainz [April 11, 2017]

byUnknown

in Ancient Environment, Climate Change, Earth Science, Geology, Germany, Morocco, Palaeontology

April 11, 2017

Stalagmites store paleoclimate data

The North Atlantic Oscillation (NAO) is the dominant atmospheric pressure mode over the North Atlantic that plays a significant role in dete...

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Could a Colorado earthquake have been triggered by dinosaur extinction impact?

Researchers have found signs of fault displacement at well-known rock outcrops in Colorado that mark the end-Cretaceous asteroid impact that may have hurried the extinction of the dinosaurs. They will present their results in a poster at the 2017 Seismological Society of America's (SSA) Annual Meeting.

Longs Canyon area of Colorado's Trinidad Lakes State Park [Credit: WikiCommons]

Norm Sleep of Stanford University and colleagues suggest that the impact, which occurred near the Yucatán Peninsula of Mexico, could have generated massive seismic waves that triggered earthquakes as far away as Colorado, in the center of a tectonic plate where no previous fault had existed.

Sleep and his colleagues found evidence for the fault in two areas in Colorado's Trinidad Lakes State Park, where a layer of iridium generated by the asteroid impact clearly marks the boundary between Cretaceous and Tertiary-age rocks, at the time of the dinosaurs' extinction about 65 million years ago. At the Long's Canyon and Madrid Canyon roadcuts, "there is a fault that slipped about a meter at the time of the impact," Sleep said. "It offset the material below the impact layer but not above, but it's not something that would be obvious to the casual observer."

The researchers suggest that the Colorado earthquake may have been as large as magnitude 6. Very strong seismic waves from the impact -- much larger than would be generated by a regular earthquake, Sleep said -- would be necessary to trigger an earthquake in this location, in the middle of a tectonic plate with no previous faults.

The end-Cretaceous asteroid strike, however, could have generated ground velocities of a meter or two per second, Sleep said. "The ground would be moving up and down and sideways like a ship in a strong storm."

At the time of the earthquake, the area in Colorado was a swampy, delta-like environment, crossed by large braided streams that ran from the young Rocky Mountains. Sleep and his colleagues saw signs that the earthquake had diverted a small stream in the area.

This summer, the researchers will be checking in New Mexico near the Raton Basin for further signs of intraplate quakes that may have been triggered by the asteroid strike.

Source: Seismological Society of America [April 11, 2017]

byUnknown

in Dinosaurs, Earth Science, Geology, North America, Palaeontology, USA

April 11, 2017

Could a Colorado earthquake have been triggered by dinosaur extinction impact?

Researchers have found signs of fault displacement at well-known rock outcrops in Colorado that mark the end-Cretaceous asteroid impact that...

Read More

'Nesting doll' minerals offer clues to Earth's mantle dynamics

Recovered minerals that originated in the deep mantle can give scientists a rare glimpse into the dynamic processes occurring deep inside of Earth and into the history of the planet's mantle layer. A team led by Yingwei Fei, a Carnegie experimental petrologist, and Cheng Xu, a field geologist from Peking University, has discovered that a rare sample of the mineral majorite originated at least 235 miles below Earth's surface. Their findings are published by Science Advances.

The fragment of the metamorphic rock eclogite in which the garnet that encased the ferric-iron-rich majorite 
sample was found in Northern China [Credit: Courtesy of Yingwei Fei]

Majorite is a type of garnet formed only at depths greater than 100 miles. Fascinatingly, the majorite sample Fei's team found in Northern China was encased inside a regular garnet -- like mineralogical nesting dolls. It was brought to surface as an eclogite xenolith in the North China Craton, one of the oldest cratonic blocks in the world. What's more, the majorite was rich in ferric iron, an oxidized form of iron, which is highly unusual for the mineral.

All of these uncommon factors prompted the team to investigate the majorite's origins.

They used several different kinds of analytical techniques to determine the chemistry and structural characteristics of this majorite formed deep inside Earth. In order to determine the exact depth of its origin, Carnegie's postdoc Renbiao Tao conducted high-pressure experiments that mimicked the formation conditions of natural majorite. The team pinpointed its origin to a depth of nearly 250 miles (400 kilometers), at the bottom of the soft part of the upper mantle, called the asthenosphere, which drives plate tectonics.

It is extremely unusual that a high-pressure majorite could survive transportation from such a depth. Adding to the strange circumstances is the fact that it was later encased by a garnet that formed at a much shallower depth of about 125 miles (200 kilometers). The nesting-doll sample's existence required two separate geological events to explain, and these events created a time capsule that the researchers could use to better understand Earth's deep history.

"This two-stage formation process offers us important clues about the mantle's evolutionary stage at the time when the majorite was first formed," Fei explained.

The sample's location and depth of origin indicate that it is a relic from the end of an era of supercontinent assembly that took place about 1.8 billion years ago. Called Columbia, the supercontinent's formation built mountain ranges that persist today.

"More research is needed to understand how the majorite became so oxidized, or rich in ferric iron, and what this information can tell us about mantle chemistry. We are going back to the site this summer to dig deeper trenches and hope to find fresh rocks that contain more clues to the deep mantle," Fei added.

Source: Carnegie Institution for Science [April 07, 2017]

byUnknown

in China, Earth Science, Geology, Palaeontology

April 07, 2017

'Nesting doll' minerals offer clues to Earth's mantle dynamics

Recovered minerals that originated in the deep mantle can give scientists a rare glimpse into the dynamic processes occurring deep inside of...

Read More

Tibet sediments reveal climate patterns from late Miocene, six million years ago

The Tibetan Plateau in China experiences the strongest monsoon system on Earth, with powerful winds—and accompanying intense rains in the summer months—caused by a complex system of global air circulation patterns and differences in surface temperatures between land and oceans.

Stratification in Tibet sediment. Climate variations are reflected in color variations with the red sediment typically
 indicating a wetter climate and the white indicating a drier climate. "You can literally walk up time 
as you sample the sediment," Garzione says [Credit: Qingquan Meng]

These extreme weather patterns make this area an ideal location for climate scientists to study the delicate interconnected web of the global climate system.

Carmala Garzione, a professor of earth and environmental sciences at the University of Rochester, and Junsheng Nie, a visiting research associate at the University, surveyed sediment samples from the northern Tibetan Plateau's Qaidam Basin and were able to construct paleoclimate cycle records from the late Miocene epoch of Earth's history, which lasted from approximately 11 to 5.3 million years ago. They recently published their findings in Science Advances.

Reconstructing past climate records can help scientists determine both natural patterns and the ways in which future glacial events and greenhouse gas emissions may affect global systems.

Based on previous research on ice core, marine, and sediment records, researchers determined that for the past 800,000 years, Northern Hemisphere ice ages—in which vast areas of North America, Europe, and Asia are covered with thick sheets of ice—occurred about every 100,000 years. Prior to that period, ice ages occurred more frequently, on cycles of 41,000 years, and scientists believed this was the norm.

Using the sediment samples from the Qaidam Basin, Nie and Garzione show that the East Asian monsoon patterns in the late Miocene also follow similar 100,000 year cycles, with stronger monsoons peaking at 100,000 years and diminishing in the periods in between. This reveals a greater than 6 million earlier onset of these 100,000 year cycles than was previously documented.

"People have been thinking that the 100,000 year cycle was a later Quaternary [present-day] climate anomaly," Nie says. "But from our results, we see that it's not an anomaly, it was present many years before."

Several factors affect these cycles, but they are ultimately determined by orbital forcing—the Sun's radiation received by the Earth due to variations in the Earth's orbit in the solar system. There are three types of variations that occur simultaneously, known as the Milankovitch Cycles:

1. Eccentricity: How the Earth rotates around the Sun—the shape of Earth's orbit gradually changes from being more oval to more round over a period of 100,000 years.
2. Axial tilt: The Earth tilts toward the Sun at an angle that changes from an approximate 22-degree tilt to a 24.5-degree tilt over a period of 41,000 years.
3. Precession of equinox: The Earth slowly wobbles as it spins, much like a toy top, while at the same time, the Earth's rotational axis—the line from the north to south poles—rotates. The interaction of these two processes results in cyclical movement of equinoxes over a period of approximately 23,000 years.

"Each of these factors influences incoming solar radiation and how the earth is absorbing heat,"Garzione says.

Mysteries remain because eccentricity is the weakest cycle, so should logically not be the dominant cycle for climatic events. It is not only sunlight that plays a role in these cycles, but the influence of glaciers and atmospheric carbon dioxide.

For the past one million years, the waxing and waning of Northern Hemisphere ice sheets—mainly those in Canada—have controlled the climate cycles, by affecting ocean currents, temperatures, and wind patterns. Southern Hemisphere ice in Antarctica has remained relatively fixed, without any major glacial melting to catalyze advances and retreats.

During the late Miocene, this was the opposite, with ice in Antarctica in the Southern Hemisphere waxing and waning. Nie and Garzione suggest that the fluctuating Antarctic ice sheet in the late Miocene, at a time when there was minimal ice in the Northern Hemisphere, exerted the dominant control on the 100,000 year cycles observed in the Qaidam Basin record.

"If one hemisphere sees major advances and retreats in ice sheets, that's when we get into this pattern of 100,000 year cycles dominating," Garzione says. "The question is, will we push carbon dioxide high enough in the future that the Northern Hemisphere remains ice free and the advances and retreats begin again with the Southern Hemisphere ice sheets."

If so, the Southern Hemisphere ice sheets may once again exert dominant influence on climate cycles.

Source: University of Rochester [April 06, 2017]

byUnknown

in Asia, Earth Science, Geology, Palaeontology, Tibet

April 06, 2017

Tibet sediments reveal climate patterns from late Miocene, six million years ago

The Tibetan Plateau in China experiences the strongest monsoon system on Earth, with powerful winds—and accompanying intense rains in the su...

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Study finds ancient Earth's fingerprints in young volcanic rocks

Earth's mantle is made of solid rock that nonetheless circulates slowly over millions of years. Some geologists assume that this slow circulation would have wiped away any geochemical traces of Earth's early history long ago. But a new study led by University of Maryland geologists has found new evidence that could date back more than 4.5 billion years.

A fountain of lava erupts from Hawaii's Kilauea Iki crater on Dec. 5, 1959. Two rock samples from this eruption 
contain geochemical anomalies that could date back 4.5 billion years, shortly after the Earth first formed 
[Credit: USGS/J.P. Eaton]

The authors of the research paper, published in the journal Science, studied volcanic rocks that recently erupted from volcanoes in Hawaii and Samoa. The rocks contain surprising geochemical anomalies -- the "fingerprints" of conditions that existed shortly after the planet formed.

The researchers are not yet sure how Earth's mantle preserved these anomalies. But the group's results suggest that some of these rocks contain material that survived through all of Earth's history -- and that the planet's interior may not be well mixed after all.

"We found geochemical signatures that must have been created nearly 4.5 billion years ago," said Andrea Mundl, a postdoctoral researcher in geology at UMD and the lead author of the study. "It was especially exciting to find these anomalies in such young rocks. We don't yet know how these signatures survived for so long, but we have some ideas."

The anomalous signatures are found in the ratios of key isotopes of two elements: tungsten and helium.

In the case of tungsten, which has many isotopes, the important ratio is tungsten-182 to tungsten-184. The heavier isotope, tungsten-184, is stable and has existed since the planet first formed. Tungsten-182, on the other hand, results from the decay of hafnium-182, which is highly unstable. All naturally occurring hafnium-182 decayed within the first 50 million years of Earth's history, leaving tungsten-182 in its place.

Tungsten and hafnium behaved very differently during the planet's first 50 million years. Tungsten tends to associate with metals, so most of it migrated to Earth's core, while hafnium, which tends to associate with silicate minerals, stayed in Earth's mantle and crust. Most of the rocks on Earth have a similar ratio of tungsten-182 to tungsten-184, and this ratio serves as a global baseline. Geologists can learn a lot from rocks with an unusually high or low amount of tungsten-182 -- which indicates how much hafnium-182 was present in the rock long ago.

"Nearly all of these anomalies formed within the first 50 million years after the solar system formed," Mundl said. "Higher than normal levels of tungsten-182 are seen in very old rocks that most likely contained a lot of hafnium long ago. But lower levels of tungsten-182 are rare, and resemble what we might expect to see deep beneath the surface, in or near the planet's metallic core."

Sure enough, Mundl and her colleagues observed an unusually low amount of tungsten-182 in some of the rocks from Hawaii and Samoa. On its own, the tungsten isotope ratio is interesting, but not enough to make any convincing conclusions. But the researchers also observed that the same rocks contain an unusual ratio of helium isotopes.

Helium-3 is extremely rare on Earth, and tends to show up in samples of rock that have not been melted or otherwise recycled since the planet first formed. Helium-4, on the other hand, can form from the radioactive decay of uranium and thorium. A higher than normal ratio of helium-3 to helium-4 typically indicates very old rocks that have not been significantly altered since the planet formed.

"Variations in the isotopic composition of helium have been long known, but have never been correlated with other geochemical parameters," said Richard Walker, professor and department chair of geology at UMD and a co-author of the paper. "Rocks with high helium-3 to helium-4 ratios have commonly been speculated to contain 'primitive' mantle material, but how primitive was not known. Our tungsten data show that it is very primitive indeed, with the source region most likely forming within the first 50 million years of solar system history."

Mundl, Walker and their co-authors suggest a few different scenarios that could have produced the tungsten and helium anomalies they observed in volcanic rocks from Hawaii and Samoa. Perhaps the volcanoes are drawing material from Earth's core, where the ratios are expected to favor low tungsten-182 and high helium-3.

Alternatively, the rocky outer surface of Earth might have formed in patches, with vast magma oceans in between. Parts of these magma oceans may have crystallized and sunk to the boundary between the mantle and the core, preserving the ancient tungsten and helium signatures.

"Each of these scenarios contain some inconsistencies that we can't yet explain," Mundl said. "But this is an exciting result that is sure to generate lots of interesting new research questions."

Source: University of Maryland [April 06, 2017]

byUnknown

in Earth Science, Geology, Palaeontology

April 06, 2017

Study finds ancient Earth's fingerprints in young volcanic rocks

Earth's mantle is made of solid rock that nonetheless circulates slowly over millions of years. Some geologists assume that this slow ci...

Read More

Brexit 1.0: Scientists find evidence of Britain's original separation from Europe

Researchers have found evidence of how ancient Britain separated from Europe, which happened in two stages, they report in Nature Communications.

An illustration of what the land bridge connecting Britain to Europe may have looked like before the formation of the 
Dover Strait. The foreground is around where the port of Calais is today and way in the distance (the background 
of this illustration) is early Britain. Huge waterfalls cascading over the land bridge represents the beginning 
of physical separation of Britain from Europe [Credit: Imperial College London/Chase Stone]

Nearly 450,000 years ago, when Earth was in the grip of an ice age, ice stretched right across the North Sea, from Britain to Scandinavia. The low sea levels meant that the entire English Channel was dry land, a frozen tundra landscape, crisscrossed by small rivers.

Britain's separation from mainland Europe is believed to be the result of spill over from a proglacial lake - a type of lake formed in front of an ice sheet - in the North Sea, but this has remained unproven. Now, researchers from Imperial College London and their colleagues from institutes in Europe show that the opening of the Dover Strait in the English Channel occurred in two episodes, where an initial lake spill over was followed by catastrophic flooding.

Ten years ago, the researchers from Imperial College London revealed geophysical evidence of giant valleys on the seafloor in the central part of English Channel. They believed these valley networks were evidence of a megaflood gouging out the land, which they speculated may have been caused by a catastrophic breach in a chalk rock ridge joining Britain to France.

The new study by the team, working with their colleagues in Europe, now shows for the first time the details of how this chalk ridge in the Dover Strait, between Dover and Calais, was breached. New geophysical data collected by colleagues from Belgium and France has been combined with seafloor data from the UK showing evidence of huge holes and a valley system located on the seafloor.

The team show that the chalk ridge acted like a huge dam and behind it was a proglacial lake. This lake was first hypothesised by scientists more than 100 years ago and the authors of today's study show how the lake overflowed in giant waterfalls, eroding the rock escarpment, weakening it and eventually causing it to fail and release huge volumes of water onto the valley floor below.

Bathymetry map of the Dover Strait showing prominent valley eroded through center of Strait. Note the rock ridge made 
of chalk in southern Britain and northern France which would have connected across Strait prior to breaching 
[Credit: Imperial College London]

The team believe that the huge holes that they analysed on the seafloor are plunge pools, created when water cascading over an escarpment hit the ground and eroded rock. The plunge pools in the Dover Strait are huge - up to several kilometres in diameter and around 100 metres deep and were drilled into solid rock. Around seven plunge pools run in a line from the ports of Calais to Dover. The researchers suggest these plunge pools are evidence of an overflow of water from the lake in the southern North Sea.

The straight line of the plunge pools suggests they were cascading off one single rock ridge perhaps 32 kilometres long and 100 metres high- the land bridge between Europe and the UK.

The researchers have also found evidence that a second event fully opened the Dover Strait. Later on, perhaps hundreds of thousands of years later, a new valley system, the Lobourg Channel, was carved by megaflood processes that crossed the Dover Strait. The researchers demonstrate that this valley system is connected to the giant valley network in the central English Channel. They suggest that a spill over of other, smaller lakes in front of the ice sheets in the North Sea may have been responsible for the later episode of flood erosion.

It has taken ten years, but by pulling all the pieces of the geological jigsaw puzzle together the team say they are more confident about what may have caused the megaflood in the English Channel thousands of years ago.

Dr Jenny Collier, a co-author of the study from the Department of Earth Science and Engineering at Imperial College London, said: "Based on the evidence that we've seen, we believe the Dover Strait 450,000 years ago would have been a huge rock ridge made of chalk joining Britain to France, looking more like the frozen tundra in Siberia than the green environment we know today. It would have been a cold world dotted with waterfalls plunging over the iconic white chalk escarpment that we see today in the White Cliffs of Dover.

3-D perspective view of bathymetry in Dover Strait showing prominent valley in central part of Strait and the chalk 
escarpment in southern Britain that would have connected to northwest France prior to breaching of the Strait. 
The Strait is ~33 km in width [Credit: Imperial College]

"We still don't know for sure why the proglacial lake spilt over. Perhaps part of the ice sheet broke off, collapsing into the lake, causing a surge that carved a path for the water to cascade off the chalk ridge. In terms of the catastrophic failure of the ridge, maybe an earth tremor, which is still characteristic of this region today, further weakened the ridge. This may have caused the chalk ridge to collapse, releasing the megaflood that we have found evidence for in our studies."

Engineers first found evidence of the plunge pools when they were carrying out geological surveys of the Dover Strait seafloor back in the 1960s. No one knew what caused them, but they were called the Fosse Dangeard. The loose gravel and sand infilling these plunge pools meant that the engineers had to move the route of the Channel Tunnel to avoid them. In 1985 a marine geologist named Professor Alec Smith, from Bedford College in London, first proposed that the holes were created by ancient waterfalls, but the lack of hard evidence meant that the assertions were largely forgotten. Now, the authors of today's study say Smith's original assertions were right.

The scientists say if it wasn't for a set of chance geological circumstances, Britain may have still remained connected to mainland Europe, jutting out into the sea similarly to Denmark.

Professor Sanjeev Gupta, a co-author from the Department of Earth Science and Engineering at Imperial, added: "The breaching of this land bridge between Dover and Calais was undeniably one of the most important events in British history, helping to shape our island nation's identity even today. When the ice age ended and sea levels rose, flooding the valley floor for good, Britain lost its physical connection to the mainland. Without this dramatic breaching Britain would still be a part of Europe. This is Brexit 1.0 - the Brexit nobody voted for."

The team still do not have an exact timeline of events. In the next step, the researchers would like to take core samples of the in-filled sediments in the plunge pools, which they will analyse to determine the timing of erosion and infill of the plunge pools, the environments represented by these sediments, and the source of the sediments. Developing a timeline of events would enable them to learn more about the distinctive evolution of Britain, compared to mainland Europe. However, this will be a real challenge for the team as getting sediment core samples in the Dover Strait means dealing with huge tidal changes and traversing the world's busiest shipping lane.

Source: Imperial College London [April 04, 2017]

byUnknown

in Earth Science, Europe, Geology, Palaeontology, UK, Western Europe

April 04, 2017

Brexit 1.0: Scientists find evidence of Britain's original separation from Europe

Researchers have found evidence of how ancient Britain separated from Europe, which happened in two stages, they report in Nature Communicat...

Read More

Future CO2 and climate warming potentially unprecedented in 420 million years

New research led by the University of Southampton suggests that, over the next 100 to 200 years, carbon dioxide concentrations in the Earth's atmosphere will head towards values not seen since the Triassic period, 200 million years ago. Furthermore, by the 23rd century, the climate could reach a warmth not seen in 420 million years.

A living Ginkgo leaf (left) and fossil (right). Density of stomata in such leaves is proxy of atmospheric CO2 in past 
[Credit: Dana Royer]

The study, published in Nature Communications, compiled over 1200 estimates of ancient atmospheric carbon dioxide (CO2) concentrations to produce a continuous record dating back nearly half a billion years. It concludes that if humanity burns all available fossil fuels in the future, the levels of CO2 contained in the atmosphere may have no geologically-preserved equivalent during this 420 million year period.

The researchers examined published data on fossilised plants, the isotopic composition of carbon in soils and the oceans, and the boron isotopic composition of fossil shells. Gavin Foster, lead author and Professor of Isotope Geochemistry at the University of Southampton, explains: "We cannot directly measure CO2 concentrations from millions of years ago. Instead we rely on indirect 'proxies' in the rock record. In this study, we compiled all the available published data from several different types of proxy to produce a continuous record of ancient CO2 levels."

This wealth of data shows that CO2 concentrations have naturally fluctuated on multi-million year timescales over this period, from around 200-400 parts per million (ppm) during cold 'icehouse' periods to up to 3000 ppm during intervening warm 'greenhouse' periods. Although evidence tells us our climate has fluctuated greatly in the past (with the Earth currently in a colder period), it also shows the current speed of climate change is highly unusual.

Carbon dioxide is a potent greenhouse gas and in the last 150 years humanity's fossil fuel use has increased its atmospheric concentration from 280 ppm in the pre-industrialisation era to nearly 405 ppm in 2016. However, it's not just CO2 that determines the climate of our planet, ultimately it is both the strength of the greenhouse effect and the amount of incoming sunlight that is important. Changes in either parameter are able to force climate change.

"Due to nuclear reactions in stars, like our sun, over time they become brighter," adds co-author Dan Lunt, Professor of Climate Science at the University of Bristol. "This means that, although carbon dioxide concentrations were high hundreds of millions of years ago, the net warming effect of CO2 and sunlight was less. Our new CO2 compilation appears on average to have gradually declined over time by about 3-4 ppm per million years. This may not sound like much, but it is actually just about enough to cancel out the warming effect caused by the sun brightening through time, so in the long-term it appears the net effect of both was pretty much constant on average."

This interplay between carbon dioxide and the sun's brightness has fascinating implications for the history of life on Earth. Co-author Professor Dana Royer, from Wesleyan University in the US, explains: "Up until now it's been a bit of a puzzle as to why, despite the sun's output having increased slowly over time, scant evidence exists for any similar long-term warming of the climate. Our finding of little change in the net climate forcing offers an explanation for why Earth's climate has remained relatively stable, and within the bounds suitable for life for all this time."

This long-term view also offers a valuable perspective on future climate change. It is well recognised that the climate today is changing at rates well above the geological norm. If humanity fails to tackle rising CO2 and burns all the readily available fossil fuel, by AD 2250 CO2 will be at around 2000 ppm - levels not seen since 200 million years ago.

Professor Foster adds: "However, because the Sun was dimmer back then, the net climate forcing 200 million years ago was lower than we would experience in such a high CO2 future. So not only will the resultant climate change be faster than anything the Earth has seen for millions of years, the climate that will exist is likely to have no natural counterpart, as far as we can tell, in at least the last 420 million years."

Source: University of Southampton [April 04, 2017]

byUnknown

in Ancient Environment, Climate Change, Environment, Fossils, Geology, Natural Heritage

April 04, 2017

Future CO2 and climate warming potentially unprecedented in 420 million years

New research led by the University of Southampton suggests that, over the next 100 to 200 years, carbon dioxide concentrations in the Earth...

Read More

Rock exposed in World War I trenches offers new fossil find

An unusual fossil find is giving scientists new ideas about how some of the earliest animals on Earth came to dominate the world's oceans.

A dark patch of rock on the left marks the Rauchkofel Boden trench, dug in the Cardiola Formation 
of the Austrian Alps during World War I. Many fossils have been found at the site
[Credit: Annalisa Ferretti, University of Modena and Reggio Emilia]

An international research team found 425-million-year-old fossilized remnants of juvenile crinoids, a distant ancestor of today's sea lilies, encased in iron oxide and limestone in the Austrian Alps.

Researchers collected the rock from a formation on the border between Italy and Austria known as the Cardiola Formation, which was exposed in trenches dug during World War I.

Crinoids were abundant long ago, when they carpeted the sea floor. Most stalked crinoid fossils depict spindly, plantlike animals anchored to sea floor rocks, explained William Ausich, professor of earth sciences at The Ohio State University and co-author of the study in the open-access journal Geologica Acta.

Fossils of juvenile crinoids are rare, he said.

Rarer still is that these newly uncovered crinoids weren't attached to rocks when they died. Whatever they were attached to during their young lives didn't survive fossilization.

Microscope image of one fossilized holdfast from a juvenile crinoid discovered in the Alps 
[Credit: The Ohio State University]

"The fossils indicate that they were either attached to objects floating in the water at the time, or attached to another bottom dweller that lacked preservable hard parts," said Ausich said.

They might have clung to free-floating algae beds or swimming cephalopods, either of which could have carried them far away from where they formed as larvae.

Modern sea lilies reproduce by ejecting sperm and eggs into the water. Larvae grow into free-floating juvenile animals and eventually attach to the ocean bottom, where they grow to adulthood within 18 months.

At least, that's what sea lilies do today. This fossil find suggests that their distant ancestors sometimes settled on objects that carried them far from home before they reached reproductive age.

"We now have important information about the behavior of these ancient organisms, and a clue as to why they had such a wide geographic distribution," Ausich said.

Holdfasts from very young crinoids that lived 450 million years ago, fossilized in iron oxide. An international team 
of researchers discovered the tiny fossils, measuring one to four millimeters across, in rock exposed 
by World War I trenches dug in the Alps [Credit: The Ohio State University]

With long, stem-like bodies topped with feathery fronds, crinoids resembled flowers, though the center of the "flower" was a mouth, and the "petals" were arms that captured plankton for food. At the other end of the creature was star-shaped organ called a holdfast, which gripped the seafloor.

While some of today's sea lilies are able to detach their holdfasts from the seafloor and walk short distances on their arms, they don't do it often. If their crinoid ancestors spent their entire adult lives similarly anchored to one spot, they couldn't have spread worldwide without help.

Fossilized holdfasts are all that remain of the young crinoids uncovered in the Alps, and that's not unusual, Ausich said.

"The hard part about studying the fossils that I study is that they need to be buried alive in order to be completely preserved," he explained. "Crinoids and other echinoderms have a skeleton comprised of innumerable individual calcite plates held together by various connective soft tissues. These tissues begin to decompose within a day of an organism's death. So, having only parts [of crinoids] rather than whole organisms is actually the norm -- as frustrating as that may be."

The sediment that eventually covered these young crinoids must have been rich in iron, because the holdfasts were preserved as minerals of iron oxide -- and that detail is unusual, he added.

Today, the fossil holdfasts look like rusty star-shaped rings. The stars measure only 1 to 4 millimeters across, meaning they came from very young, post-larval juveniles.

The tiny fossils might have been hard to isolate from the surrounding rock, but researchers were able to take advantage of the presence of iron oxide to dissolve the limestone and pull the fossils from the resulting slurry with a magnet.

Researchers had actually collected rock samples from the Cardiola Formation long ago, Ausich said. The area contains abundant fossils, including ancient corals and trilobites. But only recently did anyone discover that these particular rock samples also contained the crinoid holdfasts.

Researchers are interested in crinoids not just because they're part of Earth's history, but because the various crinoid species were able to survive millions of years of climate changes to become the sea lilies we know today.

Author: Pam Frost Gorder | Source: Ohio State University [April 03, 2017]

byUnknown

in Austria, Fossils, Geology, Palaeontology

April 03, 2017

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