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TOPIC: The ancient sea


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RE: The ancient sea

Did comets help create Earth's seas?
For years scientists thought comets slamming against the newborn Earth helped deliver water to a once dry planet. But roughly a decade ago this view was shaken by the discovery that the water in comets and Earth's oceans did not match up in terms of hydrogen isotopes.
Calculations then showed it was highly improbable that enough icy rocks from the suspected homes of comets the Kuiper belt past Neptune and the Oort cloud past that could have collided with Earth to supply its oceans.
In the last two years, however, researchers have discovered comets in the outer part of the asteroid belt. These "main-belt comets" may have the right levels of hydrogen isotopes, and are perhaps close enough to Earth to have realistically brought us the seas that life emerged from.

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Ancient Oxygen

Scientists may have to rethink accepted theories of how the prehistoric earth's atmosphere developed after new discoveries in ancient sulphur raised serious questions, researchers said on Wednesday.
Up to now it has been generally accepted that the earth's atmosphere was devoid of oxygen for some 80 percent of its existence.

"The popular model is that there was little oxygen in the earth's atmosphere before about 2.4 billion years ago" - Hiroshi Ohmoto, Pennsylvania State University.

But Ohmoto's team has cast doubt on the theory after finding sulphur isotopes, indicating prevalent oxygen, that predate the accepted start of atmospheric oxygenation.
The key lies in the fact that while all isotopes of sulphur behave the same chemically, they have slightly differing masses according to the amount of atmospheric oxygen at the time.
Isotopes from two sulphur samples the team analysed -- one 2.76 billion years old from a lake bed and the other 2.92 billion years old from the sea bed -- did not indicate an oxygen-starved atmosphere.

"We analysed the sulphur composition and could not find the abnormal sulphur isotope ratio (indicating no oxygen). This is the first time that sediment that old was found to contain no abnormal sulphur isotope ratio" - Hiroshi Ohmoto.

The team concluded that there were two possible explanations -- either that prehistoric atmospheric oxygen levels fluctuated wildly over the millennia, or that sulphur showing no oxygen might have been produced in an oxygenated atmosphere as long ago as 3.8 billion years by violent volcanic activity.
Either way, the accepted theories needed to be re-evaluated.
The findings were published in the science journal Nature.



Posts: 131433
RE: The ancient sea

NASA Scientists Confirm Toxic Seas During Earth's Evolution

Exobiology researchers confirmed Earth's oceans were once rich in sulphides that would prevent advanced life forms, such as fish and mammals, from thriving.
The research was funded in part by NASA's exobiology program.

A team of scientists from the Massachusetts Institute of Technology and Harvard University, working with colleagues from Australia and the United Kingdom, analyzed the fossilized remains of photosynthetic pigments preserved in 1.6 billion-year-old rocks from the McArthur Basin in Northern Australia.

They found evidence of photosynthetic bacteria that require sulphides and sunlight to live. Known as purple and green sulphur bacteria because of their respective pigment colorations, these single-celled microbes can only live in environments where they simultaneously have access to sulphides and sunlight.

The researchers also found very low amounts of the fossilized remains of algae and oxygen-producing cyanobacteria. The relative scarcity of these organisms is due to poisoning by large amounts of sulphide.

"This work suggests Earth's oceans may have been hostile to animal and plant life until relatively recently. If so, this would have profound implications for the evolution of modern life" - Dr. Carl Pilcher, NASA's senior scientist for astrobiology.

"The discovery of the fossilized pigments of purple sulphur bacteria is totally new and unexpected. Because they need fairly high intensity sunlight, it means the pink bacteria, along with their essential source of sulphide, close to the surface, perhaps as close as 20 to 40 meters. The sulphide would have come from bacteria that reduces sulphate carried into the oceans by the weathering of rocks" - Roger Summons, Massachusetts Institute of Technology professor of geobiology.

"The McArthur Basin rocks were deposited over a very large area and over many millions of years, so it's likely they formed under water that was intermittently connected to or actually part of an ocean. In turn, this implies the ocean had an abundant and continuous supply of hydrogen sulphide and must have been quite toxic to any oxygen-breathing organisms. In fact, for seven-eighths of Earth's 4.5 billion-year history, there was probably little oxygen in the oceans and certainly not enough to support oxygen-breathing marine animals" - Jochen Brocks, team member.

This research continued the efforts of NASA and partner institutions to understand the early history of the Earth. Research results were published in the Oct. 6, 2005, edition of Nature magazine.

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source NASA



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Global Warming

Open University researchers have uncovered startling new evidence about an extreme period of a sudden, fatal dose of global warming some 180 million years ago during the time of the dinosaurs. The scientists' findings could provide vital clues about climate change happening today and in the future.

The Open University Department of Earth Sciences team, PhD student Dave Kemp and supervisors Drs. Angela Coe and Anthony Cohen, along with Dr. Lorenz Schwark of the University of Cologne, discovered evidence suggesting that vast amounts of methane gas were released to the atmosphere in three massive 'methane burps' or pulses. The addition of methane, a greenhouse gas, to the atmosphere had a severe impact on the environment, warming Earth about 10°C, and resulting in the extinction of a large number of species on land and in the oceans.

"We've known about this event for a few years through earlier work by our team and others, but there's been a great deal of uncertainty about its precise size, duration, and underlying cause. What our present study shows is that this methane release was not just one event, but 3 consecutive pulses. Importantly, our data demonstrate that each individual pulse was very rapid. Also, whilst the methane release was very quick, we've found that the recovery took much longer, occurring over a few hundred thousand years" - Dr Angela Coe.

The methane came from gas hydrate, a frozen mixture of water and methane found in huge quantities on the seabed. This hydrate suddenly melted, allowing the methane to escape. The Open University researchers based their findings on geochemical analyses of mudrocks that are preserved along the
Yorkshire coast near Whitby, UK, and date from the Jurassic Period of geological time.

"The methane was released because slight wobbles in the Earth's orbit periodically bring our planet closer to the Sun, warming the oceans sufficiently to melt the vast reserves of hydrate. We believe that this effect was compounded by warming from greenhouse gases from volcanoes. After the methane was released into the atmosphere from the seabed it reacted rapidly with oxygen to form carbon dioxide. Carbon dioxide is also a powerful greenhouse gas that persists in the atmosphere for many hundreds of years, and it was this gas which caused such a massive global warming effect" - Dave Kemp.

"One of the most important aspects of the study is that it provides an accurate timescale for how the Earth, and life, reacted to a sudden increase in atmospheric carbon dioxide. Today we are releasing large amounts of carbon dioxide to the atmosphere, primarily through the burning of fossil fuels. It is possible that the rate at which carbon dioxide is being added to the atmosphere now actually outstrips the rate at which it was added 180 million years ago. Given that the effects were so devastating then, it is extremely important to understand the details of past events in order to better comprehend present-day climate change. With this information, we are better informed about what action needs to be taken to mitigate or avoid some of the potential detrimental future effects" - Dr Anthony Cohen.

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Ancient climate

An oxygen-free ocean from bottom to surface is probably the worst scenario that marine higher life can experience.
Are processes and feedbacks linking the atmosphere to the deep ocean capable to cause a rapid change from an oxygen-rich to an oxygen-free deep ocean? And what are the consequences for the global carbon cycle that ultimately drive marine and terrestrial ecosystems and climate variation?

These are fundamental and burning questions on the society's agenda. Hurricane Katrina and other natural catastrophes in recent years have shown how vulnerable mankind is in the face of nature. Professor Tom Wagner of Newcastle University, England, led a cross-disciplinary study of geological records combined with climate modelling to shed new light on the mechanisms and processes that led to repetitive rapid climatic change with major impact on the ocean during past greenhouse conditions.

By analysing sediments laid down on the ocean floor about 85m years ago in the Cretaceous, the research team found evidence that Cretaceous greenhouse climate was highly variable and repeatedly resulted in major changes in ocean chemistry and deep circulation causing disastrous consequences for marine ecosystems. These extreme conditions fostered massive burial of dead organic matter from marine species, such as algae and plankton, at the sea floor, leading to the formation of distinct sediments, "marine black shale", also well known as the world's primary source for oil and gas.

Professor Wagner and colleagues uncovered evidence of the mechanisms that drove rapid and repetitive climate change by studying the quantity and content of proxy parameters in black shale in a core of sedimentary rock drilled out of the ocean bed, off Africa's Ivory Coast, and comparing these results with data from a global climate model.

The model data were used to quantify the freshwater run-off from tropical Africa into the equatorial Atlantic, where the core has been drilled, and to specify the role of orbital configuration and the water cycle on climate and oceanographic variation. With these data, it was possible to explain the formation of the sedimentary succession of black shale and carbonate-rich sediments, indicating alternation between oxygen-depleted and oxygen-rich conditions in the deep ocean. All life other than simple organisms like bacteria would have been seriously depleted in the deeper ocean as oxygen became progressively scarce. On land, the climate variability would cause strong regional contrasts, with widespread deserts at mid-latitudes and extremely humid areas in the tropics.

Processes in the atmosphere driven by cyclic changes in the amount of energy from the sun entering the top of the atmosphere (insolation) have been identified to be the cause for the observed dramatic changes in ocean chemistry that resulted in the formation of black shale. This contributes to the current discussion on whether the atmosphere drives the oceans or vice-versa.
Higher rainfall would have caused increased amounts of fresh water running off the land, carrying large quantities of nutrients into the oceans, resulting in an increase in marine productivity and supporting oxygen depletion and a change in circulation patterns in the deep ocean.

Climate modelling identified that specific periods of extremely high river discharge occurred during maxima in seasonal contrasts when the northern equinox (when the sun is directly over the earth's equator) coincided with perihelion (when the earth passes closest to the sun). It was only during this specific orbital configuration that freshwater run-off exceeded a certain threshold, finally to result in a rapid change to ocean anoxia.
The findings, reported in Nature, the international weekly journal of science, suggest that variations in the water cycle, once they have exceeded a certain threshold, are capable of inducing major environmental change in the oceans.
The researchers conclude: 'The results of this study demonstrate how sensitively and rapidly tropical marine areas close to continental margins react to even relatively moderate increases in continental freshwater discharge.

'The freshwater threshold required to shift sheltered and semi-enclosed areas of the modern ocean into an anoxic mode are unknown but the progressive emission of greenhouse gases to the modern atmosphere is gradually shifting Earth towards a greenhouse mode with an accelerated hydrological cycle'

'At present it is hardly possible to estimate where we are on the long-term climate trend but once the freshwater threshold is passed, a substantial impact on biochemical cycling of continental margins may be expected.'

Commenting on the Nature paper, Professor Wagner said that the majority of the world's population live in coastal areas, which were the most vulnerable to natural catastrophes as recorded in the geological record.

'Understanding the processes and feedbacks controlling carbon and nutrient cycling in the modern world and during past periods of extreme warmth is therefore critical to separate human impact on climate from natural variability and underpins the ability to adapt to future conditions'.




Posts: 131433
The ancient sea

The ancient sea was more like a giant salty lake than a rolling ocean, report scientists from Imperial College London in the May edition of the Journal of the Geological Society.
A new computer model that simulates how tides in North West Europe would have behaved 300 million years ago shows a sea with so little movement that it was unlike any on Earth today.
Using information on the ancient land masses and the tidal pull of the Moon, the new computer modelling system reveals a picture of a Palaeozoic ocean in which even basic life forms would have struggled to survive. Without tides, shallow coastal water is not mixed up, preventing life-saving oxygen from being circulated.
This shortage of oxygen causes life forms such as plankton to die and the decay of these life forms uses up further oxygen, contributing to the creation of an environment unable to support life. The Palaeozoic period lasted from 570 to 245 million years ago.
"It is very difficult to understand how these huge ancient seas behaved, since we have no examples of this sort of water body on Earth today. We have used a new computer model to deduce the tidal range in ancient seas and show that they were almost tide less. Understanding the behaviour of these vast shallow expanses is critical to our knowledge of the ancient climate and environments and to understand how early marine life evolved and diversified." - Dr Peter Allison, from the Department of Earth Sciences and Engineering and one of the authors of the study.

According to the researchers' estimates, the new computer programme can model the behaviour of the sea many times faster than existing modelling systems. The model, developed by Dr Chris Pain, Dr Matthew Piggott and Martin Wells, has great potential for examining other patterns of ocean behaviour.
"The modelling technology developed here at Imperial is a novel and fascinating means of investigating the ancient Earth. Although this is 'blue-skies' research now, we are validating an exciting new modelling technology which will ultimately help us to predict climate change." - Martin Wells, Graduate student.
SOURCE: Imperial College Press Release

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