The rise of oxygen caused Earth’s earliest ice age

Geologists may have uncovered the answer to an age-old question – an ice-age-old question, that is. It appears that Earth’s earliest ice ages may have been due to the rise of oxygen in Earth’s atmosphere, which consumed atmospheric greenhouse gases and chilled the earth.

Alan J. Kaufman, professor of geology at the University of Maryland, Maryland geology colleague James Farquhar, and a team of scientists from Germany, South Africa, Canada, and the U.S.A., uncovered evidence that the oxygenation of Earth’s atmosphere – generally known as the Great Oxygenation Event – coincided with the first widespread ice age on the planet.

“We can now put our hands on the rock library that preserves evidence of irreversible atmospheric change,” said Kaufman. “This singular event had a profound effect on the climate, and also on life.”

Using sulfur isotopes to determine the oxygen content of ~2.3 billion year-old rocks in the Transvaal Supergroup in South Africa, they found evidence of a sudden increase in atmospheric oxygen that broadly coincided with physical evidence of glacial debris, and geochemical evidence of a new world-order for the carbon cycle.

“The sulfur isotope change we recorded coincided with the first known anomaly in the carbon cycle. This may have resulted from the diversification of photosynthetic life that produced the oxygen that changed the atmosphere,” Kaufman said.

Two and a half billion years ago, before the Earth’s atmosphere contained appreciable oxygen, photosynthetic bacteria gave off oxygen that first likely oxygenated the surface of the ocean, and only later the atmosphere. The first formed oxygen reacted with iron in the oceans, creating iron oxides that settled to the ocean floor in sediments called banded iron-formations – layered deposits of red-brown rock that accumulated in ocean basins worldwide. Later, once the iron was used up, oxygen escaped from the oceans and started filling up the atmosphere.

Once oxygen made it into the atmosphere, Kaufman’s team suggests that it reacted with methane, a powerful greenhouse gas, to form carbon dioxide, which is 62 times less effective at warming the surface of the planet. “With less warming potential, surface temperatures may have plummeted, resulting in globe-encompassing glaciers and sea ice” said Kaufman.

In addition to its affect on climate, the rise in oxygen stimulated the rise in stratospheric ozone, our global sunscreen. This gas layer, which lies between 12 and 30 miles above the surface, decreased the amount of damaging ultraviolet sunrays reaching the oceans, allowing photosynthetic organisms that previously lived deeper down, to move up to the surface, and hence increase their output of oxygen, further building up stratospheric ozone.

“New oxygen in the atmosphere would also have stimulated weathering processes, delivering more nutrients to the seas, and may have also pushed biological evolution towards eukaryotes, which require free oxygen for important biosynthetic pathways,” said Kaufman.

The result of the Great Oxidation Event, according to Kaufman and his colleagues, was a complete transformation of Earth’s atmosphere, of its climate, and of the life that populated its surface. The study is published in the May issue of Geology.

Origins of sulfur in rocks tells early oxygen story

Sedimentary rocks created more than 2.4 billion years ago sometimes have an unusual sulfur isotope composition thought to be caused by the action of ultra violet light on volcanically produced sulfur dioxide in an oxygen poor atmosphere. Now a team of geochemists can show an alternative origin for this isotopic composition that may point to an early, oxygen-rich atmosphere.

“The significance of this finding is that an abnormal isotope fractionation (of sulfur) may not be linked to the atmosphere at all,” says Yumiko Watanabe, research associate, Penn State. “The strongest evidence for an oxygen poor atmosphere 2.4 billion years ago is now brought into question.”

The researchers, who also include James Farquhar, associate professor of geology, University of Maryland and Hiroshi Ohmoto, professor of geoscience, Penn State, present the possibility that the rocks with an anomalous sulfur isotope fractionation came from locations on the ocean floor where hydrothermal fluids seeped up from submarine vents through organic carbon rich sediments and mixed with the ocean water. Watanabe used laboratory experimentation to test their theory and report on the results in today’s (Apr. 17) issue of Science.

Chemical elements often have more than one form. While the number of protons and electrons are all the same, the element may have forms with a greater or lesser number of neutrons and consequently a different atomic weight. Sulfur has four naturally occurring isotopes none of which are radioactive. Although 95 percent of sulfur has an atomic weight of 32, the other 5 percent is composed of sulfur with atomic weights of 33, 34 or 36. The relationship between the amounts of 33, 34 and 36 are predictable based on the differences in their weights, but in the early rocks examined, the relationship was often anomalous. Other scientists have previously determined that the sulfur dioxide, ultraviolet light reaction in the absence of oxygen can produce the anomalous isotope fractionation.

Watanabe looked at samples of amino acids and sodium sulfur compounds to try to recreate the anomalous sulfur isotope composition in another way. She chose amino acids as a proxy for organic material because the anomalous sulfur isotopes often come from sedimentary rock, black shale, that also contains abundant mature kerogen — a mixture of organic compounds. She chose sodium compounds because of the large amounts of sodium and sulfate in the ocean.

Initial experiments used two amino acids — alanine and glycine — and sodium sulfite, which is less oxidized compared to sulfate. When heated, these did not produce abnormal fractionation. Watanabe then tested five amino acids, adding histidine, arginine and tryptophan, and mixed them with sodium sulfate. In this case, alanine and glycine produced the anomalous isotope composition found in the rocks. In all, she ran 32 series of experiments with more than 100 individual samples.

“At high temperatures it sometimes took 24 hours for the sulfate to reduce to sulfide,” said Watanabe. “At lower temperatures it took about two months, 1,000 hours. I ran the experiments until I had enough product to test the isotopic distribution.”

Although Watanabe captured the sulfur from the experiments as hydrogen sulfide gas, she converted it to silver sulfide for analysis because it is easier to work with a solid than a gas.

“People never thought that anomalous sulfur isotope fractionation could be caused by a process other than atmospheric reactions,” said Ohmoto. “Our study significantly shifts possibilities to something different, to a biological and thermal regime. There are now at least two ways that the anomalous sulfur isotope fractionation seen in some rocks could be achieved.”

While sulfate-reducing bacteria do not produce anomalous isotope relationships, the remains of simple organisms coupled with thermal sulfate reduction does produce the anomalous isotope signature.

The researchers plan to look at dead cyanobacteria — blue green algae — next to see if their organic material will fuel the thermal reaction to produce anomalous sulfur isotope relationships.

Did a nickel famine trigger the ‘Great Oxidation Event’?

The Earth’s original atmosphere held very little oxygen. This began to change around 2.4 billion years ago when oxygen levels increased dramatically during what scientists call the “Great Oxidation Event.” The cause of this event has puzzled scientists, but researchers writing in Nature* have found indications in ancient sedimentary rocks that it may have been linked to a drop in the level of dissolved nickel in seawater.

“The Great Oxidation Event is what irreversibly changed surface environments on Earth and ultimately made advanced life possible,” says research team member Dominic Papineau of the Carnegie Institution’s Geophysical Laboratory. “It was a major turning point in the evolution of our planet, and we are getting closer to understanding how it occurred.”

The researchers, led by Kurt Konhauser of the University of Alberta in Edmonton, analyzed the trace element composition of sedimentary rocks known as banded-iron formations, or BIFs, from dozens of different localities around the world, ranging in age from 3,800 to 550 million years. Banded iron formations are unique, water-laid deposits often found in extremely old rock strata that formed before the atmosphere or oceans contained abundant oxygen. As their name implies, they are made of alternating bands of iron and silicate minerals. They also contain minor amounts of nickel and other trace elements.

Nickel exists in today’s oceans in trace amounts, but was up to 400 times more abundant in the Earth’s primordial oceans. Methane-producing microorganisms, called methanogens, thrive in such environments, and the methane they released to the atmosphere might have prevented the buildup of oxygen gas, which would have reacted with the methane to produce carbon dioxide and water. A drop in nickel concentration would have led to a “nickel famine” for the methanogens, who rely on nickel-based enzymes for key metabolic processes. Algae and other organisms that release oxygen during photosynthesis use different enzymes, and so would have been less affected by the nickel famine. As a result, atmospheric methane would have declined, and the conditions for the rise of oxygen would have been set in place.

The researchers found that nickel levels in the BIFs began dropping around 2.7 billion years ago and by 2.5 billion years ago was about half its earlier value. “The timing fits very well. The drop in nickel could have set the stage for the Great Oxidation Event,” says Papineau. “And from what we know about living methanogens, lower levels of nickel would have severely cut back methane production.”

What caused the drop in nickel? The researchers point to geologic changes that were occurring during the interval. During earlier phases of the Earth’s history, while its mantle was extremely hot, lavas from volcanic eruptions would have been relatively high in nickel. Erosion would have washed the nickel into the sea, keeping levels high. But as the mantle cooled, and the chemistry of lavas changed, volcanoes spewed out less nickel, and less would have found its way to the sea.

“The nickel connection was not something anyone had considered before,” says Papineau. “It’s just a trace element in seawater, but our study indicates that it may have had a huge impact on the Earth’s environment and on the history of life.”

Deep-sea rocks point to early oxygen on Earth

This is the location of the core drilling in the Pilbara Craton, West Australia. -  Hiroshi Ohmoto/Yumiko Watanabe
This is the location of the core drilling in the Pilbara Craton, West Australia. – Hiroshi Ohmoto/Yumiko Watanabe

Red jasper cored from layers 3.46 billion years old suggests that not only did the oceans contain abundant oxygen then, but that the atmosphere was as oxygen rich as it is today, according to geologists.

This jasper or hematite-rich chert formed in ways similar to the way this rock forms around hydrothermal vents in the deep oceans today.

“Many people have assumed that the hematite in ancient rocks formed by the oxidation of siderite in the modern atmosphere,” said Hiroshi Ohmoto, professor of geochemistry, Penn State. “That is why we wanted to drill deeper, below the water table and recover unweathered rocks.”

The researchers drilled diagonally into the base of a hill in the Pilbara Craton in northwest Western Australia to obtain samples of jasper that could not have been exposed to the atmosphere or water. These jaspers could be dated to 3.46 billion years ago.

“Everyone agrees that this jasper is 3.46 billion years old,” said Ohmoto. “If hematite were formed by the oxidation of siderite at any time, the hematite would be found on the outside of the siderite, but it is found inside,” he reported in a recent issue of Nature Geoscience.

The next step was to determine if the hematite formed near the water’s surface or in the depths. Iron compounds exposed to ultra violet light can form ferric hydroxide, which can sink to the bottom as tiny particles and then converted to hematite at temperatures of at least 140 degrees Fahrenheit.

“There are a number of cases around the world where hematite is formed in this way,” says Ohmoto. “So just because there is hematite, there is not necessarily oxygen in the water or the atmosphere.”

The key to determining if ultra violet light or oxygen formed the hematite is the crystalline structure of the hematite itself. If the precursors of hematite were formed at the surface, the crystalline structure of the rock would have formed from small particles aggregating producing large crystals with lots of empty spaces between. Using transmission electron microscopy, the researchers did not find that crystalline structure.

“We found that the hematite from this core was made of a single crystal and therefore was not hematite made by ultra violet radiation,” said Ohmoto.

This could only happen if the deep ocean contained oxygen and the iron rich fluids came into contact at high temperatures. Ohmoto and his team believe that this specific layer of hematite formed when a plume of heated water, like those found today at hydrothermal vents, converted the iron compounds into hematite using oxygen dissolved in the deep ocean water.

“This explains why this hematite is only found in areas with active submarine volcanism,” said Ohmoto. “It also means that there was oxygen in the atmosphere 3.46 billion years ago, because the only mechanism for oxygen to exist in the deep oceans is for there to be oxygen in the atmosphere.”

In fact, the researchers suggest that to have sufficient oxygen at depth, there had to be as much oxygen in the atmosphere 3.46 billion years ago as there is in today’s atmosphere. To have this amount of oxygen, the Earth must have had oxygen producing organisms like cyanobacteria actively producing it, placing these organisms much earlier in Earth’s history than previously thought.

“Usually, we look at the remnant of what we think is biological activity to understand the Earth’s biology,” said Ohmoto. “Our approach is unique because we look at the mineral ferric oxide to decipher biological activity.”

Ohmoto suggests that this approach eliminates the problems trying to decide if carbon residues found in sediments were biologically created or simply chemical artifacts.

Geologist finds clues to ancient chemistry of deep oceans





Paeleontologist Guy Narbonne at his Mistaken Point exploration site on the coast of Newfoundland - Photo by Greg Locke
Paeleontologist Guy Narbonne at his Mistaken Point exploration site on the coast of Newfoundland – Photo by Greg Locke

Queen’s researchers have moved another step closer to explaining changes in the chemistry of the deep oceans – and the sudden appearance of large animal fossils – more than 500 million years ago.



Conducted by Geological Sciences and Geological Engineering professor Guy Narbonne, with an international team of researchers, the study focuses on analyses of iron speciation (iron-bearing minerals that form under different oxygen concentrations) and sulfur isotopes during intense global ice ages. This period is commonly called the “snowball” Earth, 800 to 580 million years ago.



“Our results imply that surface ocean waters of this age were oxygenated, but that deep-sea waters were anoxic (depleted of oxygen) during most of this time,” says Dr. Narbonne, an expert in the early evolution of animals and their ecosystems. The deeper water contained abundant dissolved iron, however – a feature that had not been seen for more than one billion years of Earth history, he adds.



The team’s findings appear on-line in the current edition of Science Express.


The level of dissolved oxygen required for accelerated animal growth did not reach deep-sea waters until about 580 million years ago. This coincided with the first appearance of large, animal-like fossils in deep-water sediments of Newfoundland and northwestern Canada.



In 2002, Dr. Narbonne and his colleagues discovered the world’s oldest complex life forms between layers of sandstone on the southeastern coast of Newfoundland. This pushed back the age of Earth’s earliest known complex life to more than 575 million years ago, soon after the melting of the massive “snowball” glaciers.



The current research team, headed by Donald Canfield from the University of Southern Denmark, also includes: Simon Poulton (Newcastle University), Andrew Knoll (Harvard), Gerry Ross (Kula, Hawaii) and Harald Strauss (Justus-Liebig-Universitat Giessen).

Two Oxygenation Events In Ancient Oceans Sparked Spread Of Complex Life





The photo (field of view about 0.15 millimeter in width) is of an exceptionally preserved eukaryotic fossil from the Doushantuo Formation (635--551 million years old) in South China. High-resolution geochemical data from the Doushantuo Formation indicate that the early diversification of eukaryotes may have coupled with episodic oxygenation of Ediacaran oceans. (Credit: Photograph by Shuhai Xiao)
The photo (field of view about 0.15 millimeter in width) is of an exceptionally preserved eukaryotic fossil from the Doushantuo Formation (635–551 million years old) in South China. High-resolution geochemical data from the Doushantuo Formation indicate that the early diversification of eukaryotes may have coupled with episodic oxygenation of Ediacaran oceans. (Credit: Photograph by Shuhai Xiao)

The rise of oxygen and the oxidation of deep oceans between 635 and 551 million years ago may have had an impact on the increase and spread of the earliest complex life, including animals, according to a new study.



Today, we take oxygen for granted. But the atmosphere had almost no oxygen until 2.5 billion years ago, and it was not until about 600 million years ago when the atmospheric oxygen level rose to a fraction of modern levels. For a long time, geologists and evolutionary biologists have speculated that the rise of the breathing gas and subsequent oxygenation of the deep oceans are intimately tied to the evolution of modern biological systems.



To test the interaction between biological evolution and environmental change, an international team of scientists from Virginia Tech, the University of Maryland, University of Nevada at Las Vegas, and Chinese Academy of Sciences, examined changes in the geochemistry and fossil distribution of 635- to 551-million-year old sediments preserved in the Doushantuo Formation in the Yangtze Gorges area of South China.



Millions of years ago, the Yangtze Gorges area was an ancient sea, said Kathleen A. McFadden, a Ph.D. candidate in geobiology at Virginia Tech and the lead author of the PNAS article.



To determine when there was enough oxygen to support animal life in the ocean, the researchers asked, “What kind of geochemical evidence would there be in the rock record?” said Shuhai Xiao, associate professor of geosciences at Virginia Tech.



Scientists hypothesized that there was a lot of dissolved organic carbon in the ocean when oxygen levels were low. If oxygen levels rose, some of this organic carbon would be oxidized into inorganic forms, some of which can be preserved as calcium carbonate in the rock record. “We measured the carbon isotope signatures of organic and inorganic carbon in the ancient rocks to infer oxidation events,” said co-author Ganqing Jiang, assistant professor of geology at the University of Nevada at Las Vegas.



The layers of sediment exposed by the Three Gorges Dam represent millions of years of deposits. “We went through road cuts, bed by bed, measuring and describing the exposed rock, then took small rock samples every few feet or so,,” said McFadden. She collected about 200 samples; hundreds of samples were taken to three labs.



The researchers cleaned and crushed the small samples to powder, which they reacted with acid to release carbon dioxide from carbonate minerals, and then burned the residue to get carbon dioxide from organic matter. “The CO2 that is released was measured with mass spectrometers to gives us the isotopic signature of the carbonate and organic carbon that was present in the rock,” said McFadden.



“The relative abundances of the carbon-12 and carbon-13 isotopes, which are stable and do not decay with time, provide a snapshot of the environmental processes taking place in the ocean at the different times recorded in the layers of rock,” McFadden said.


The stratigraphic pattern of carbon isotope abundances suggested to these researchers that the ocean, which largely lacked oxygen before animals arrived on the scene, was aerated by two discrete pulses of oxygen.



“The first pulse apparently had little impact on a large organic carbon reservoir in the deep ocean, but did spark changes in microscopic life forms,” McFadden said. “The second event, which occurred around 550 million years ago, however, resulted in the reduction of the organic carbon reservoir, indicating that the ocean became fully oxidizing just before the evolution and diversification of many of Earth’s earliest animals,” she said.



“The Doushantuo Formation has a wonderful fossil record,” McFadden said. “It allows us to look at major fossil groups, when they appear and when they disappear, and to see a relationship between oxidation events and biological groups.”



“This study supports the growing view that life and environment co-evolved through this tumultuous period of Earth history,” said geochemist Alan J. Kaufman, a co-author of the study from the University of Maryland.



The researchers analyzed the fossils in the Doushantuo Formation, from microscopic life forms of 635 million years ago to large algae around 551 million years ago. Looking at data from four locations with very similar isotopic records, they report that the first oxygen spike resulted in a rise in microscopic organisms, some of which are thought to be the earliest animal embryos. The second spike in oxygen coincides with a dramatic increase in species of large complex algae.



“Both oxidation events appear to coincide with increased diversity of fossils assemblages in the Doushantuo basin, with the number of species nearly doubling,” McFadden said.



Following this second oxidation event, between 550 and 542 million years ago, there was a worldwide increase of Ediacara organisms, complex macroscopic life forms, an event recently dubbed as the Avalon Explosion. “This was when we see the first burrowing animals and biomineralizing animals in the fossil record,” McFadden said. Biomineralizing animals are the first animals to form external skeletons, or shells.



The triggers for the oxidation events remain elusive, however. “These events recorded in the ocean were probably related to oxygen in the atmosphere reacting with sediments on land,” McFadden said. “Weathering of rocks and soils on the continents would result in the release of certain dissolved ions, such as sulfate, into rivers. These would then be transported to the sea where they might be used by bacteria to oxidize the organic carbon pool in the deep oceans,” she said..



The article, “Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation,” was written by Kathleen A. McFadden; Jing Huang and Xuelei Chu of the Institute of Geology and Geophysics, Chinese Academy of Sciences; Ganqing Jiang; Alan J. Kaufman; Chuanming Zhou and Xunlai Yuan of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences; and Shuhai Xiao. The URL for the paper is www.pnas.org/cgi/content/abstract/0708336105v1. The paper will publish in the print issue of March 4 (Issue 9, Volume 105, pp. 3197-3202).



The joint research was supported by NSF Sedimentary Geology and Paleobiology Program, NASA Exobiology Program, National Natural Science Foundation of China, Virginia Tech Institute of Critical Technology and Applied Sciences, Evolving Earth Foundation, and several other funding agencies.