Sunday, February 18, 2007

 

Oldest rocks show how Earth may have dodged frozen fate of Mars

Carbon dioxide, a greenhouse gas that has become a bane of modern society, may have saved Earth from freezing over early in the planet's history, according to the first detailed laboratory analysis of the world's oldest sedimentary rocks.

Scientists have theorized for years that high concentrations of greenhouse gases could have helped Earth avoid global freezing (see "Snowball Earth") in its youth by allowing the atmosphere to retain more heat than it lost. Now a team from the University of Chicago and the University of Colorado at Boulder that analyzed ancient rocks from the eastern shore of Hudson Bay in northern Quebec, Canada, have discovered the first direct field evidence supporting this theory.

The study [1] shows carbon dioxide in Earth's atmosphere could have sustained surface temperatures above freezing before 3.75 billion years ago according to the researchers, led by University of Chicago Assistant Professor Nicolas Dauphas. Co-authors on the study, which appeared online January 16 2007 in the journal Earth and Planetary Science Letters, included Assistant Professor Stephen Mojzsis and doctoral student Nicole Cates of CU-Boulder's geological sciences department and Vincent Busigny, now of the Institut de Physique du Globe in Paris.

The new study helps explain how Earth may have avoided becoming frozen solid early in its history, when astrophysicists believe the sun was 25 percent fainter than today. Previous studies had shown liquid water existed at Earth's surface even though the weak sun should have been unable to warm the planet above freezing conditions. But high concentrations of CO2 or methane could have warmed the planet, according to the research team.

The ancient rocks from Quebec contain iron carbonates believed to have precipitated from ancient oceans, according to the study. Since the iron carbonates could only have formed in an atmosphere containing far higher CO2 levels than those found in Earth's atmosphere today, the researchers concluded the early Earth environment was extremely rich in CO2.

"We now have direct evidence that Earth's atmosphere was loaded with CO2 early in its history, which probably kept the planet from freezing and going the way of Mars," said Mojzsis.

The CO2 could even have played a role as a "planetary thermostat," since cold, icy conditions on Earth would have decreased the chemical weathering of rocks and increased the amount of CO2 moving into the atmosphere, ratcheting up Earth's surface temperatures, according to Dauphas.

In a companion article [2] that appeared online February 2 in Earth and Planetary Science Letters, Mojzsis, Cates and CU-Boulder undergraduate Jon Adam used a technique known as uranium-lead dating to establish the ancient age of the Hudson Bay rocks. Discovered by Canadian scientists in 2001, the rocks were confirmed by Mojzsis and his team to be at least as old as an isolated outcropping of West Greenland rocks previously believed by researchers to be the oldest on Earth.

The CU-Boulder team analyzed the rocks by crushing them into powder and dating zircon crystals present in the rock, said Mojzsis. The technique allowed them to calculate the geologic age of the crystals based on the radioactive decay rate of the uranium and lead isotopes in relation to each other, a technique known to be accurate to 1 percent or less.

"Zircon is nature's best timekeeper," said Mojzsis. "The tests show that the rocks in Quebec are roughly 3.75 billion years old, about the same as the West Greenland rocks."

The landscape of the Hudson Bay region under study today, marked by hills of grassland and marsh peppered by lakes, streams and craggy outcroppings, is much different from the alien Earth of 3.8 billion years ago, said Mojzsis. In much earlier times, a dense atmosphere of CO2 would have given the sky a reddish cast, and a greenish-blue ocean of iron-rich water would have lapped onto beaches, he said.

While scientists have been concerned that the limited sample of Earth's oldest known rocks from West Greenland provided a biased view of early Earth, the Hudson Bay discovery essentially doubles the known amount of extremely ancient rocks, and there appear to be a number of similar, ancient outcrops in the vicinity. "We are now finding Earth's oldest rocks are not as rare as we once thought," Mojzsis said.

The ongoing research effort by Mojzsis and his group has been funded by the NASA Astrobiology Institute, the NASA Exobiology Program and the National Science Foundation. For more information online go to isotope.colorado.edu.

Source: University of Colarado at Boulder PR "World's Oldest Rocks Show How Earth May Have Dodged Frozen Fate Of Mars" February 5, 2007

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[1] Identification of chemical sedimentary protoliths using iron isotopes in the {greater than} 3750 Ma Nuvvuagittuq supracrustal belt, Canada

Nicolas Dauphas, Nicole L. Cates, Stephen J. Mojzsis and Vincent Busigny

Earth and Planetary Science Letters Volume 254, Issues 3-4 , 28 February 2007, Pages 358-376 doi:10.1016/j.epsl.2006.11.042

An Eoarchean supracrustal belt dated at ca. 3750 Ma was recently identified in the Innuksuac Complex, northern Quebec (Canada). Rocks from the Nuvvuagittuq locality include mafic and ultramafic amphibolites, quartz-biotite and pelitic schists, orthogneisses, and banded quartz-magnetite-amphibole/pyroxene rocks of probable chemical sedimentary origin. The purported metasediments are enriched in the heavy isotopes of Fe by approximately 0.3%/amu relative to IRMM-014. They also have high Fe/Ti ratios, up to 100x that of associated amphibolite units. These signatures demonstrate that quartz-magnetite-amphibole/pyroxene rocks from Nuvvuagittuq are chemical sediments (e.g. banded iron-formations, BIFs) formed by precipitation of dissolved ferrous iron in a marine setting. All units were metamorphosed to upper amphibolite facies, which partly homogenized Fe isotopes. Variable Fe isotope compositions of bulk quartz-magnetite rocks are interpreted to reflect binary mixing between primary oxides and carbonates. Mixing relationships with major element chemistry (Ca/Fe, Mg/Fe, and Mn/Fe) are used to estimate the Fe isotope composition of the primary Fe-oxide phase (0.3 to 0.4%/amu) and the chemistry of the carbonate (siderite and ankerite). Iron isotopes can thus be used to constrain the primary mineralogy of Fe-rich chemical sedimentary precipitates before metamorphism. The possible presence of siderite in the primary mineral assemblage supports deposition under high PCO2. We developed an isotope distillation model that includes two possible abiotic oxidation paths, homogeneous and heterogeneous. The isotopic composition of Fe in the precursor phase of magnetite in BIFs can be explained by partial oxidation through oxygenic or anoxygenic photosynthesis of Fe from a hydrothermal source.

[2] Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Quebec

N.L. Cates and S.J. Mojzsis

Earth and Planetary Science Letters Volume 255, Issues 1-2 , 15 March 2007, Pages 9-21 doi:10.1016/j.epsl.2006.11.034

Geochemistry and U-Pb ion microprobe zircon geochronology guided by high-resolution mapping (1:50 scale) was used to define a minimum age of ca. 3750 Ma for supracrustal rocks from the Nuvvuagittuq supracrustal belt (NSB) in the northern Superior Province, Quebec (Canada). Mineralogy and geochemistry of critical field relationships preserved at the Porpoise Cove locality describe a supracrustal succession of (mafic) amphibolites and ultramafic rocks, finely banded quartz-magnetite units with intermixed coarse-grained ferruginous quartz-pyroxene rocks and quartz-biotite schists that superficially resemble polymict meta-conglomerates with large (up to 10 cm) deformed polycrystalline quartz and mafic clasts. All units in the mapped outcrop have sharp lithological contacts. Narrow (dominantly trondhjemitic) orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals. The total extent of the supracrustal enclave is not, vert, similar 8 km2; it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions. The quartz-biotite schists record complex zircon growth at not, vert, similar 3500 and not, vert, similar 2800 Ma, interpreted to reflect metamorphic events. Zircons separated from orthogneisses in the enclave - including one sheet that transects a banded quartz-pyroxene (plus/minus magnetite) unit - yield magmatic 207Pb/206Pb ages of ca. 3750 Ma. These ages are slightly younger than earlier provisional reports for an NSB orthogneiss from Porpoise Cove. The Nuvvuagittuq supracrustals are the oldest rocks thus far reported for the Minto Block, they overlap in age with the ca. 3.70 - 3.83 Ga Isua supracrustal belt and Akilia association rocks in West Greenland, and they represent an important new area for exploration of the early Earth.

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