Grafite recente em rochas antigas desafia os sinais mais antigos de vida

domingo, maio 22, 2011

Young Graphite in Old Rocks Challenges the Earliest Signs of Life

ScienceDaily (May 21, 2011) — Carbon found within ancient rocks has played a crucial role developing a time line for the emergence of biological life on the planet billions of years ago. But applying cutting-edge technology to samples of ancient rocks from northern Canada has revealed the carbon-based minerals may be much younger than the rock they inhabit, a team of researchers report in the latest edition of the journal Nature Geoscience.

Boston College assistant professor of Earth and environmental sciences Dominic Papineau and colleagues report in the journal Nature Geoscience that carbon laced within ancient rock formations may be millions of years younger than the rock itself, raising questions about the evidence of the earliest signs of life. (Credit: Lee Pellegrini, Boston College)

The team -- which includes researchers from Boston College, the Carnegie Institution of Washington, NASA's Johnson Space Center and the Naval Research Laboratory -- says new evidence from Canada's Hudson Bay region shows carbonaceous particles are millions of years younger than the rock in which they're found, pointing to the likelihood that the carbon was mixed in with the metamorphic rock later than the rock's earliest formation -- estimated to be 3.8 to 4.2 billion years ago.

The samples come from the Nuvvuagittuq Supracrustal Belt, a sedimentary banded iron formation located in the Archean Superior craton, one of Earth's ancient continental shields. Samples were subjected to a range of high-tech tests in an effort to more clearly characterize the carbon in the rock.

Traditional techniques used by scientists have involved collecting samples and crushing them into powder and then determining the bulk characteristics of carbon minerals. The new approach relies upon a variety of microscopy and spectroscopy methods to characterize intact micro-fabricated cross-sections of crystalline graphite removed from the rock samples. The results found that the carbon was very young compared to the age of these oldest rock samples ever unearthed.

"The characteristics of the poorly crystalline graphite within the samples are not consistent with the metamorphic history of the rock," said Boston College Assistant Professor of Earth and Environmental Sciences Dominic Papineau, a co-author of the report. "The carbon in the graphite is not as old as the rock. That can only ring a bell and require us to ask if we need to reconsider earlier studies."

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Young poorly crystalline graphite in the >3.8-Gyr-old Nuvvuagittuq banded iron formation

D. Papineau, B. T. De Gregorio, G. D. Cody, J. O’Neil, A. Steele, R. M. Stroud & M. L. Fogel 

Corresponding author

Nature Geoscience (2011) doi:10.1038/ngeo1155Received 16 December 2010 Accepted 15 April 2011 Published online 15 May 2011


Carbonaceous material present in ancient rocks can be used as an indicator of life during the time the rocks were formed. In particular, evidence for the existence of life more than 3,800 million years ago might come from mineral associations between apatite and graphite in rocks from southern West Greenland1, 2, 3, 4, 5, 6, 7. However, this interpretation is partly based on the assumption that the graphite was formed at the same time as the host rocks, an assumption that has been difficult to prove2, 3, 4, 5, 6, 7. Here we investigate the origins of poorly crystalline graphite associated with apatite in metamorphosed banded iron formations from northern Canada that are 3,750 to 4,280 million years old8, 9, 10, 11. We measured average δ13Cgraphite values of −22.8±1.9‰ (1σ), similar to values from West Greenland sedimentary rocks of comparable age1, 3, 5, 6, 7, 12, 13, 14, and that point to a biological source for this carbon. Our microscopic and spectroscopic analyses suggest, however, that the graphite experienced much lower temperatures than the host rocks during metamorphism. We conclude that the poorly crystalline graphite in these rocks was deposited by fluids after peak metamorphism of the banded iron formations. We suggest that the occurrence of carbonaceous material with low δ13C values in Eoarchaean rocks cannot be used to indicate the presence of a microbial biosphere on the earliest Earth unless the syngeneity of the carbonaceous material in the host rock can be confirmed.


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