Tuesday, October 19, 2010
We're not in Kansas anymore....
This weeks lecturer will be discussing early Earth - going back in time to the point at which the first whiff of oxygen appeared.
The Hadean Era was the first in Earth history, extending from the first formation of continental crust, which began some time around 4.4 billion years becoming persistent soon after, to approximately 3.5 billion years. The earliest terrestrial environments were harsh: Levels of atmospheric oxygen around 1% were too low to sustain an ozone layer, without which there would have been little protection from solar radiation. High levels of atmospheric carbon dioxide and methane would have created a strong greenhouse effect, with global temperatures estimated to have been between 30 and 50°C. Oceans formed early, between 4.4 and 3.9 billion years, from condensation of atmospheric water vapor. Estimates suggest that the earliest oceans were hot (between 80 and 100°C) and acidic.
University of Colorado researcher Stephen Mojzsis explains what Earth might have looked like back then:
"Before 4 billion years ago, the Earth would not be recognizable for the pale blue world that we are familiar with today. Indeed, although we now understand that there were significant landmasses already present by that time, the denser carbon dioxide-rich atmosphere would have given the sky a reddish-tinge. The oceans, with a much higher concentration of iron than our contemporary oceans, would look a dark greenish-blue and these oceans would have bathed hundreds of small continents akin to New Zealand or the Japan arc,"
This weeks reading is a paper published by Mojzsis about evidence for life on earth at 3.4 billion years ago. Mojzsis measured carbon isotopes in inclusions within apatite grains. The isotopes (δ13C) were light – meaning they were enriched in 12C relative to 13C. Organisms turning CO2 into organic matter favor the 12C atoms over the 13C atoms because metabolic processes involving the lighter isotope occur faster. So inorganic carbon (C not fixed by organisms) has a δ13C of ~-10‰, carbonaceous fossils have a δ13C of ~-20 to -35‰ and photosynthesizers, ~-50 to -60‰.