SIO40lecture5

SIO40lecture5 - SIO 40 Life and Climate on Earth Monday Oct...

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SIO 40 – Life and Climate on Earth Monday Oct 4, 2010 Lecture 5 - How the planet changed with life, Part I http://www.humboldt.edu/~natmus/lifeThroughTime/PreCam.web/ Banded Iron Formation, a geological indicator of changing redox conditions on the Earth's surface.
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Outline - Early life and the atmosphere - Methane production - Nitrogen fixation - The rise of O 2 - Geological evidence for cyanobacteria - Geological evidence for increasing O 2 - The timing of the O 2 increase - Summary
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How did the first life affect Earth's atmosphere? Answering this question depends on the nature of the metabolism of early life forms. The fossils in the Apex Chert (left) were originally thought to be photosynthetic cyanobacteria, but these fossils are now questioned. Some suggest that deep-sea hydrothermal vents hosted the first life forms, which must have been non-photosynthetic. Today, many organisms at hydrothermal vents obtain energy by reacting H 2 S from the vents with O 2 in the surrounding water. On the early Earth, however, O 2 was not present. How might non-photosynthetic life have survived on the early Earth?
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Methanogens Some information about the metabolic nature of the earliest life forms can be gained from the "Tree of Life" based on ribosomal RNA. Note the abundance of methanogenic bacteria (underlined in red) among the earliest Archaea.
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CO 2 + 4 H 2 CH 4 + 2 H 2 O Methanogenic bacteria Methanogens use versions of the following reaction to to produce energy: What would be the effect of adding methane to the atmosphere? We can see an example of a methane-dominated atmosphere on Saturn's moon Titan (left) which is enveloped in an orange haze. The haze is a result of photochemical reactions involving methane. Recent laboratory experiments have replicated the haze and indicate that similar conditions could have prevailed on early Earth, serving as a source of organic matter both before and after the emergence of life, providing a UV-shield to the surface, and helping to regulate climate.
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Isotopes of an element have the same # of protons but different numbers of neutrons. THEREFORE, the chemical properties of different isotopes are identical but the mass of each is different. Concept of Isotopes Isotopes are important as tracers of natural processes - eg. 14 C dating Also 13 C 6 protons plus 7 neutrons. 12 C and 13 C are stable isotopes, whereas 14 C undergoes radioactive decay.
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Isotopic evidence for widespread methanogenesis on the early Earth δ 13 C = 1000[{ 13 C/ 12 C} sample /{ 13 C/ 12 C} standard -1] Recall that carbon has several isotopes, including 12 C and 13 C. Methanogenic organisms produce methane that is strongly depleted in 13 C relative to 12 C. This methane may have left a distinctive carbon isotopic signature behind in organic carbon preserved from the early Earth. The ratio of 12 C to 13 C in different materials is usually expressed using the δ ("del") notation, as follows, in units of parts per thousand, ‰, also called parts per mil, relative to a standard substance:
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SIO40lecture5 - SIO 40 Life and Climate on Earth Monday Oct...

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