Join us for the next University of Washington Astrobiology Seminar! George Shaw of Union College will be speaking on the topic "A (not so) Brief History of Carbon on Earth." Date/Time: Tuesday, October 24, 2:30PM PDT (11:30am HT/3:30pm MDT/4:30pm CDT/5:30pm EDT)
Abstract: A (not so) Brief History of Carbon on Earth
It is widely agreed that carbon first arrived on Earth in a reduced form, as found in almost all meteorites, and was abiotic in origin. For more than thirty years, the prevailing view has been that the carbon in Earth's early atmosphere (and near surface environment) was virtually all in the form of carbon dioxide, the oxidized chemical state found in volcanic gases that are thought to be the source of atmospheric carbon compounds resulting from degassing of Earth's interior.
For about the same period of time there has also been broad agreement that a large fraction of the near-surface volatiles, including both water and carbon compounds, were degassed very early in Earth's history, implying a carbon dioxide rich early atmosphere. This has been thought by many to be a suitable explanation of the necessary enhanced greenhouse effect required to compensate for the early faint sun.
On the other hand there are several lines of evidence strongly at odds with this model for the early atmosphere:
1) Very early, Rubey pointed out the drastic geochemical and sedimentological consequences of a large CO2-rich atmosphere, including both severe weathering effects and consequent massive deposition of carbonate rocks, for which there is little or no evidence in the early Archean.
2) The delay in oxygenation of the atmosphere following the advent of oxygenic photosynthesis in cyanobacteria, as early as 2.8 BYBP (perhaps even earlier) is a long recognized (if often ignored) problem. Analysis of various sinks and nutrient constraints does not eliminate this problem.
3) The record of carbon isotopes in sediments points to a longstanding (at least since ca. 3.5 BYBP) balance between carbonate carbon and biogenic (fixed organic) carbon at a ratio of about 4 to 1. This implies substantial (and very early) fixation of large amounts of biogenic carbon and release of proportional amounts of free oxygen, which is inconsistent with geologic and isotopic evidence for an anoxic surface environment until ca. 2.1-2.3 BYBP.
These problems could be solved if one could identify a reservoir to hold the degassed carbon and release it into the biosphere on a geologic time scale. The lack of residual early Archean carbonate sediments (or metasediments) from such a hypothetical reservoir speaks against carbonate as the reservoir substance. The likelihood of early degassing precludes a deeper (e.g. upper mantle) reservoir. The only remaining choice is a reduced carbon reservoir at or near the surface. This reservoir cannot be atmospheric methane (or other gaseous hydrocarbon) because photochemical reactions rapidly remove such compounds from the atmosphere.
An early ocean with a high concentration of photochemically (and electrically) produced complex organic compounds solves all of these problems, with the added attraction that it is a favorable environment for the emergence of life. The oxidation of subducted organic rich sediments during upper mantle magmagenesis slowly provides CO2 to the surface environment, on a time scale consistent with the time scale for oxygenation of the surface environment by photosynthetic cyanobacteria, with the record of carbon isotopes in sedimentary rocks, and with the record of carbonate sedimentation.
An early reduced carbon reservoir at/near Earth's surface follows directly from early degassing, under reducing conditions, of the original (and/or hydrogenated) meteoritic carbon compounds. The largely methane atmosphere so produced is short lived, but the photochemical products accumulate in the ocean and are continuously recycled into the atmosphere as methane by low temperature hydrothermal activity. This model provides a suitable source of the early (methane) enhanced greenhouse effect.
For connection information and more, please visit http://nai.arc.nasa.gov/seminars/seminar_detail.cfm?ID=93 [Source: NAI Newsletter]
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