Biology
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p52b..03m&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P52B-03
Biology
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1094 Instruments And Techniques, 4840 Microbiology, 4870 Stable Isotopes, 5494 Instruments And Techniques
Scientific paper
The general approach to understanding the early development of life on Earth has been to establish the antiquity of rocks by identifying radiometrically dateable sequences containing morphologically classifiable fossils (morphofossils). While this approach has served us well for nearly a century, classical micropaleontological methods used to interpret the Proterozoic (2.5 - 0.54 Ga) record of life are unsatisfactory in studying the early Archean (>3.2 Ga) record, which has been obscured by metamorphism. To gather insights into earlier traces of life, we must see past the limitations of the morphofossil record and recognize the value of chemical fossils. One such approach has been to utilize the strong fractionation that metabolic activity of organisms imparts to light stable isotope ratios (13C/12C, 15N/14N, 34S/32S). The importance of such searches is that they provide a natural test bed for planning the kinds of analyses to be performed on samples returned from elsewhere, even if such searches have had mixed success in ancient terrestrial rocks. For example, the atmosphere, carbon products of mantle degassing and carbonate in water on Earth define the inorganic pool of carbon from which bioorganic carbon is isotopically fractionated. Mass balance calculations demonstrate that the average isotopic composition of terrestrial carbon is ~ δ13C = -6 ‰ and therefore typical metabolic fractionations from this starting value results in δ13C (biomass) < -27‰ . However, problems arise when applying the assumptions based on the terrestrial chemofossil record to another planet. Mars is the strongest candidate for a second planetary biosphere in the solar system. If an ancient biosphere did exist on Mars, returned samples might be expected to yield data that challenge many assumptions about what constitutes an isotopic biosignature. Mars appears to be different from the Earth. The isotopic values for the various reservoirs of carbon, nitrogen and sulfur on Mars have been extrapolated from the study of martian meteorites and from remote spectroscopic measurements of the atmosphere. Carbonate carbon in martian meteorites has values that range at least between δ13C = +7 to +42‰ , which are interpreted to represent the isotopic values of carbonate formed in equilibrium with isotopically heavy (~ +40‰ ) CO2 in the Mars atmosphere and carbon released from martian meteorites at the high temperature stage of step-heating experiments (above 700 C) has δ13C values between -30 and -15‰ , which may represent magmatic values. Depending on the different starting values of the inorganic reservoirs of carbon on Mars described above, possible martian biotic carbon residues would be expected to be either isotopically heavier on average (δ13C > +5‰ ?) or much lighter (δ13C < -40‰ ?) than terrestrial life (average δ13C = -40‰ in the early Archean). Interpreting isotopic values for N and S on Mars present their own unique difficulties. Furthermore, all of these values may have changed over time as the martian atmosphere evolved, which behooves us to link the age of a sample and its geologic context with a specific isotopic signal as we would do with ancient terranes on the Earth. A silver-lining to all of this is that exploring the above possibilities might have the added benefit of differentiating between terrestrial contamination of a returned sample of Mars and the true remains of past martian biota.
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