Biology
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.v42a..03o&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V42A-03
Biology
0315 Biosphere/Atmosphere Interactions (0426, 1610), 0325 Evolution Of The Atmosphere (1610, 8125), 0406 Astrobiology And Extraterrestrial Materials, 1041 Stable Isotope Geochemistry (0454, 4870), 1055 Organic And Biogenic Geochemistry
Scientific paper
Many recent geoscientists have accepted that the record of mass independent fractionation of sulfur isotopes (MIF-S) is a smoking gun for the dramatic change from an anoxic (pO2 < 10-5 PAL) to oxic (pO2 > 10-5 PAL) atmosphere ~2.4 Ga ago, based on the acceptance of two dogmas: (I) atmospheric photochemical reactions are the only mechanisms to create MIF-S, and (II) only rocks older than 2.4 Ga in age bear MIF-S signatures. We question the validity of these dogmas because of the following recent discoveries: (1) the presence of MIF-S in some rocks younger than 1.7 Ga; (2) the absence of MIF-S in many rocks older than 2.7 Ga; (3) a systematic geochemical signature coincident with strong MIF-S in shales; (4) large MIFs in the isotopes of many elements (e.g., U, Cr, Mg) accompanying redox reactions, which are attributed to nuclear field shift effect (i.e., nuclear size and shape effects); (5) MIF-S in the H2S generated from the reduction of sulfate by simple amino acids in our experiments at 150-200°C, while a lack of sulfate reduction by more complex amino acids occurred (Watanabe et al., preceeding paper); and (6) agreements in the Δ33 S(sulfide)- Δ33 S(sulfate) relationships predicted from the nuclear field shift theory with those observed in (a) geologic samples, (b) photochemical experimental products, and (c) reaction products in our amino acids - sulfate experiments. The above discoveries suggest the following three possible reasons for a MIF-S signature in a sedimentary rock: (i) an anoxic atmosphere, (ii) a regional explosive volcanic event, or (iii) unique diagenetic reactions (e.g., sulfate reduction by amino acids) in organic-rich sediments. If (i) was the case, it would imply a yo-yo atmosphere, where the atmospheric pO2 fluctuated from anoxic to oxic, during the Archean. However, we suggest the combinations of (ii) and (iii) were the main reasons for MIF-S signatures in sedimentary rocks for the following reasons: (a) reactive amino acids were probably more abundant in organic-rich sediments of Archean age, compared to younger sediments, because of the probable change in relative abundances of simple/complex amino acids through geologic time (e.g., Jordan et al., 2005); and (b) the Earth's interior was probably hotter and more intensive and extensive volcanic activity took place during the Archean, as indicated by the high abundances of Archean komatiites, granitoids, and mantle plumes. Therefore, the MIF-S record of sedimentary rocks may reflect the changes in thermal structure and types of dominant organisms through geologic time, rather than a change in the atmospheric oxygen level.
Lasaga Antonio C.
Ohmoto Hiroshi
Watanabe Yu
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