Sulphur isotope evidence for an oxic Archaean atmosphere.
The presence of mass-independently fractionated sulphur isotopes (MIF-S) in many sedimentary rocks older than approximately 2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr ago. This is because the only mechanism known to produce MIF-S has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen-poor atmosphere. Here we report the absence of MIF-S throughout approximately 100-m sections of 2.76-Gyr-old lake sediments and 2.92-Gyr-old marine shales in the Pilbara Craton, Western Australia. We propose three possible interpretations of the MIF-S geologic record: (1) the level of atmospheric oxygen fluctuated greatly during the Archaean era; (2) the atmosphere has remained oxic since approximately 3.8 Gyr ago, and MIF-S in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere; or (3) MIF-S in rocks was mostly created by non-photochemical reactions during sediment diagenesis, and thus is not linked to atmospheric chemistry.
Ohmoto
Hiroshi
H
NASA Astrobiology Institute and Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. ohmoto@geosc.psu.edu
Watanabe
Yumiko
Y
Ikemi
Hiroaki
H
Poulson
Simon R
SR
Taylor
Bruce E
BE
D001105:Archaea / Q000378:metabolism*; D001272:Atmosphere / Q000737:chemistry*; D001315:Australia; D005618:Fresh Water; D019015:Geologic Sediments / Q000737:chemistry; D010100:Oxygen / Q000378:metabolism*; D010777:Photochemistry; D012623:Seawater; D013455:Sulfur / Q000032:analysis*; D013460:Sulfur Isotopes; D013997:Time Factors; D018707:Volcanic Eruptions
D013460:Sulfur Isotopes; D013455:Sulfur; D010100:Oxygen