Ohmoto Hiroshi, Watanabe Yumiko, Ikemi Hiroaki, Poulson Simon R, Taylor Bruce E
NASA Astrobiology Institute and Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Nature. 2006 Aug 24;442(7105):908-11. doi: 10.1038/nature05044.
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.
在许多年龄超过约24亿年（Gyr）的沉积岩中存在质量独立分馏的硫同位素（MIF-S），而在较年轻的岩石中不存在MIF-S，这被认为是约24亿年前从缺氧大气急剧转变为富氧大气的最佳证据。这是因为已知产生MIF-S的唯一机制是在缺氧大气中对火山二氧化硫气体进行紫外光解。在此，我们报告在西澳大利亚皮尔巴拉克拉通约100米厚的27.6亿年历史的湖相沉积物和29.2亿年历史的海相页岩中均不存在MIF-S。我们对MIF-S地质记录提出了三种可能的解释：（1）太古代时期大气氧含量波动极大；（2）自约38亿年前以来大气一直保持富氧状态，沉积岩中的MIF-S代表了将大量二氧化硫喷入平流层的剧烈火山喷发的时期和区域；或者（3）岩石中的MIF-S大多是在沉积物成岩过程中由非光化学反应产生的，因此与大气化学无关。
