Pavlov A A, Kasting J F
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA.
Astrobiology. 2002 Spring;2(1):27-41. doi: 10.1089/153110702753621321.
Mass-independent fractionation (MIF) of sulfur isotopes has been reported in sediments of Archean and Early Proterozoic Age (> 2.3 Ga) but not in younger rocks. The only fractionation mechanism that is consistent with the data on all four sulfur isotopes involves atmospheric photochemical reactions such as SO2 photolysis. We have used a one-dimensional photochemical model to investigate how the isotopic fractionation produced during SO2 photolysis would have been transferred to other gaseous and particulate sulfur-bearing species in both low-O2 and high-O2 atmospheres. We show that in atmospheres with O2 concentrations < 10(-5) times the present atmospheric level (PAL), sulfur would have been removed from the atmosphere in a variety of different oxidation states, each of which would have had its own distinct isotopic signature. By contrast, in atmospheres with O2 concentrations > or = 10(-5) PAL, all sulfur-bearing species would have passed through the oceanic sulfate reservoir before being incorporated into sediments, so any signature of MIF would have been lost. We conclude that the atmospheric O2 concentration must have been < 10(-5) PAL prior to 2.3 Ga.
在太古宙和早元古代（>23亿年前）的沉积物中已报道了硫同位素的质量无关分馏（MIF）现象，但在更年轻的岩石中未发现。唯一与所有四种硫同位素数据相符的分馏机制涉及大气光化学反应，如二氧化硫光解。我们使用一维光化学模型研究了在低氧和高氧大气中，二氧化硫光解过程中产生的同位素分馏如何转移到其他气态和含硫颗粒物种中。我们表明，在氧气浓度低于当前大气水平（PAL）的10^(-5)倍的大气中，硫会以多种不同氧化态从大气中去除，每种氧化态都有其独特的同位素特征。相比之下，在氧气浓度大于或等于10^(-5) PAL的大气中，所有含硫物种在被纳入沉积物之前都会经过海洋硫酸盐储库，因此任何MIF特征都会消失。我们得出结论，在23亿年前之前，大气中的氧气浓度必定低于10^(-5) PAL。
