Rye R, Kuo P H, Holland H D
Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Nature. 1995 Dec 7;378(6557):603-5. doi: 10.1038/378603a0.
The composition of the Earth's early atmosphere is a subject of continuing debate. In particular, it has been suggested that elevated concentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar luminosity in early Earth history. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (pO2) before 2.2 Gyr ago. Here we use the same well studied palaeosols to constrain atmospheric pCO2 between 2.75 and 2.2 Gyr ago. The observation that iron lost from the tops of these profiles was reprecipitated lower down as iron silicate minerals, rather than as iron carbonate, indicates that atmospheric pCO2 must have been less than 10(-1.4) atm--about 100 times today's level of 360 p.p.m., and at least five times lower than that required in one-dimensional climate models to compensate for lower solar luminosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in pCO2 than has been thought, or that one or more greenhouse gases other than CO2 contributed significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons.
地球早期大气的组成一直是个备受争议的话题。特别是有人提出，鉴于地球早期历史中太阳光度较低，要维持正常的地表温度，大气中二氧化碳的浓度必须升高。被称为古土壤的化石风化剖面为22亿年前之前的大气氧分压（pO2）提供了半定量限制。在此，我们利用同样经过深入研究的古土壤来限制27.5亿年至22亿年前的大气pCO2。从这些剖面顶部流失的铁在下部重新沉淀为硅酸铁矿物，而非碳酸铁，这一观察结果表明，大气pCO2必定小于10^(-1.4) 个大气压——约为如今360 ppm水平的100倍，且至少比一维气候模型中为补偿27.5亿年前较低太阳光度所需的水平低五倍。我们的结果表明，要么地球早期气候对pCO2增加的敏感度比此前认为的高得多，要么在太古宙晚期和元古宙早期，除二氧化碳之外的一种或多种温室气体对大气辐射平衡有显著贡献。
