Hessler Angela M, Lowe Donald R, Jones Robert L, Bird Dennis K
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA.
Nature. 2004 Apr 15;428(6984):736-8. doi: 10.1038/nature02471.
The quantification of greenhouse gases present in the Archaean atmosphere is critical for understanding the evolution of atmospheric oxygen, surface temperatures and the conditions for life on early Earth. For instance, it has been argued that small changes in the balance between two potential greenhouse gases, carbon dioxide and methane, may have dictated the feedback cycle involving organic haze production and global cooling. Climate models have focused on carbon dioxide as the greenhouse gas responsible for maintaining above-freezing surface temperatures during a time of low solar luminosity. However, the analysis of 2.75-billion-year (Gyr)-old palaeosols--soil samples preserved in the geologic record--have recently provided an upper constraint on atmospheric carbon dioxide levels well below that required in most climate models to prevent the Earth's surface from freezing. This finding prompted many to look towards methane as an additional greenhouse gas to satisfy climate models. Here we use model equilibrium reactions for weathering rinds on 3.2-Gyr-old river gravels to show that the presence of iron-rich carbonate relative to common clay minerals requires a minimum partial pressure of carbon dioxide several times higher than present-day values. Unless actual carbon dioxide levels were considerably greater than this, climate models predict that additional greenhouse gases would still need to have a role in maintaining above-freezing surface temperatures.
对太古宙大气中存在的温室气体进行量化，对于理解大气氧的演化、地表温度以及早期地球的生命条件至关重要。例如，有人认为，两种潜在温室气体二氧化碳和甲烷之间平衡的微小变化，可能决定了涉及有机霾形成和全球变冷的反馈循环。气候模型一直将二氧化碳视为在低太阳光度时期维持地表温度高于冰点的温室气体。然而，对27.5亿年历史的古土壤（保存在地质记录中的土壤样本）的分析最近对大气二氧化碳水平给出了一个上限，该上限远低于大多数气候模型中防止地球表面冻结所需的水平。这一发现促使许多人将目光投向甲烷，将其作为满足气候模型的另一种温室气体。在这里，我们利用32亿年历史的河砾石上风化外皮的模型平衡反应表明，相对于常见粘土矿物而言，富含铁的碳酸盐的存在需要二氧化碳的最低分压比当前值高出几倍。除非实际二氧化碳水平远高于此，气候模型预测额外的温室气体仍需在维持地表温度高于冰点方面发挥作用。
