Walker J C
Department of Atmospheric, Oceanic, and Space Sciences, The University of Michigan, Ann Arbor 48109, USA.
Glob Planet Change. 1990;82:261-89.
Climate is an important environmental parameter of the early Earth, likely to have affected the origin and evolution of life, the composition and mineralogy of sedimentary rocks, and stable isotope ratios in sedimentary minerals. There is little observational evidence constraining Precambrian climates. Most of our knowledge is at present theoretical. Factors that must have affected the climate include reduced solar luminosity, enhanced rotation rate of the Earth, an area of land that probably increased with time, and biological evolution, particularly as it affected the composition of the atmosphere and the greenhouse effect. Cloud cover is a major uncertainty about the early Earth. Carbon dioxide and its greenhouse effect are the factors that have been most extensively studied. This paper presents a new examination of the biogeochemical cycles of carbon as they may have changed between an Archean Earth deficient in land, sedimentary rocks, and biological activity, and a Proterozoic Earth much like the modern Earth, but lacking terrestrial life and carbonate-secreting plankton. Results of a numerical simulation of this transition show how increasing biological activity could have drawn down atmospheric carbon dioxide by extracting sedimentary organic carbon from the system. Increasing area of continents could further have drawn down carbon dioxide by encouraging the accumulation of carbonate sediments. An attempt to develop a numerical simulation of the carbon cycles of the Precambrian raises questions about sources and sinks of marine carbon and alkalinity on a world without continents. More information is needed about sea-floor weathering processes.
气候是早期地球的一个重要环境参数，可能影响了生命的起源与演化、沉积岩的成分与矿物学以及沉积矿物中的稳定同位素比率。几乎没有观测证据能限制前寒武纪的气候。目前我们的大部分认识都是理论性的。必定影响气候的因素包括太阳光度降低、地球自转速率加快、陆地面积可能随时间增加以及生物演化，尤其是其对大气成分和温室效应的影响。云层覆盖是早期地球的一个主要不确定因素。二氧化碳及其温室效应是研究最为广泛的因素。本文对碳的生物地球化学循环进行了新的审视，这些循环在一个缺乏陆地、沉积岩和生物活动的太古宙地球与一个非常类似于现代地球但缺乏陆地生命和分泌碳酸盐浮游生物的元古宙地球之间可能发生了变化。这种转变的数值模拟结果表明，生物活动的增加如何通过从系统中提取沉积有机碳来消耗大气中的二氧化碳。大陆面积的增加通过促进碳酸盐沉积物的积累可能进一步消耗二氧化碳。尝试对前寒武纪的碳循环进行数值模拟引发了关于一个没有大陆的世界中海洋碳和碱度的源与汇的问题。关于海底风化过程还需要更多信息。
