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Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints

机译：化石雨滴的印记使27亿年前的空气密度限制在现代水平的两倍以下

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在距今大约27亿年前的太古代末期,一场降雨rn在来自今天南非草原上一次火山喷发的火山灰rn沉积物上留下了其标记。随着火山灰硬化形成rn凝灰岩，由个个雨点留下的与火山坑相似的rn印迹也被变成化石了。对这些印迹所做的一项rn分析，结合与2010年冰岛Eyjafjallajoekull火山喷rn发所形成的类似印迹进行的对比，表明太占代rn空气密度不到今天空气密度的两倍。与此同rn时，太阳也要比今天暗些，但气候却比较温rn暖。用来解释这种“暗淡年轻太阳”悖论的大rn多数理论都假设，太古代的大气要比今天的大rn气密度大，当时的温室效应也更强，但这项最rn新工作却排除了当时二氧化碳水平较高的可rn能性，“氮压力加宽”可能性仍不大，但却是有rn可能的。%According to the 'Faint Young Sun' paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening or enhanced greenhouse gas concentrations2. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen2"7. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt. Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m~(-3), compared to today's 1.2 kg m~(-3), but because such drops rarely occur, air density was more probably below 1.3 kgm~(-3). The upper estimate for air density renders the pressure broadening explanation1 possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO2 levels required for hot Archaean climates.
机译：在距今大约27亿年前的太古代末期,一场降雨rn在来自今天南非草原上一次火山喷发的火山灰rn沉积物上留下了其标记。随着火山灰硬化形成rn凝灰岩，由个个雨点留下的与火山坑相似的rn印迹也被变成化石了。对这些印迹所做的一项rn分析，结合与2010年冰岛Eyjafjallajoekull火山喷rn发所形成的类似印迹进行的对比，表明太占代rn空气密度不到今天空气密度的两倍。与此同rn时，太阳也要比今天暗些，但气候却比较温rn暖。用来解释这种“暗淡年轻太阳”悖论的大rn多数理论都假设，太古代的大气要比今天的大rn气密度大，当时的温室效应也更强，但这项最rn新工作却排除了当时二氧化碳水平较高的可rn能性，“氮压力加宽”可能性仍不大，但却是有rn可能的。%According to the 'Faint Young Sun' paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening or enhanced greenhouse gas concentrations2. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen2"7. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt. Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m~(-3), compared to today's 1.2 kg m~(-3), but because such drops rarely occur, air density was more probably below 1.3 kgm~(-3). The upper estimate for air density renders the pressure broadening explanation1 possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO2 levels required for hot Archaean climates.

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  • 来源
    《Nature》 |2012年第7394期|p.359-362C3|共5页
  • 作者

    Sanjoy M. Som; David C. Catling; Jelte P. Harnmeijer; Peter M. Polivka; Roger Buick;

  • 作者单位

    Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA,Blue Marble Space Institute of Science, Seattle, Washington 98145,USA;

    Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA;

    Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA,Sustainable Community Energy Network, Edinburgh Centre for Low Carbon Innovation, Edinburgh EH8 9AA, UK;

    Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA,Department of Civil and Environmental Engineering, University of Washington,Seattle, Washington 98195, USA;

    Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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