Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints
Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints
复制标题
27亿年前的空气密度因化石雨滴印记而限制在现代水平的两倍以下
作者：
S. Som;D. Catling;J. Harnmeijer;P. Polivka;R. Buick
根据“微弱的年轻太阳”悖论，在太古宙晚期，太阳亮度降低了大约20%，使早期的地球变暖，因此它有液态水和温和的气候。对这一现象的解释是，大气密度更大，通过氮压力扩大或增加温室气体浓度来提供温暖。地球化学研究和数值调查对太古代大气气体（包括甲烷、二氧化碳和氧气）给出了近似的浓度限制，这是允许的。但是没有关于地面空气密度和气压的实地数据报道，使得这些不同假设的合理性受到怀疑。在这里，我们展示了南非Ventersdorp超群凝土中的雨滴印记，将27亿年前的地表空气密度限制在不到现代水平的两倍。我们通过实验来解释雨滴化石，在实验中，已知大小的水滴以终端速度落在新鲜和风化的火山灰中，从而定义了印记大小与雨滴撞击动量之间的关系。雨滴变平后的破碎将雨滴的大小限制在与空气密度无关的最大值，而雨滴的终端速度则与空气密度的平方根成反比。如果太古代的雨滴达到现代测量的最大尺寸，那么空气密度一定小于2.3 kg m - 3，而今天的雨滴为1.2 kg m - 3，但由于这样的雨滴很少发生，空气密度更可能低于1.3 kg m - 3。对空气密度的较高估计使压力扩大的解释成为可能，但在可能的较低估计下则是不可能的。我们的研究结果也排除了太古宙炎热气候所需的极端二氧化碳水平。
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 concentrations. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen. 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 kg m−3. The upper estimate for air density renders the pressure broadening explanation possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO2 levels required for hot Archaean climates.