Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere
Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere
复制标题
古代微陨石暗示太古宙高层大气富含氧气
作者：
A. Tomkins;L. Bowlt;M. Genge;S. Wilson;H. Brand;J. Wykes
人们普遍认为，地球早期的大气中含有的氧气（O2）水平不到今天的0.001%，直到大约24亿年前的大氧化事件导致O2浓度大幅上升。有多条证据表明早期地球上的氧气浓度较低，但所有先前的观测都与侏罗纪时期的低层大气成分有关;迄今为止，还没有开发出对侏罗纪高层大气进行采样的方法。我们从石灰岩沉积岩中提取了微陨石化石，这些化石在27亿年前缓慢积累，然后在澳大利亚皮尔巴拉地区保存下来。我们认为，这些微陨石是在沙子大小的颗粒进入地球大气层并在大约75至90公里的高度（考虑到与今天相似的大气密度）融化时形成的。在这里，我们表明，在所产生的宇宙球粒中的FeNi金属被氧化，而熔融，淬火结晶，形成主要是磁铁矿（Fe 3 O 4）的联锁树枝状晶体的球体，与wüstite（FeO）+金属保存在一些颗粒。我们的大气微陨石氧化模型表明，火星上层大气中的氧气浓度可能与现在的地球接近，而且氧气与一氧化碳的比例足够高，可以防止一氧化碳对氧化的明显抑制。这一时期海底沉积物中黄铁矿（FeS 2）的异常硫同位素（Δ 33 S）特征表明，在侏罗纪时期，上层和下层大气之间可能存在最小的混合。
It is widely accepted that Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O2) level, until the Great Oxidation Event resulted in a major rise in O2 concentration about 2.4 billion years ago. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia’s Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth’s atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wüstite (FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Δ33S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.