Want an oxygen-rich atmosphere? Stuff oxygen’s friends in the mantle.
Want an oxygen-rich atmosphere? Stuff oxygen’s friends in the mantle.
想要富含氧气的大气层？把氧气的“朋友”塞进地幔里吧。
Planet Earth has some pretty great qualities going for it. Pretty high on the list of positives is a richly oxygenated atmosphere. But that’s something that evolved and built up over a couple billion years, only eventually resulting in a world conducive to animal life like us. 地球拥有许多得天独厚的优良品质，其中最重要的一项就是富含氧气的大气层。但这并非与生俱来，而是经过了数十亿年的演化与积累，最终才造就了一个适合像我们这样的动物生存的世界。
Scientists have many ideas about what could have caused oxygen to increase, and it seems that a number of them are probably correct. No one thing in isolation seems to explain it. Life is part of the story, with photosynthetic life pumping out oxygen. The chemistry of the solid Earth also had a role to play, both through supporting photosynthetic life and through reactions that can shuttle oxygen between the atmosphere and rocks deep inside the Earth. 科学家们对于氧气含量增加的原因提出了许多见解，其中不少观点可能都是正确的。单一因素似乎无法完全解释这一现象。生命是其中的一部分，光合生物不断释放出氧气。固体地球的化学作用也发挥了重要作用，它既支持了光合生物的生存，也通过化学反应在地球大气层与深层岩石之间进行氧气的交换。
A new study led by Wei Shi of the Chengdu University of Technology suggests that evidence of changes in the subduction of tectonic plates—the process by which they disappear down into Earth’s interior—lines up with the timing of jumps in oxygen levels. 由成都理工大学的施炜（音译）领导的一项新研究表明，构造板块俯冲（即板块沉入地球内部的过程）的变化证据，与大气氧含量激增的时间节点高度吻合。
Cooling off
冷却过程
The Earth has gradually cooled over time, and the scant remnants of its earliest history show us that major geologic processes evolved quite a bit as a result. Early in its history, cold, dense surface rock would have sunk through hot mantle rock in ways that bear little resemblance to modern plate tectonics. And the continents around us are 4.5 billion-year-long construction projects, so imagination is required to picture what was present early on. 随着时间的推移，地球逐渐冷却，其早期历史中留下的稀少遗迹向我们展示了主要地质过程是如何随之演化的。在地球历史早期，寒冷且致密的表层岩石穿过炽热的地幔岩石下沉的方式，与现代板块构造几乎毫无相似之处。我们周围的大陆是历经45亿年构建而成的“工程”，因此我们需要发挥想象力来还原早期的景象。
It wasn’t a smooth, linear evolution—there seem to be transition points in that geologic history. The oxygenation of Earth’s atmosphere wasn’t linear, either. It started with a jump during the Great Oxygenation Event about 2.4 to 2.0 billion years ago. But then it stalled out until resuming between 800 and 500 million years ago. A third increase between 450 and 250 million years ago brought us up to modern oxygen levels. 这并非一个平滑的线性演化过程——在地质史上似乎存在着多个转折点。地球大气的氧化过程也并非线性的。它始于约24亿至20亿年前“大氧化事件”期间的一次激增，随后陷入停滞，直到8亿至5亿年前才恢复增长。而在4.5亿至2.5亿年前的第三次增长，最终将氧气含量提升到了现代水平。
The research team’s idea was that changes in subduction might have influenced atmospheric oxygen by controlling how much carbon and sulfur—both of which love to bond with oxygen—were being carried into the deep interior of the Earth. 研究团队认为，俯冲作用的变化可能通过控制碳和硫（这两种元素都极易与氧结合）被带入地球深处的数量，从而影响了大气中的氧含量。
When the mantle is hotter, carbon and sulfur don’t make it very far down with the subducted rock. They’re released into the shallow mantle and can soon come back into the atmosphere via volcanoes, ready to scavenge any plucky molecules of oxygen present in the atmosphere. The converse is that a plate diving into cooler mantle will hang on to more of its sulfur and carbon. 当地幔温度较高时，碳和硫无法随俯冲岩石深入地底。它们会被释放到浅层地幔，并很快通过火山活动回到大气层，随时准备“清除”大气中存在的氧分子。反之，当板块俯冲进入较冷的地幔时，则会携带更多的硫和碳。
Tectonic shifts
构造变迁
Running this history of subduction through a basic chemical model, the researchers found they could roughly reproduce the timeline of oxygenation. The beginning of the story, they say, could be the assembly of an early “supercontinent” (think Pangaea) called Columbia. With an appreciable amount of land above sea level, erosion could deliver enough nutrients to the oceans to support a large amount of photosynthetic cyanobacteria. 研究人员将这段俯冲历史代入一个基础化学模型中，发现他们可以大致重现氧化的时间线。他们认为，故事的开端可能是早期“超大陆”（类似于盘古大陆）——哥伦比亚超大陆的形成。随着大量陆地露出海平面，侵蚀作用将足够的营养物质输送到海洋，从而支持了大量光合蓝细菌的繁衍。
The breakup of Columbia aligns with the first signs of lower-temperature subduction. That would have enabled more of this organic carbon—and carbonate accumulating in shallow water around Columbia—to be subducted deep into the mantle. Then comes the Boring Billion, when even mantle convection and tectonic plate movement seem to have been sluggish. 哥伦比亚超大陆的解体与低温俯冲的最初迹象相吻合。这使得更多的有机碳以及在哥伦比亚超大陆周围浅水区积累的碳酸盐能够被俯冲带入地幔深处。随后便是“枯燥的十亿年”（Boring Billion），当时地幔对流和板块运动似乎都处于迟滞状态。
But after that, the formation and breakup of the supercontinents Gondwana and Pangaea move us toward a map of tectonic plate boundaries that looks like our present world, with lots of low-temperature subduction. The “Ring of Fire” around the Pacific Ocean today, for example, marks a huge zone of subduction that continuously carries carbon and sulfur-rich sediments deep into the mantle. Once this sort of subduction became common, the balance of Earth’s oxygen was able to tilt more toward the atmosphere. 但在此之后，冈瓦纳大陆和盘古大陆的形成与解体，使板块边界的分布逐渐演变成我们今天所见的样子，并伴随着大量的低温俯冲。例如，今天环绕太平洋的“火环”标志着一个巨大的俯冲带，它不断地将富含碳和硫的沉积物带入地幔深处。一旦这种俯冲变得普遍，地球氧气的平衡就能够更多地向大气层倾斜。
There certainly is a lot more to the story, both in terms of biology and geology. Our oxygen-rich atmosphere is the product of a rich set of interactions. But, the researchers write, “These processes all operated on top of the baseline defined by the net flux of carbon (and sulfur) between Earth’s interior and exterior, which we argue was controlled by the evolving efficiency of cold subduction on a cooling Earth.” 无论是在生物学还是地质学层面，这个故事远不止于此。我们富含氧气的大气层是一系列复杂相互作用的产物。但研究人员写道：“所有这些过程都是在地球内部与外部之间碳（和硫）净通量所定义的基准之上运作的，我们认为，这一基准是由冷却中的地球上不断演变的低温俯冲效率所控制的。”