Investigating the Precambrian atmosphere, ocean and biosphere with selenium isotopes.

多细胞生命是由我们富氧的大气层维持的，但我们星球的早期历史标志着漫长的缺氧期，在这个时期，这样的生命永远不可能存在。古代和现代地球之间的地质差异表明，早期大气中的氧气不足，无法留下今天普遍存在的氧化痕迹，例如裸露的富铁岩石变红。从古老的缺氧大气到部分含氧大气的转变发生在大约2400-2200年前。在这一点上，氧气水平足以产生平流层臭氧层，保护生命免受紫外线辐射，但氧气水平显然不足以支持动物类生命的呼吸。后来，大约在8亿至6亿年前，氧气水平明显再次上升，随后出现了动物生命。尽管有这样的总体情况，但第一次和第二次氧气上升后达到的确切氧气水平仍然是一个分歧的问题。然而，氧气水平对于确定地球的基本方面至关重要，例如，在氧气第一次上升后，深海是否仍然缺乏氧气，或者确切地说，在阿米巴类生命或多细胞生命中，何时有足够的氧气支持广泛的呼吸。事实上，为什么氧气在地球45亿年的历史中途出现，并在后来增加，仍然是悬而未决的问题。微生物在新陈代谢中使用某些化学元素。硒就是这样一种元素，当微生物将不同形式的硒转化为不溶状态以获得能量时，它们就会区分不同的硒同位素。只有当周围有足够的氧气时，硒才是可溶的，所以沉积物中硒化合物的同位素组成应该反映环境的氧化作用。此外，根据含氧量的不同，硒有两种可溶状态，因此我们应该能够推断出氧水平达到迄今难以捉摸的程度。我们将分析沉积岩，这些岩石采样了地球大氧过渡之前、之后和期间的时代。通过寻找硒同位素的趋势和变化，我们可以推断大气中是否存在氧气，并表征氧气水平的大小。众所周知，硒气体是通过微生物释放到大气中的。当这些气体在古老的缺氧大气中时，可能是大气化学反应产生了反常的同位素信号，即同位素的“与质量无关的分馏”。我们还将测试这一信号，这可能是氧气第一次上升的强大信号。在古代岩石中发现的硒同位素信号也将表明存在利用硒的细菌群。这一推断将意味着这些细菌在岩石沉积时已经进化。如果未来的基因研究显示当今利用硒的细菌与其他细菌之间的关系，那么我们将推进对地球上微生物进化的理解。这项工作的总体结果将有助于解决地球历史上环境的氧气状态。这个问题对于理解生命的共同进化以及大气、海洋和陆地表面的化学是基本的。

Multicellular life is sustained by our oxygen-rich atmosphere, yet our planet's early history is marked by a lengthy anoxic period when such life could never have existed. Geological differences between the ancient and modern Earth show that there was insufficient oxygen in the early atmosphere to leave traces of oxidation that today are ubiquitous, such as the reddening of exposed iron-rich rocks. The transition from the ancient anoxic atmosphere to a partly oxygenated atmosphere occurred around 2400-2200 million years ago. At this point, oxygen levels were sufficient to produce a stratospheric ozone layer that protected life from ultraviolet radiation but oxygen levels were apparently insufficient to support respiration in animal-like life. Later, around 800-600 million years ago, oxygen levels evidently rose again and animal life subsequently appeared. Despite this general picture, the exact levels of oxygen that were reached after the first and second rises of oxygen are still a matter of disagreement. However, the level of oxygen is critical for determining basic aspects of the Earth, such as whether the deep ocean remained devoid of oxygen after the first rise of oxygen or exactly when there was enough oxygen to support widespread respiration in amoeba-like life or multicellular life. In fact, exactly why oxygen appeared about halfway through Earth's 4500 million-year history and increased later are still open questions. Microbes use certain chemical elements in their metabolism. Selenium is one such element, and when microbes convert selenium between different soluble forms or to an insoluble state to gain energy, they discriminate between different selenium isotopes. Selenium is only soluble when there is sufficient oxygen around, so the isotopic composition of selenium compounds in sediments should reflect the oxygenation of the environment. Moreoever, selenium has two soluble states depending upon the amount of oxygen, so we should be able to deduce the oxygen level to a degree that has hitherto proved elusive. We will analyse sedimentary rocks that sample the times before, after and during Earth's great oxic transitions. By looking for trends and changes in the selenium isotopes, we can deduce whether there was oxygen in the atmosphere or not, and characterise the magnitude of the oxygen level. Selenium gases are known to be released to the atmosphere from microbes. When these gases were in the ancient anoxic atmosphere, it is possible that atmospheric chemical reactions produced anomalous isotopic signals, or 'mass-independent fractionation' of isotopes. We will also test for this signal, which may prove a powerful signal of the first rise of oxygen. The discovery of selenium isotope signals in ancient rocks will also indicate the presence of groups of bacteria that use selenium. This inference will mean that such bacteria had evolved by the time that the rocks were deposited. If future genetic studies show how present-day selenium-utilizing bacteria relate to other bacteria, then we will advance the understanding of microbial evolution on Earth. The overall outcome of this work will be to help resolve the oxygenation state of the environment through Earth history. This issue is fundamental in understanding the co-evolution of life and the chemistry of the atmosphere, oceans and land surface.