Chemical evolution of the proterozoic biosphere

当今地球表面的特点是氧气含量高。这对维持地球上许多形式的生命至关重要。相比之下，当生命第一次进化时，大气和海洋基本上不含氧气。在这种条件下，只有某些类型的细菌能够存活。各种证据表明，大约在23亿年前，大气中的氧气含量才开始上升，但此时大气中的氧气含量仍然比现在低得多。直到最近，人们还认为这也导致了海洋的氧化(就像现在的情况一样)，这是早期生命形成的环境。然而，更高级的生命形式，如藻类(最终是人类)，直到很晚才开始进化。最近有人提出，在大气氧气最初上升后，海洋并没有变得有氧。相反，氧气的增加导致陆地上硫化物矿物的风化，这导致河流向海洋输送的硫增加。然后海洋变得富含硫化氢，而不是氧气(类似的情况在现代黑海中也有发现)。硫化氢对许多生命形式都是剧毒的，因此，丰富的硫化氢海洋的发展有助于解释更晚的高等生命形式的进化。事实上，海洋可能只是在氧气第二次上升之后才变得氧气充足。氧气的第二次增加与地球上生命的“爆炸”大致一致，因此表明了氧合作用和生物进化之间的剂量联系。然而，在地球早期历史的很长一段时间里，海洋中含有硫化氢的想法是非常有争议的，需要进一步的研究来测试这种条件是否确实存在。同样重要的是要确定这些条件的普遍程度(即整个海洋是否富含硫化氢)，并确定海洋最终成为氧气的确切时间。幸运的是，通过对当时沉积在海洋中的岩石进行详细的化学检查，有可能回答这些问题。在地球早期历史的很长一段时间里，海洋中沉积了一种被称为“带状铁建造”的岩石。这些岩石含有很大比例的富铁矿物，这种岩石不是在现代环境中形成的。据信，这些岩石只能在含氧量非常低的海洋中形成。大约23亿年前，大气中氧气含量上升后的一段时间，带状铁层的沉积突然停止。该项目将研究在地球上带状铁建造沉积的最后阶段形成的岩石。这类岩石之所以可用于科学研究，很大程度上是因为以前曾在地球表面下的相当深的地方钻探岩芯，以便找到适合经济开采矿物的地点。通过研究条带状铁建造和上覆海洋沉积物中含铁和含硫矿物的类型，将评估目前海洋化学变化的性质。此外，这种情况持续的时间长度将通过检查在两个大气氧气上升时期之间沉积的岩石来探索。将开发新的技术，并将其应用于所有研究的岩石，特别是要确定全球范围内海洋化学变化的性质。这项研究最终应该会更好地理解大气氧气、海洋化学和地球生命进化之间的联系。通过这样做，将实现对宇宙其他地方生命存在的必要条件的更好理解。

The surface of the present day Earth is characterised by high levels of oxygen. This is vital to sustain many forms of life on Earth. By contrast, when life first evolved the atmosphere and oceans contained essentially no oxygen. Under these conditions, only certain types of bacteria were able to survive. Various lines of evidence suggest that the oxygen content of the atmosphere only began to rise about 2.3 billion years ago, but at this time the atmosphere still contained much lower amounts of oxygen than at present. Until recently it was thought that this also led to oxygenation of the ocean (as in the present day), which is the environment where early life formed. However, higher life forms such as algae (and ultimately humans), only began to evolve much later. Recently it has been proposed that the oceans did not become oxygenated after the initial rise in atmospheric oxygen. Instead, the increase in oxygen led to the weathering of sulfide minerals on the land, which resulted in increased riverine delivery of sulfur to the oceans. The oceans then became rich in hydrogen sulfide rather than oxygen (similar conditions are found in the modem day Black Sea). Hydrogen sulfide is highly toxic to many life forms, and thus the development of an ocean rich in hydrogen sulfide helps to explain the much later evolution of higher life forms. In fact, the ocean may only have became oxygenated following a second, much later rise in oxygen. This second rise in oxygen broadly coincides with an 'explosion' of life on Earth, and thus indicates a dose link between oxygenation and biological evolution. However, the idea that the oceans contained hydrogen sulfide for a long period of Earth's early history is highly controversial, and further studies are required to test whether such conditions did in fact exist. It is also important to determine how widespread these conditions were (i.e. was the entire ocean rich in hydrogen sulfide), and to determine the precise time when the ocean eventually became oxygenated. Fortunately, it is possible to answer these questions by a detailed chemical examination of rocks which were deposited in the oceans at this time. For a large period of Earth's early history, a type of rock called a 'banded iron formation' was deposited in the oceans. These are rocks which contain a large proportion of iron-rich minerals, and such rocks do not form in the modem environment. It is believed that these rocks could only form in an ocean containing very low amounts of oxygen. Some time after the rise in atmospheric oxygen around 2.3 billion years ago the deposition of banded iron formations abruptly stopped. This project will examine rocks formed during the final stages of the deposition of banded iron formations on Earth. Such rocks are available for scientific research largely because of the previous drilling of rock cores from significant depths below the Earth's surface, in order to find suitable sites for the economic exploitation of minerals. By examining the type of iron-and sulfur-containing minerals in banded iron formations and in overlying oceanic sediments, the nature of the change in ocean chemistry at this time will be evaluated. In addition, the length of time that such conditions lasted will be explored by examining rocks deposited between the two periods of rising atmospheric oxygen. New techniques will be developed and applied to all of the rocks studied, with a particular aim to identify the changing nature of ocean chemistry on a global scale. This research should ultimately provide a better understanding of the links between atmospheric oxygen, ocean chemistry, and the evolution of life on Earth. In doing so, a better understanding of the conditions necessary for life to exist elsewhere in the universe will be achieved.