Encyclopedia of geochemistry. A comprehensive reference source on the chemistry of the Earth / Геохимическая энциклопедия. Всеобъемлющий справочный источник по химии Земли

Few fields within the earth sciences have grown as rapidly or had as great an impact as geochemistry. The impact of geochemistry has been vast and greatly influenced our understanding of virtually every aspect of this planet, its neighbors, and the life that makes it so unique.As an example, consider what Darwin wrote in 1859 in Origin of Species: “If the theory [evolution] be true, it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed. . .and that during these vast periods, the world swarmed with living creatures. But to the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer.” Darwin understood that the Precambrian comprised “long periods” but had no idea how long. Geochemistry, in particular analysis of radioactive elements and their daughter products, provided the answer: four billion years. While the answer to “ how long” could have been given more than half a century ago, the answer to his question “why” has only become apparent within the last decade, and that answer also comes from geochemistry. Recent studies of redox-sensitive metals such as uranium and molybdenum in ancient marine sediments have revealed that although the Earth’s atmosphere first became oxidizing about 2.3 billion years ago, oxygen levels remained too low to support animal life for nearly another 2 billion years. Animals appear in the fossil record shortly, geologically speaking, after that rise. <...>