foot or so to arrive at this darker regime, whose only animal life is made up of worms or clams with tubes or burrows that penetrate upward into the sea itself when the tide is high. If we continue to dig, we soon lose even these hardy colonists and find ourselves in a world devoid of animals. It is not devoid of life: rich colonies of microbial bacteria inhabit this world, but animals are nonexistent.
The blackness of this subterranean world tells us volumes about the chemical nature of this region. The black compounds and minerals found here are characteristic of a reducing environment as contrasted to an oxidizing one. A reducing environment is one in which oxygen is in short supply and any that somehow arrives is quickly involved in a chemical reaction. The reduction of chemical compounds in the sediment leads to the distinctive color, because many of the compounds contain carbon, which in substances like coal has a black color, and metals such as iron and lead, which in reduced states have minerals that are also dark in color, caused by the near absence of oxygen. It is the lack of oxygen that precludes animal life.
It is not just the sand that covers a reducing, anoxic world. Water itself can be virtually devoid of dissolved oxygen. Today there are large regions of the Gulf of Mexico that are composed of water volumes essentially lacking in oxygen, and the Black Sea in Asia, between Turkey and the old Soviet Union, is the largest known water body that has oxygen at very low levels. In both areas animal life is rare or absent, depending on the level of dissolved oxygen in the water.
We live in an oxygenated atmosphere. So how could places like the bottom of the Black Sea have little or no oxygen? It has an anoxic bottom because, unlike the larger oceans of the present-day, the Black Sea is composed of highly stratified, rather than mixed (from top to bottom), packages of seawater. Its relatively small size and more importantly, a low number of highly energetic storms or constant strong winds allows the seawater to settle into distinct strata based on their density. The lesson from the Black Sea is that, because of oceanographic conditions, even in a world with a highly oxygenated atmosphere there can be anoxic seas. And even large stretches of ocean considered “oxic” today could change. For example, because of phosphate- and