As we look deep into the Earth's past, the sparse evidence from old rocks suggests that conditions were once very different.
In particular, the rocks indicate that the early atmosphere contained little or no oxygen. What are these rocks, how do they tell us about a past atmosphere that no longer exists, and how did the atmosphere evolve to its present state?
Banded iron formation: dark layers are rich in the mineral iron oxide magnetite and red layers are rich in jasper. ©AMNH
What questions do banded iron formations answer and pose?
The rocks in question are known as banded iron formations (BIFs). BIFs consist of alternating iron-rich and iron-poor layers, typically only millimeters to centimeters thick. They are chemical precipitates, meaning that they formed when minerals precipitated directly from seawater. (Sedimentary rocks like sandstone and shale form as suspended particles accumulate. Precipitation is the formation of solid matter from material dissolved in the liquid. A familiar chemical precipitate is salt.) Most BIFs are strikingly colorful. In the BIF on display in the Museum's Hall of Planet Earth, which is typical of many such formations, the dark layers are made up mainly of the iron oxide mineral magnetite (Fe3O4) and red layers of jasper, a variety of chalcedony, or very fine-grained quartz (SiO2). The red color comes from the presence of minute grains of iron oxides. The most ancient banded iron formation known is 3.8 billion years old, but most are 2.6 to 1.7 billion years old, and no true BIFs are any younger. BIFs also happen to be of more than just scientific interest—they are the source of more than 90% of our iron.
An illustration of a Warrawoona microfossil. ©AMNH
BIFs give rise to a number of questions. If we were to collect and analyze present-day seawater, we would discover that it contains essentially no iron. So, the first question is, how could these iron-rich chemical sediments have formed in the oceans? The answer lies in the fact that the iron content of seawater is negligible because the water is oxygenated: i.e., it contains dissolved oxygen (which is, of course, what fish breathe). Only in water that contains no oxygen—anoxic water—can iron dissolve in any significant quantities. (In anoxic water iron dissolves in the ferrous state as ions of hydrous Fe2+, or FeOH+.) Therefore, in order for iron-rich chemical precipitates to form, the early oceans must have been sufficiently anoxic to dissolve iron. Since the ocean and atmosphere exchange oxygen rapidly, the atmosphere could not have contained much oxygen, either. Furthermore, the presence of iron-rich sediments indicates that there must have been a source of oxygen to transform the dissolved ocean water iron into iron oxide minerals.
This brings us to a second question: Why did BIFs form only in the distant past? The iron now in BIFs and once dissolved in the ocean must have represented an enormous sink for oxygen (i.e., this iron reacted with and removed oxygen from the ocean and atmosphere). The theory is that oxygen was continually supplied to the ocean and atmosphere, but as quickly as it was supplied it was consumed by the oxidation of organic carbon and volcanic gas, and by the precipitation of iron oxide minerals in the ocean. This continued to happen until the ocean simply ran out of iron. A slow buildup of oxygen in the atmosphere/ocean began when the rate of oxygen production began to exceed the rate of its removal. This probably began to happen about 2.2 billion years ago. By the end of the Precambrian, 543 million years ago, the oxygen content of the atmosphere was 2% to 20% of its present level. This made possible the evolution of organisms that utilize oxygen for their metabolisms, as all animal life does.
The oxygen necessary to form BIFs almost certainly came from photosynthesis of early organisms. But let's consider a couple additional questions first. Where did the enormous amount of iron present in BIFs come from? Some may have washed into the oceans from the continents, but some of it, perhaps even most of it, was introduced into the ocean from submarine hot springs. Today, heat from magma bodies drives the circulation of hot water through the ocean crust at mid-ocean ridges. The hot water leaches iron and other metals out of the rocks. When this hydrothermal fluid, as it is called, is injected into the ocean, the metals in the fluid immediately precipitate to form sulfides and other minerals around the vents.
Finally, it is impossible to look at BIFs and not wonder why they are so delicately laminated, or layered. One theory is that the alternating layers reflect seasonal variations—but how seasonal variations might cause different minerals to precipitate on the ocean floor remains a mystery.
This view of the shallows of Shark's Bay, Australia, shows a colony of living stromatolites. ©Isao Inouye (University of Tsukuba), Mark Schneegurt (Wichita State University), and Cyanosite
What early organisms produced oxygen, and when?
The geologic record suggests that life was the source of the oxygen that formed BIFs and subsequently built up in the atmosphere.
Fossils indicate that life on Earth was entirely microbial until about a billion years ago. (The Proterozoic, the time from 2.5 billion to 543 million years ago, is informally known as the "Age of Bacteria.") The oldest fossils are mere microscopic forms in rocks of the 3.3 to 3.5 billion year old Warrawoona Group, a sequence of metamorphosed sediments in Western Australia. The organisms appear to have grown in shallow seas near the surface, and represent a diverse group of microbes resembling cyanobacteria (blue-green algae). Cyanobacteria are light-sensitive. If this interpretation is correct, photosynthesis—the process by which plants use sunlight to convert carbon dioxide to food and energy, and of which oxygen is a byproduct—must have been extensive in the early Archean (3.8 to 2.5 billion years ago).
The complexity and diversity of the Warrawoona microfossils implies that life had existed for long enough to have time to evolve. How long? We don't know. The craters on the Moon suggest that the Earth also experienced heavy bombardment during its first 500 million years of existence. That in turn suggests that life may not have gained a permanent foothold until about 3.8 billion years ago. The evidence of such ancient life is indirect and sketchy. It amounts to old sedimentary rocks from west Greenland that contain carbon in a ratio of 12C to 13C , to be expected if the carbon were a remnant of life.
Before the beginning of the Paleozoic 543 million years ago, when hard-shelled metazoans (animals, or muliticellular organisms) appeared, fossils are almost exclusively limited to structures known as stromatolites. Present-day stromatolites are mounds of microbial masses that trap or precipitate sediment. They build their way upward and laterally on the floor of shallow seas and lakes, and are made of a variety of microbes, such as cyanobacteria. Early Precambrian stromatolites must have also inhabited this niche. Since the atmosphere contained little or no oxygen, there was no protective ozone layer. Stromatolites could have lived only in water shallow enough for sunlight to penetrate but deep enough to block out most of the sun's harmful ultraviolet radiation.
Beginning about 2.6 billion years ago, fossil stromatolites became relatively common and diverse in form. This may have related to the more rapid growth of continents which started at that time. The increase in the area of continents probably also increased the areas of shallow seas on their margins, thus greatly extending stromatolite habitats. The dominance of stromatolites ended abruptly between 600 and 700 million years ago, for reasons we don't understand. It occurred during a period of long and extensive glaciation, and perhaps the colder climate substantially reduced stromatolite habitat.