Posted byShige Abe
April 27, 2005
Iron Record: Ancient Rocks Tell the Story of Oxygen, and Life
Banded Iron Formations
The Rise and Fall and Rise and Fall and Rise and Fall and Rise… of the Cyanobactia Empire
Banded Iron Formation
Beautiful striped rocks dating billions of years ago tell story of the dramatic risings and fallings of the cyanobacteria. Two billion years ago, the Earth had no plants and no animals. Single-celled organisms, like cyanobacteria, ruled the lakes and oceans.
Cyanobacteria make energy from sunlight by photosynthesis, creating oxygen as a waste product. As the cyanobacteria prospered, they made more and more oxygen. When oxygen concentrations reached a certain level, it poisoned the cyanobacteria, killing the cells. The oxygen concentration then decreased until it attained a level compatible with cyanobacteria growth. The populations then rose again, starting another cycle.
Evidence of the growth and death cycles is in the rocks formed at this time. During the periods of cyanobacteria growth, high concentrations of oxygen caused chemical changes in the water, including the transformation of iron into a form that sinks to the bottom, where rock is formed. During periods with little cyanobacteria, depleted oxygen in the water caused iron to stop falling to the ocean floor, and rock was formed without an iron component.
The rocks that formed at the bottom of these ancient oceans can be read like the rings of a tree trunk. Instead of recording years, the layers of rock record the cycles of ocean chemistry. These rocks show the concentration of iron gradually building as the cyanobacteria populations grew, then the iron band abruptly stops, suggesting that when the oxygen levels became too high, the cyanobacteria were quickly killed off.
Today’s waters still contain cyanobacteria, commonly known as blue-green algae. This name is a misnomer; true algae belong to the domain eukaryotes, not bacteria. The larger and more complex true algae cells contain organelles that originated from cyanobacteria. In fact, all plants today owe their ability to make energy from the sun to the cyanobacteria.
Humans are not the first Earthlings to pollute the environment. Billions of years ago, chemical waste produced by a smaller Earth inhabitant changed the environment, causing many extinctions.
The earliest life on Earth enjoyed an atmosphere free of oxygen. For about 2,000 million years, life was anaerobic, meaning that it thrived without oxygen. Indeed, oxygen, produced as a waste product of energy production by photosynthesis, was poisonous to the cells that produced it as well as their neighbors.
The first photosynthesizing cells, the cyanobacteria commonly known as blue-green algae, started polluting the air with oxygen about 2.2 billion years ago. As life flourished, more and more oxygen was produced and released into the atmosphere. In 200 million years levels of oxygen in the air rose from 1 percent to 15 percent. This caused pronounced global changes.
How the rising oxygen levels affected the oceans is debated. While it’s clear that the pH of the ocean changed and that there were numerous repeated poisoning/extinctions then revivals for ocean life (see side bar), details about whether different parts of the ocean responded differently or the phases in which the change took place are not clear.
Scientists are acting as detectives, finding clues to reconstruct how the atmosphere and oceans on Earth changed so drastically billions of years ago. Many clues can be found in rocks that formed at this time. Because rock composition depends on the environment, oceans that contain more dissolved oxygen will create different rocks than oxygen-free waters.
Evidence of life in iron pyrite
Oxygen reacts with many metal ions, including iron. The formation of rocks from iron in ocean water and the absorption of iron from rock into ocean water is called iron cycling and is important to life, since cells need iron to live.
A team led by NASA Astrobiology Institute investigators hypothesized that the oxygen content in the ocean billions of years ago is recorded in iron pyrite rocks. They measured the iron isotope ratios in ancient rocks to reconstruct the history of oxygen in the Precambrian ocean, the time of oxygen pollution by cyanobacteria.
“Our most exciting discovery is that the rise of atmospheric oxygen had a direct affect on iron cycling in the ocean,” said lead author Oliver Rouxel of the Woods Hole Oceanographic Institution. This means that pyrite does contain a record of the oxygen in the ocean. The researchers analyzed the pyrite clues by using their knowledge of iron chemistry.
Oxygen prefers to form compounds with a particular form of iron called “ferric iron” or “iron (III).” In an ocean without much oxygen, ferric iron precipitates as iron oxide. This iron (III) oxide falls to the ground and becomes incorporated in rock while another form, ferrous iron or iron (II), mostly stays in the water. Some iron (II) will react with sulfur, forming iron pyrite, which gets incorporated into rock. In oxygenated water, all the iron (III) forms iron oxide and the iron (II) forms iron pyrite, leaving very little iron in the water.
In addition to coming in different forms, iron atoms can also have different weights. These isotopes generally act the same chemically, but, for reasons not well understood, heavier atoms of iron (II) are more likely to be transformed into iron (III) than light atoms. The iron (II) left over will be relatively lighter. Therefore, the pyrite formed in an oxygen-rich ocean will have lighter iron than pyrite formed in an ocean without oxygen. This separation of the isotopes is called isotope fractionation.
Rouxel’s study, published in the 18 February 2005 issue of Science, is the third leg of a stool, confirming previous results found through measuring carbon and sulfur, other elements that react with oxygen. Their results support the modern theory that describes a three-stage process of ocean evolution two billion years ago.
Rocks older than 2.3 billion years have low ratios of heavy to light iron isotopes, suggesting that there was little oxygen and a lot of iron in the water. In the second stage, between 2.3 and 1.8 billion years ago, the ratio increased, suggesting that while the atmosphere gained oxygen, the oceans remained mostly oxygen-free. Scientists are currently pondering the reason for the lagging ocean oxygenation. Rocks formed after about 1.6 billion years suggest conditions similar the modern oxygenated air and water environments.
Having found that iron isotope fractionation is a dependable measurement of the oxygen in oceans, Rouxel and his colleagues are now looking into why and how isotope fractionation occurs.
Towards finding life beyond Earth
NAI researcher Abby Kavner of the University of California at Los Angeles is also interested in isotope fractionation.
“Olivier’s paper is based on observations of natural systems—showing changes in iron isotope composition of oceans,” she said. “However, it does not address questions of the fundamental physical mechanisms by which fractionation may be occurring.”
Fractionation is thought to partially be due to chemical processes and partially due to reactions occurring within cells. Why life prefers a lighter form of iron is a mystery that grabbed Kavner’s interest.
The members of Kavner’s group have their eyes set beyond Earth’s oceans. They believe that understanding how life alters the environment on this planet could help people find evidence of life on other planets.
“I had a hypothesis, Electrochemical processes (or electron transfer processes) fractionate isotopes, and I tested it systematically in the lab,” said Kavner. “It turned out to be true.”
Kavner fractionated specific isotopes through electroplating, the same basic technique that coats costume jewelry with a thin layer of metal. This is the first time this has been done.
Kavner wondered if a similar process causes isotope fractionation by cells. She found that the small amount of electricity needed in her electroplating experiments is consistent with the amount produced in the cellular process of electron transfer, according to current electron transfer theories. It is possible that the cells prefer the lighter form of iron because of its particular electrochemical properties.
Iron’s high abundance in the solar system, its ability to take varying forms, and its high stability make it a good candidate when scientists look for biological signatures in rock from other planets. If scientists find a rock from Mars, for example, that has areas with a skewed ratio of iron isotopes, this could be evidence that living systems affected the rock’s formation.
While scientists are still far from being able to test for Martian life from the iron composition of a rock, the work of Rouxel and Kavner brings this goal closer.
Life goes on
After cyanobacteria “poisoning” its environment, life found ways to adapt and change, eventually evolving into the oxygen loving organisms we are familiar with.
The primitive anaerobic cells still exist in deep, dark places where there is little oxygen. They are both living fossils of a younger Earth and essential members of the modern global ecology.