Banded Iron Formations -

Telling the Story of Earth’s Oxygen

(Published in The EarthSphere Blog. Cover Image: Banded Rocks by WM House; ArcheanArt)

Prologue

My last article in the Forgotten Origins series discussed evidence of when Earth’s first free oxygen arrived. But the story of oxygen is nuanced and the path to an oxygenated world, where multi-celled life could survive, was a long one.Geology has helped to unravel the story.

Rocks Tell the Story

One of the great mysteries of Earth’s early development lies in the seemingly late oxygenation of our atmosphere and oceans. Between 2.4 and 2.3 billion years ago, our planet experienced a dramatic rise in the levels of free oxygen. This saturation of the atmosphere and oceans with significant concentrations of oxygen is dubbed “The Great Oxidation Event.” Before then, free oxygen was sparse, and low concentrations of this key ingredient limited the ability of life to evolve into complex multi-cellular organisms. Today almost all life depends on free oxygen for cellular respiration.

Understanding the history of Earth’s oxygen is intimately tied to ancient geological formations called “banded iron formations.” These rocks display beautiful layering, alternating between black and red bands. The importance of this layering derives from how the red bands get their color. The red comes from the inclusion of tiny iron oxide particles in the original sedimentary matrix.

Iron oxide is exactly what the name implies; a compound formed when iron reacts with free oxygen. Think of it as ancient rust. Banded iron formations are major sources of iron ore for modern industry, but the last of these chemical sedimentary rocks formed over 800 million years ago.

The history of banded iron formations is also the history of our planet’s journey from anaerobic to aerobic ecosystems. These formations trace the fluctuating levels of free oxygen in Earth’s atmosphere and oceans.

Banded Iron

Banded iron formations are chemically derived sedimentary deposits. We often think of sedimentary rocks as sands, silts, and clays carried by rivers and streams into oceans and lakes. The sediments accumulate in layers and eventually harden to form solid rocks under high temperatures and pressure from burial.

But chemically derived deposits don’t originate on land; they form through chemical reactions in the oceans. The small particles from these reactions slowly drift to the ocean bottom, forming stratigraphic layers.

Banded Iron formations rely on the presence of iron and the availability of free oxygen to chemically react with it. The oxidized iron particles precipitate out of solution and drop to the ocean bottom. When the presence of free oxygen is sporadic, there are distinct periods where iron oxides form, interspersed with periods where no free oxygen is available, and other types of sediments are deposited on the ocean bottom. This leads to the visual red and black banding these rocks are famous for.

So, banded iron formations only occur when oxygen levels are low enough for all free oxygen to be periodically depleted. The oceans must alternate between aerobic and anoxic conditions. This characteristic makes banded iron formations direct indicators of oceanic oxygen levels, providing us with a record of Earth’s oxygenation.

History of Earth’s Oxygenation

The geologic record shows how Earth slowly became oxygenated. The oldest known banded iron formations are found in Greenland’s Isua Greenstone Belt and are tentatively dated at 3.7 to 3.8 billion years. But these types of deposits did not become widespread until the late Archean from about 2.8 to 2.5 billion years ago. The global disappearance of banded iron formations just after the end of the Archean is critical evidence for the Great Oxidation Event. Once oxygen was constantly available in the oceans, the banded iron deposits could no longer form.

Interestingly two periods of banded iron deposition occurred after the Great Oxidation Event and serve to remind us that Earth’s environment was still in a state of flux. Unknown environmental conditions drained the oceans of free oxygen at 1.8 billion years and again at about 800 million years ago.

The history of these banded iron formations is intriguing and raises questions about the 1.4 billion years between the first hints of free oxygen and the Great Oxidation Event. Various theories are proposed to explain this mystery. One idea posits that most free oxygen in this period was absorbed during the oxygenation of Earth’s mantle. Another focuses on the possible role of supercontinents in providing nutrients to oxygen-producing algae. We will look at these theories in the next Forgotten Origins article.

What we do know is multi-cellular life did not make an appearance on the stage of life until after free oxygen was readily available.