The Secret to Life on Earth Was What Made It Stop Being a Poisonous Ice Ball

Here’s what you’ll learn when you read this story:

• 2.4 billion years ago, there wasn’t much oxygen to be found in a poisonous atmosphere of methane and carbon dioxide, but that began to change as a frozen Earth began to melt.
• Meltwater carried sulfates from land into the ocean, and when organisms used them to break down organic matter, carbon was buried in sediment, allowing oxygen levels to rise.
• Atmospheric oxygen wasn’t always steady. For 200 million years, levels fluctuated drastically before they became stable enough for life to really flourish.

Earth was a vastly different planet 2.4 billion years ago. Shrouded in a haze of carbon dioxide, methane, and water vapor, its atmosphere was more similar to that of Saturn’s moon Titan than to the nitrogen-and-oxygen mix that dominates today. Any atmospheric oxygen would react with methane and with rock at the surface, locking it into the crust and leaving it unavailable to early life-forms.

Nascent Earth was also atmospherically unstable. Asteroid impacts released immense amounts of hydrogen and methane, and that methane reacted with sunlight and trace amounts of oxygen, slowly oxidizing to carbon monoxide and carbon dioxide over millions of years—a mix still lethal for most living things to breathe. As impacts grew less frequent, the climate cooled, and the carbon dioxide that remained was eventually drawn down into the crust, leaving a frozen surface. At some point, however, oxygen began to accumulate in the atmosphere. How that oxygen managed to keep the planet habitable has remained a mystery, but now, researchers from the University of California, Riverside, and the University of Leeds have found evidence for how it emerged and endured.

Life as we know it needs phosphorus to thrive, and 2.4 billion years ago, a thawing planet finally began to deliver it. Based on rocks from the Transvaal Supergroup in South Africa, the researchers think the chain reaction began when the ice sheets encasing Earth started to melt, and meltwater washed phosphorus and sulfur off the newly exposed land and into the oceans. There, the sulfur bonded with oxygen to form sulfates, which fueled microbes as they broke down organic matter on the seafloor. That decomposition freed the phosphorus locked inside the dead cells—and phosphorus is what life had been waiting for.

The freed phosphorus fueled a surge in biological productivity, and as those organisms died and sank, they carried carbon into seafloor sediments. Carbon burial is the key step, because decaying organic matter normally consumes oxygen, so sealing it beneath the seafloor instead let oxygen accumulate in the atmosphere. Better yet, the cycle fed itself. Each bloom delivered more organic matter to the seafloor, where microbes recycled still more phosphorus back into the water. That self-reinforcing loop drove the Great Oxidation Event.

“Following an enhanced…phosphorus influx that stimulated oxygenation in the aftermath of glaciation, sedimentary phosphorus recycling was controlled by feedback-driven fluctuations in sulphate availability and oceanic [chemical reactions],” the researchers said in a study recently published in Nature Communications.

Through diagenesis, that sediment slowly turned to rock. The rock locked in a record of the phosphorus released from organic matter, and together with the carbon in the Transvaal shale, that record shows how much oxygen made it into the atmosphere. Previous studies had gauged phosphorus levels in ancient rock, but those measurements didn’t necessarily show how much of that phosphorus was actually bioavailable. What this research team wanted to know was how much phosphorus organisms could actually access, as opposed to the phosphorus locked in minerals, which was out of reach. To find out, they isolated minerals from rock samples and dissolved them one by one to separate the phosphorus inside. The ages of the samples then revealed when phosphorus was most abundant (and atmospheric oxygen highest), and when oxygen dwindled in its absence.

Inside the rocks was a history of shifting oxygen levels that made Earth more habitable during some stretches and less habitable during others. After the initial spike in atmospheric oxygen, more and more pyrite was buried in ocean sediment, until that burial outpaced the sulfate delivered by the weathering of land exposed by the ice melt. Pyrite locked sulfur away as iron sulfide instead of leaving it in the water as sulfate, and with fewer sulfates available, there was less microbial processing of organic matter to bury carbon and free phosphorus. Less recycled phosphorus meant less carbon burial, and less carbon burial meant oxygen could no longer build up in the atmosphere. Only after the planet went through another glaciation, and another influx of phosphorus followed in its aftermath, did oxygen concentrations hold relatively stable. Those concentrations grew more consistent with the eons that followed.

“Extreme climatic fluctuations that were a defining characteristic of the Great Oxidation Event drove dramatic variability in atmospheric and seawater oxygenation levels,” the researchers said, “with this instability persisting for over 200 million years.”