Earth’s ancient oceans held pockets of oxygen long before the atmosphere finally filled with it, and new rock cores reveal when and how those early “oases” sprang to life. In a 2.93-billion-year-old drill core from Canada’s Red Lake area, scientists found that these fleeting bursts of oxygen were driven by sudden surges in phosphorus recycling beneath an iron-rich, oxygen-poor sea.
“Our data document one of the earliest known intervals of surface water oxygen accumulation, predating the first accumulation of atmospheric oxygen by about 500 Ma,” the team reports—showing that oxygen oases flickered in shallow waters half a billion years before the Great Oxidation Event .
To piece together this hidden chapter, researchers measured redox-sensitive metals like molybdenum and vanadium, tracked iron mineral types, and mapped out how different forms of phosphorus locked into the sediments. They discovered that before each oxygen spike, the ocean bottom cycled from iron-rich (ferruginous) conditions to moments with extra sulfide in the water.
“These intervals were preceded by ferruginous intervals and intervals of enhanced sulfide availability, which led to pulsed increases in oceanic phosphorus bioavailability via anoxic recycling from sediments,” they write, underlining that nutrient release from the seafloor stoked bursts of life.
Phosphorus is a key fuel for photosynthesis, and when it built up suddenly in seawater, ancient microbes could gorge themselves on nutrients and pump out oxygen. The core’s alternating layers of chert, iron formations and siltstones record these cycles: periods of low phosphorus, then flashes of higher phosphorus tied to organic-rich layers.
“Enhanced phosphorus bioavailability would have helped stimulate photosynthetic primary productivity and organic carbon burial, probably exerting a major control on the episodic development of oxygen oases in the late Archaean ocean,” the authors state, highlighting a nutrient feedback loop that lit up the seas.
Although the atmosphere stayed largely anoxic until about 2.43 billion years ago, these local oxygen pockets would have offered early life a taste of breathable waters—perhaps allowing the first complex microbes to emerge. By recycling phosphorus from buried organic matter and iron minerals, the Archaean seafloor set the stage for gradual increases in oxygen.
This work not only pushes back the timeline for surface-water oxygenation but also connects geochemistry and biology in deep time. It suggests that Earth’s first step toward a world rich in oxygen was neither a single event nor a simple switch, but a series of nutrient-fueled breathers in an otherwise breathless ocean.
The study has been published in Nature Geoscience.
