Flooding Earth’s atmosphere with oxygen may not have needed a triggering event
Just the circulation of basic nutrients may have been enough to affect the balance of gases
![algal bloom](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2019/12/121119_cg_oxygenation_feat.jpg?fit=1028%2C579&ssl=1)
Much like it does today, the nutrient phosphorus helped drive ancient algal blooms in the oceans. That nutrient cycling could ultimately have oxidated Earth’s atmosphere, new research suggests.
Alexandr Trubetskoy/Wikimedia Commons (CC BY-SA 3.0)
SAN FRANCISCO — Maybe the trigger for the rise of oxygen on Earth was nothing special. Maybe that oxidation didn’t need large tectonic shifts or the evolution of land plants. Instead, the circulation of carbon dioxide, oxygen and phosphorus between Earth’s atmosphere, oceans, rocks and the simplest of photosynthesizing life forms is sufficient to produce the dramatic shifts in atmospheric gases that occurred in Earth’s history, new research suggests.
“The [oxygen] transitions we see are driven by Earth’s nutrient cycles,” said Benjamin Mills, a biogeochemist and computer modeler at the University of Leeds in England, who presented the research December 10 at the American Geophysical Union’s annual meeting. The findings, led by Leeds geologist Lewis Alcott, were also published online December 10 in Science.
Early Earth’s atmosphere was a steamy mix of water vapor, CO₂, ammonia, hydrogen sulfide and methane. Then, about 2.4 billion years ago, oxygen in the atmosphere suddenly skyrocketed, a surge known as the Great Oxidation Event (SN: 2/6/17). After another billion years or so, two more large pulses of oxygen to the atmosphere followed. One, called the Neoproterozoic Oxidation Event, which occurred from about 800 million to 540 million years ago, brought oxygen levels to within 10 to 50 percent of modern levels and oxidated the surface ocean. During a final pulse called the Paleozoic Oxidation Event, from about 450 million to 400 million years ago, oxygen rose to modern levels in the atmosphere and penetrated down into the deep ocean.
Such dramatic pulses beg an explanation. Researchers have considered tectonics such as the formation of supercontinents, the uplift and weathering of mountains and the eruption of vast lava fields known as large igneous provinces. Such processes, the idea goes, might have funneled massive amounts of nutrients into the oceans in a short period of time, fueling sudden, world-changing blooms of algae. Other researchers propose that the three stepwise increases correspond to three big evolutionary advances: the rise of photosynthesizing algae, the flourishing and diversifying of those algae and the rise of land plants.