Earth's gradual slowing rotation over billions of years might be a surprising factor behind the increase of oxygen in our atmosphere, according to new research published in Nature Geoscience. Since its formation about 4.5 billion years ago, Earth's day length has steadily increased due to the Moon's gravitational pull slowing its spin.
According to the study, scientists know that 1.4 billion years ago, days lasted about 18 hours, compared to 24 hours today, with the lengthening measured at roughly 1.8 milliseconds per century. This slowdown has had profound effects on life, particularly on cyanobacteria blue-green algae that produce oxygen through photosynthesis and caused the Great Oxidation Event around 2.4 billion years ago.
The study highlights how longer days allowed cyanobacteria to photosynthesise more oxygen because their activity depends not just on sunlight but on the timing of their metabolic processes. Researchers observed microbial mats in Lake Huron, where oxygen-producing cyanobacteria compete with sulphur-metabolising microbes. Cyanobacteria need a "warm-up" period after sunrise, limiting oxygen production during short days.
Oceanographer Brian Arbic and his team conducted experiments and modelling that linked day length to oxygen output by these microbes. Their results suggest longer days created longer "oxygen windows", increasing atmospheric oxygen not just during the Great Oxidation Event but also the Neoproterozoic Oxygenation Event 550 to 800 million years ago.
"This research connects physical planetary changes with microbial life on a molecular level," said marine scientist Arjun Chennu. "It's exciting to see how the Earth's rotation and Moon's influence helped shape the breathability of our atmosphere."
