Ever since its formation around 4.5 billion years ago, Earth's rotation has been gradually slowing down, and its days gotten progressively longer as a result.
While Earth's slowdown is not noticeable on human timescales, it's enough to work significant changes over eons. One of those changes, new research suggests, is perhaps the most significant of all, at least to us: lengthening days have now been linked to the oxygenation of Earth's atmosphere.
Specifically, the blue-green algae (or cyanobacteria) that emerged and proliferated about 2.4 billion years ago would have been able to produce more oxygen as a metabolic by-product because Earth's days grew longer.
"An enduring question in Earth sciences has been how did Earth's atmosphere get its oxygen, and what factors controlled when this oxygenation took place," said microbiologist Gregory Dick of the University of Michigan.
"Our research suggests that the rate at which Earth is spinning - in other words, its day length - may have had an important effect on the pattern and timing of Earth's oxygenation."
There are two major components to this story that, at first glance, don't seem to have a lot to do with each other. The first is that Earth's spin is slowing down.
The reason Earth's spin is slowing down is because the Moon exerts a gravitational pull on the planet, which causes a rotational deceleration since the Moon is gradually pulling away.
We know, based on the fossil record, that days were just 18 hours long 1.4 billion years ago, and half an hour shorter than they are today 70 million years ago. Evidence suggests that we're gaining 1.8 milliseconds a century.
The second component is something known as the Great Oxidation Event - when cyanobacteria emerged in such great quantities that Earth's atmosphere experienced a sharp, significant rise in oxygen. Without this oxidation, scientists think life as we know it could not have emerged; so, although cyanobacteria may cop a bit of side-eye today, the fact is we probably wouldn't be here without them.
There's still a lot we don't know about this event, including such burning questions as why it happened when it did and not sometime earlier in Earth's history.
It took scientists working with cyanobacterial microbes to connect the dots. In the Middle Island Sinkhole in Lake Huron, microbial mats can be found that are thought to be an analog of the cyanobacteria responsible for the Great Oxidation Event.
Purple cyanobacteria that produce oxygen via photosynthesis and white microbes that metabolize sulfur, compete in a microbial mat on the lakebed. At night, the white microbes rise to the top of the microbial mat and do their sulfur-munching thing. When day breaks, and the Sun rises high enough in the sky, the white microbes retreat and the purple cyanobacteria rise to the top.
"Now they can start to photosynthesize and produce oxygen," said geomicrobiologist Judith Klatt of the Max Planck Institute for Marine Microbiology in Germany.
"However, it takes a few hours before they really get going, there is a long lag in the morning. The cyanobacteria are rather late risers than morning persons, it seems."
This means the window of daytime in which the cyanobacteria can pump out oxygen is very limited - and it was this fact that caught the attention of oceanographer Brian Arbic of the University of Michigan. He wondered if changing day length over Earth's history had had an impact on photosynthesis.
"It's possible that a similar type of competition between microbes contributed to the delay in oxygen production on the early Earth," Klatt explained.
To demonstrate this hypothesis, the team performed experiments and measurements on the microbes, both in their natural environment and a laboratory setting. They also performed detailed modelling studies based on their results to link sunlight to microbial oxygen production, and microbial oxygen production to Earth's history.
"Intuition suggests that two 12-hour days should be similar to one 24-hour day. The sunlight rises and falls twice as fast, and the oxygen production follows in lockstep," explained marine scientist Arjun Chennu of the Leibniz Centre for Tropical Marine Research in Germany.
"But the release of oxygen from bacterial mats does not, because it is limited by the speed of molecular diffusion. This subtle uncoupling of oxygen release from sunlight is at the heart of the mechanism."
These results were incorporated into global models of oxygen levels, and the team found that lengthening days were linked to the increase in Earth's oxygen - not just the Great Oxidation Event, but another, second atmospheric oxygenation called the Neoproterozoic Oxygenation Event around 550 to 800 million years ago.
"We tie together laws of physics operating at vastly different scales, from molecular diffusion to planetary mechanics. We show that there is a fundamental link between day length and how much oxygen can be released by ground-dwelling microbes," Chennu said.
"It's pretty exciting. This way we link the dance of the molecules in the microbial mat to the dance of our planet and its Moon."
The research has been published in Nature Geoscience.
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