Abundance of oxygen may be linked to speed of Earth’s rotation

A new hypothesis by scientists opens up fresh avenues for further investigation into the oxygenation of our planet

August 15, 2021 by Sandipan Talukdar
A new hypothesis by scientists opens up fresh avenues for further investigation into the oxygenation of our planet

Oxygen is a remarkable component of Earth’s air –  it is the component that fuels life. Oxygen on Earth has not always been abundant. About three million years ago, oxygen in the atmosphere and oceans was scarce. So, how did oxygen become abundant? This question has intrigued scientists for a long time, and they are still struggling to find a satisfactory explanation.

Recently, a research team proposed something new while explaining how oxygen became an abundant component in Earth’s air. They hypothesized that there may be a link between how fast the Earth spins on its axis and the production of excess oxygen in ancient times. The speed of rotation determines the length of a day. 

The team of researchers modeled the early days of the Earth and incorporated evidence from microbial mats that coated the bottom of a shallow, sunlit sinkhole in Lake Huron in North America. The team came out with an intriguing conclusion. As the spin of the Earth gradually decreased and the days became longer, it triggered more photosynthesis from the similar microbial mats, resulting in the building up of oxygen in ancient seas, which eventually diffused into the atmosphere.

The new hypothesis was published in Nature Geoscience on August 2 and has been received with excitement by some scientists. Woodward Fischer, a geobiologist at the California Institute of Technology, who was not involved in the study, was quoted to have commented in a statement that “the rise of oxygen [on Earth] is easily the most substantial environmental change in the history of our planet. The study offers a totally new flavor of an idea. It’s making a connection that people haven’t made before.”

Life appeared on Earth about some four billion years ago. The only living creatures at that time were single-celled and were present in the vast and shallow seas. Many of these earliest life forms were cyanobacteria which can form mats (thin films) on sediments and rock surfaces. Cyanobacteria are present today, too, and these sometimes cause algal blooms, a deadly phenomenon for fish and other aquatic animals.

Cyanobacteria evolved earliest to photosynthesize and could convert carbon dioxide and water into oxygen and sugar. It has been thought that these microbes that can photosynthesize were the earliest suppliers of oxygen for over eons, which ultimately created an environment favorable for aerobic life forms. But a question remains — why did it take a billion years between the emergence of the first photosynthetic microbes (fossils indicate it as 3.5 billion years ago) and the first evidence of a build-up of oxygen.

It was known to scientists that early Earth used to rotate much faster on its axis. It has been estimated that about 4.5 billion years ago, the Earth’s rotation on its axis was so fast that a day was only six hours long. Such estimations come from modeling studies involving the Moon’s distance from Earth and the atmospheric and oceanic tides.

But, about 2.4 billion years ago, the Earth’s rotation slowed down, and days got longer, with about 21 hours in a day.  Again, the rotational speed slowed, and about 700 million years ago and the Earth attained today’s speed, which has been stable since with each day having 24 hours.

The team of researchers that proposed the new hypothesis, led by Judith Klatt, a biogeochemist at the Max Planck Institute of Marine Microbiology, collected samples of the microbial mats. In their lab, they tracked how much oxygen the mats released under various day lengths. They found the longer the exposure to light, the more oxygen they released.

This preliminary tracking of oxygen release depends on the length of a day. Klatt and Arjun Chennu,  co-author of the study and a modeler at the Leibniz Center for Tropical Marine Research, set up a numerical model to estimate how much oxygen the ancient cyanobacteria could have generated.

The cyanobacteria forming microbial mats consumed as much oxygen at night as they did during the day. With Earth’s spin getting gradually slower, the additional oxygen produced by these photosynthetic microbes slowly built up and was released into the water.

The slowing of Earth’s rotation and the big leaps in atmospheric oxygen in the history of the Earth was known to Klatt. The first leap in oxygen was about 2.4 billion years ago when the rotation slowed and daytime elongated. And during the Paleozoic era, which is about 400 million years ago, the final major leap in atmospheric oxygen has been estimated to have occurred.

The new idea opens up many new avenues for further investigation of the oxygenation of our planet. This also helps bridge the missing link of why it took billions of years from the first appearance of the photosynthetic microbes to build-up of oxygen in the atmosphere.