A longer day for a breathing planet: how Earth’s slowing spin may have helped oxygen fill the air

The length of Earth’s day has changed enormously over geological time, and so has the oxygen content of the atmosphere. This article asks whether the two histories are connected, that is, whether the slowing of Earth’s spin could have helped oxygen-producing microbes raise oxygen levels in the air. It explains the three natural records that allow the length of ancient days to be measured, namely tidal rhythmites, climate-driven cycles in layered rocks and daily growth lines in fossil shells, which together indicate a day of about 13 hours some 3.2 billion years ago rising to 24 hours today. It then describes the two competing tidal effects that set the day’s length: the Moon’s gravitational brake on the ocean tides, which lengthens the day, and the Sun’s pull on the atmosphere’s daily thermal tide, which slightly shortens it. Whether the two effects once came close to balancing each other, holding the day near 19 hours for a billion years, is still disputed. Finally, the article shows how day length can change the amount of oxygen released by microbial mats, drawing on published laboratory work with mats from a low-oxygen sinkhole in Lake Huron and on a simple reaction–diffusion model of a mat that the authors wrote themselves. It closes by setting out the measurements that could put the idea to the test.