This is on a Need to Know Basis: 70 Million Years Ago the Year Was 372 Days Long

April 3, 2020 – The Earth’s orbit was roughly the same 70 million years ago. But a day lasted 23.5 of our hours back then. At least that’s what a new study reported in the journal Paleoceanography and Paleoclimatology is telling us. Base on trace chemical records in a particular species of bivalve, Torreites sanchezi, the researchers were able to determine the pace of daily laminations in fossil shells. The laminations represent subtle chemical changes associated with daily changes to light intensity to create a subtly accurate annual calendar.

The Torreites sanchezi clam lived during the Cretaceous 70 million years ago. It is extinct today but it is ideally suited to figuring out the number of days in the year back before the dinosaurs went extinct. Very fast-growing, the clam laid down daily growth rings making it possible for the paleoclimatologists to reconstruct the length of the year.

The Late Cretaceous was a period when the oceans were warmer than today. In the summer ocean waters where the clams thrived reached temperatures of 40 Celsius (104 Fahrenheit). In winter they remained warm at 30 Celsius (86 Fahrenheit). The warmer temperatures played a role in the rapid growth of these clams which grew in beds similar to where we find oysters today.

Torreites sanchezi died out during the same Cretaceous-Tertiary boundary event associated with the death of the dinosaurs. But its fossilized records produce a fascinating pattern of daily and seasonal growth. So fine is the detail that researchers can see in the fossils the layers that show faster growth during daylight and slower growth at night. The variability suggests Torreites sanchezi harboured a resident algae much the way giant clams do today. The fine details in growth laid down over the years show a common pattern in the fossil clam samples that indicate an Earth year back then that ran 372 days long.

Should we be surprised?

Scientists have sufficient evidence in the geological record let alone the historical record that shows Earth’s day was shorter in the past then it is today. In a 2016 paper published by the Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, it reports on an investigation of ancient and medieval eclipses from 720 BCE to 1600 CE that indicate the length of each solar day increased at a rate of 1.78 milliseconds per century. That doesn’t sound like a lot but in geological time that rate of change becomes significant.

A University of Wisconsin study published in 2018 indicated a day on Earth lasted 18 hours 1.4 billion years ago. And shortly after the Earth formed approximately 4.5 billion years ago, a day was much shorter approximately 4.5 hours.

The Moon and oceans govern the length of Earth’s day. The young Earth-Moon system placed the latter in very close proximity, probably no more than a 25,000 kilometer (15,500 mile) separation between the two in its earliest formative history. But since then the Moon has been spiraling outward which today amounts to 4 centimeters (1.57 inches) per year. At such a close proximity the young Moon would have exerted considerable tidal forces on the Earth. This was before the formation of Earth’s surface oceans which appear to have formed when the Moon was approximately half the distance from where it orbits today 380,000 kilometers (235,000 miles) distant from the planet.

The gravitational force exerted by the early Moon would have created extraordinary tides back in that early geological history inundating the shorelines of the planet’s continents twice daily. The extreme tides would have acted like brakes reducing Earth’s rotational rate of spin. And as the Moon continued its retreat from Earth that braking influence would have weakened accounting for a slowing of the rate in the change of Earth’s rotational spin to where we are today.

A final fact related to the Earth-Moon relationship and the determination of the length of each day on our planet. Scientists hypothesize that all of these celestial mechanics in the Moon-Earth relationship jump-started biology in our early oceans some 3.8 billion years ago with the tides playing a significant role in life’s emergence.