Researchers learn the day has not always been 24 hours

The days haven’t always been 24 hours long, according to a study published on Feb. 5 in Paleoceanography and Paleoclimatology.

Researchers used high-resolution mass spectrometry to count daily growth rings in fossil mollusk shells by measuring subtle changes in trace elements like Magnesium and Lithium of the shell. They call this new method chemical layer counting and combine it with more traditional growth layer counting and spectral analysis to argue that in the Late Cretaceous period, 100.5-66 million years ago, days were shorter and years were longer.

“A combination of layer counting, spectral analysis of chemical cyclicity and chemical layer counting shows that the rudist precipitated 372 daily laminae per year, demonstrating that length of day has increased since the Late Cretaceous, as predicted by astronomical models,” said study authors Niels J. de Winter, Steven Goderis, Stijn J.M. Van Malderen, Matthias Sinnesael, Stef Vansteenberge, Christophe Snoeck, Joke Belza, Frank Vanhaecke, and Philippe Claeys. 

Researchers used Micro X‐ray Fluorescence to map the entire shell surface so that the best preserved areas could be selected for mass spectrometry sampling. Mass spectrometry measurements were applied along the whole shell length using a circular area 10 micrometers in diameter for sampling. They also used light microscopes with 250x and 400x magnification for visual layer counting before creating an age model to estimate how many day-night cycles each shell had recorded in its layers.

Using visual layer counting, chemical layer counting, and cycle length, researchers came up with the hypothesis that these shells added a layer of laminae per day, likely not affected by tidal change, but affected by the lunar day.

The new approach of chemical layer counting described in this study provides a better tool to determine how long a day lasted in Earth's history. “This independently constrains the evolution of the Earth‐Moon system over geologic time,” researchers wrote. “The proposed method allows a more complete and careful discussion, leading to more reliable boundary conditions for astronomical models for the Earth‐Moon system through geologic time.”