In a new study in the Proceedings of the National Academy of Sciences, researchers used carbonate muds to piece together a more complete timeline of climate changes during the Earth’s Paleozoic era, about 500 million years ago. This era was when animals began to dominate over microbes.

The technical side of this analysis gets into chemistry (sorry, Pamela), so let’s start with a few basics before we dig into the science.

One of the ways in which we measure the age of stuff is called radiometric dating. Most of you have heard of this method in the context of carbon-14, which is an isotope of carbon and used to measure the ages of organic matter. Carbon-14 decays into nitrogen-14, and we can measure the ratio of the two isotopes in a bit of organic matter like wood or bone. We know what the starting isotope ratio should be, and we know the half-life of the carbon-14, so we can figure out the object’s age. But for carbon-14, the half-life is only about 6000 years, much too young for paleontology.

So how did we find out how old all those ancient fossils were? We used different isotope pairs that have longer half-lives, like uranium and lead. And then we used those ages to create a timeline of what creatures lived when and found we can even go further into chemical analysis to learn about the climate at various times.

Enter two other isotopes: oxygen-18 and oxygen-16. Oxygen-18 is the heavier of the two, and it will be the more dominant of the two isotopes at lower temperatures. Co-author Kristin Bergmann explains: As an example, if carbonate precipitates at 4 degrees Celsius, more oxygen-18 ends up in the mineral, from the same starting composition of water, [compared to] carbonate precipitating at 30 degrees Celsius. So, the ratio of oxygen-18 to -16 increases as temperature cools.

Of course, using the oxygen ratio depends on having carbonate shells, and scientists wanted to find another method. In this case, they used “clumped isotope geochemistry”, pairing two isotopes — oxygen-18 and carbon-13. This pairing is dependent on temperature but isn’t affected by alterations due to other ocean chemistry, which is excellent news.

Now here is the easy part, so if you tuned out on the chemistry, come on back. The team used samples of carbonate muds taken in Norway and western Newfoundland, where exposed rocks date back to this early Paleozoic era. Since these rocks had experienced little chemical change since formation, they could be used to test this new method. The temperatures calculated based on the new oxygen-carbon clumps matched the lower-resolution oxygen ratio results, proving the method a success and showing that temperature had an effect on the diversity of life.

This new timeline is more detailed than previous versions because by analyzing a combination of isotopes in these carbonate muds, scientists were able to base their calculations on incredibly common carbonate-rich sediments rather than relatively scarce carbonate shells. It’s an important step to creating an even more complete timeline going back further in the history of our planet since we don’t have fossils to date before about a billion years ago. That leaves us with four billion years of unknown climate data and a handful of potential hypotheses.

More Information

MIT press release

“A high-resolution record of early Paleozoic climate,” Samuel L. Goldberg et al., 2021 February 9, PNAS