About 470 million years ago, Earth was in a period known as the Middle Ordovician, during which occurred an explosion of biodiversity never seen before or since. Scientists have long hypothesized that the reason for this massive increase in forms of life has to do with an ancient ice age that started about the same time. Life likes it when the planet cools down a bit, and Earth becoming a bit of an icehouse apparently helped nudge evolution along. But the evidence for this ancient ice age has been indirect.

You see, the planet didn’t look the way it does now. Continents were in different places, water levels weren’t the same. In fact, what is now the South Pole was where today’s north-western Africa is now. And with everything moving around and being buried under a polar ice cap, we haven’t found physical evidence of glacial deposits or even sedimentary features that can help us state for certain, “Yes, we entered an ice age right here.”

So scientists have focused on fossil evidence, which can indirectly reveal changes in ocean temperatures, and on oxygen isotopes, which also show changes in climate. In a new paper published in Paleoceanography & Paleoclimatology, scientists managed to combine both of these techniques by looking at the oxygen isotopes found in calcite crystals in the interior of fossils. They had to use crystals from the interior of the skeletons because oxygen is easily altered in rocks. Lead author Christian Mac Ørum explains: To minimize the effects of diagenesis we have carefully extracted crystals of the mineral calcite from the interior of fossil brachiopods which – much like bi-valves are today – were abundant and diverse during the Ordovician where they lived on the seafloor, filtering seawater for nutrients.

It’s a tedious process, and the team performed it hundreds of times on samples from a Swedish island in order to increase the resolution of the data. Color me impressed. That is a lot of work for one project.

As for the results, they added to the mounting indirect evidence for that ancient ice age. Mac Ørum further explains: With this new study, albeit still indirect evidence, we have now reached a stage where we are beginning to see distinct shorter, unstable pulses of ice sheet growth followed by longer, more stable, colder phases which correspond to increases in continental ice sheet growth. This is exactly the scenario we would expect based on evidence from the last ice age.

More Information

University of Copenhagen press release

“A Baltic Perspective on the Early to Early Late Ordovician δ13C and δ18O Records and Its Paleoenvironmental Significance,” Oluwaseun Edward et al., 2022 February 22, Paleoceanography and Paleoclimatology