Two Millimetres of a Year: Iris Interviews William on Fossil Coral

## Two millimetres of a year

Morning light catches the edge of a sliced fossil coral. Iris leans toward the table as William turns the specimen beneath the lamp; faint bands emerge, then soften again.

**Iris:** How does a reef animal that builds its own skeleton keep a calendar of seas from forty million years ago? Where does it put the months?

**William:** In what it adds as it lives. A stony coral lays down aragonite, a form of calcium carbonate. The chemistry of that skeleton carries information about the water in which it grew. Follow the growth direction, sample successive thin layers, and you can recover a sequence instead of a single average.

William traces a faint band with a fingertip, keeping the cut face tilted toward Iris.

**Iris:** So those lines are the calendar grid?

**William:** A guide, not twelve neatly printed boxes. The visible bands and the chemical measurements aren't the same thing. Researchers establish the growth sequence and interpret repeating seasonal signals. In this study, they obtained oxygen and carbon isotope series at monthly resolution from unusually well-preserved fossil corals in France, from the Middle Eocene Climatic Optimum, about forty million years ago.

**Iris:** And the skeleton grew only about two millimetres a year. That's a very small filing cabinet.

**William:** It is. Spread that annual growth across twelve months and you're dealing, on average, with roughly a sixth of a millimetre per month. Actual growth needn't be even. The sampling has to distinguish very small increments, and sampling across too much skeleton can blend neighboring parts of the record.

**Iris:** What does an isotope measurement tell us that a band can't?

**William:** Oxygen isotope ratios help track conditions during skeleton formation. Temperature affects them, but so does the isotopic composition of the seawater. They're not a thermometer you can read without assumptions. Carbon isotopes add another line of evidence about the coral's biology and environment.

**Iris:** So neither one is a weather report by itself.

**William:** Exactly. And preservation matters. If later alteration changes the original skeleton, you may be reading what happened after burial rather than what happened while the coral lived. That's why these well-preserved aragonite specimens are valuable.

Iris watches the narrow patch of brightness travel across the slice.

**Iris:** Does growing slowly make the seasons look quieter than they were?

**William:** It can. The measured seasonal swing can be attenuated—smaller than the underlying signal. After correcting for growth-rate attenuation, the study inferred sea-surface temperature seasonality of about seven to eight degrees Celsius. That's the seasonal range, not the average temperature. The isotope evidence also supported the presence of symbiotic zooxanthellae, the photosynthetic partners familiar from many reef corals today.

**Iris:** A hothouse ocean, but not a seasonless one.

**William:** Right. And this is a record from a particular place, not a calendar for every sea on Earth.

He steadies the specimen beneath the light. Two millimetres looks almost like nothing—until Iris follows the direction of growth, and the small distance becomes a year.

Source: “Slow-growing reef corals as climate archives: A case study of the Middle Eocene Climatic Optimum 40 Ma ago,” Science Advances (2022). https://pmc.ncbi.nlm.nih.gov/articles/PMC9122318/