When Freya Sykes and her team at iC3 grew tiny plankton shells in a lab in Tromsø, they unlocked a more honest way to read the Arctic's climate secrets. Foraminifera—single-celled marine creatures no bigger than a grain of sand—build their shells from the ocean itself, layering magnesium and calcium like a diary written in chemistry. For decades, scientists have used these shells to reconstruct what polar oceans were like thousands of years ago. But there was a problem: nearly all the scientific calibrations for reading them came from warm water. Using a warm-water equation to decode a cold-water shell is like trying to read a story written in the wrong language.
This gap matters profoundly for anyone trying to understand how our climate has changed. The Nordic Seas, the subpolar North Atlantic, and other high-latitude oceans are where the climate system's most dramatic shifts happen—where ocean currents redistribute heat and where carbon gets locked away in the deep. If the tools for reading the past are miscalibrated, the whole story gets distorted. The new study, published in Geochimica et Cosmochimica Acta, extends laboratory-based magnesium-to-calcium calibration for Globigerina bulloides down to 6°C, a temperature range directly relevant to where these creatures actually live in cold seas.
The research reveals something striking: specimens from the Norwegian Sea are more sensitive to temperature than earlier warm-water studies suggested. "Calibrations aren't universal, and need to be developed for the environment they're applied in," Sykes explains. "If we use a warm-water equation to read a cold-water shell, we risk building a climate story on the wrong scale." The team grew living plankton under controlled conditions—precise temperatures, salinities, and seawater chemistry—in the Foraminifera Culturing Lab in Tromsø. They labeled the shells with barium to track newly grown material, then used laser-based mass spectrometry to measure elements in microscopic sections. This methodical work created something scientists had lacked: a direct link between shell chemistry and known ocean conditions in the cold.
The findings also include a cautionary tale. Sodium-to-calcium ratios, which scientists had hoped might signal salinity changes, did not perform as expected. Sodium fell as temperature rose and shifted with seawater chemistry, making it unreliable as a simple salinity tool. Yet it might still help as a cross-check on temperature estimates, especially when combined with magnesium data and solid knowledge of ocean chemistry. Adele Westgård, a co-author, captures the deeper principle: "A good proxy is not just a number. It is a tested relationship between biology, chemistry and the environment."
The work opens a new era in paleoceanography—one that treats shells as living archives rather than passive recorders. Mohamed Ezat, who leads the Tromsø lab, sees broader potential. "Our goal is to develop the Tromsø culturing laboratory into an international hub for experimentally grounded proxy development. By combining culturing experiments, geochemistry and paleoceanography, we can better understand the biological and environmental processes behind the climate signals recorded in marine archives."
For scientists working with sediment cores pulled from the seafloor, the practical benefit is immediate: better, more accurate reconstructions of cold-ocean temperatures. But the larger gift is philosophical—a reminder that understanding the past requires understanding not just the ocean, but the organisms that write its story.