Researchers at iC3 have found a way to improve records of past high latitude ocean change using tiny plankton shells called foraminifera.
By growing these foraminifera under controlled cold-water conditions, the team has extended a key temperature tool into the range most relevant for subpolar and polar oceans. The study's results matter for anyone using marine sediments to reconstruct past climate, ocean circulation and carbon cycle change.
A calibration for colder seas
The study led by Freya Sykes focused on Globigerina bulloides, a widespread species of foraminifera whose habitat ranges from the sub-tropics to the sub-polar regions.
Foraminifera are single-celled plankton that build their shells from elements within the seawater. When foraminifera die, their shells can sink to the seafloor and become part of the sediment archive. Scientists later analyse the shell chemistry to infer past ocean conditions.
One of the most useful chemical signals in Globigerina bulloides shells is the ratio between magnesium and calcium. In simple terms, more magnesium often points to warmer water.
However, most existing calibrations were developed using warmer-water specimens. That makes them less reliable in the Nordic Seas, the subpolar North Atlantic and other cold regions.
The new study extends laboratory-based magnesium-to-calcium calibration for Globigerina bulloides down to 6°C. It also shows that specimens from the Norwegian Sea are more sensitive to temperature than expected from earlier warm-water studies.
Local calibration matters
"The main message for other scientists is that calibrations aren't universal, and need to be developed for the environment they're applied in," says Freya. "If we use a warm-water equation to read a cold-water shell, we risk building a climate story on the wrong scale."
The research builds on Freya's earlier iC3 work showing that Globigerina bulloides can follow different life strategies that influence the chemical signals in the shells that they leave behind. That earlier finding matters because shells of different sizes may record different time windows in the ocean.
The new paper adds the chemical calibration needed to read those windows more accurately.
Beyond one-size-fits-all proxies
The study also gives researchers a useful warning.
Sodium-to-calcium ratios did not provide a clear salinity signal under the tested conditions. Instead, sodium fell as temperature rose, and it was also affected by seawater chemistry. That means sodium should not be used as a simple salinity tool in this species.
However, sodium may still be useful. It could provide an independent check on temperature estimates, especially when combined with magnesium and when scientists have good information about seawater chemistry.
"A good proxy is not just a number," says Adele Westgård, a co-author on the new study. "It is a tested relationship between biology, chemistry and the environment. Our publication helps researchers see where the relationship is strong, and where it needs more caution."
Adele recently published a separate study showing that another polar foraminifera species can grow an outer shell crust with a different chemical signal from the shell beneath it.
Biology inside the climate archive
Better shell calibrations help scientists to explore potential climate links with greater confidence.
"This study reminds us that the shell is a living archive, not a passive recorder," says Freya. "To use shells well, we need to understand both the ocean conditions and the organism that made the shell."
The team grew living plankton in the Foraminifera Culturing Lab in Tromsø at controlled temperatures, salinities and seawater chemistry.
They used a barium label to identify shell material grown during the experiment, and laser-based mass spectrometry to measure magnesium, sodium and strontium in small parts of individual shells. This allowed them to link shell chemistry directly to known growth conditions.
"Our broader goal is to develop the Tromsø culturing laboratory into an international hub for experimentally grounded proxy development," says Mohamed Ezat, who leads the laboratory.
"By combining culturing experiments, geochemistry and paleoceanography, we can better understand the biological and environmental processes behind the climate signals recorded in marine archives."
How other scientists can use the findings
For scientists working with sediment cores, the most immediate use is practical. The new calibration can improve cold-ocean temperature reconstructions using Globigerina bulloides. The study also helps researchers choose better proxy combinations, test older records, and avoid using sodium as a simple salinity indicator in this species.
For laboratory scientists, the paper points to the next experiments. Future work can test different genetic types, other ocean basins, and more tightly separated effects of temperature, salinity and seawater chemistry.
For modellers and policy-facing researchers, the benefit is better evidence. More reliable past ocean records can help establish how polar oceans responded to earlier warm periods, freshwater input and changing circulation. That improves the long-term context for today's rapid Arctic and subpolar change.
