Tiny plankton shells used to reconstruct past polar ocean temperatures may contain two different chemical stories, a new study by iC3 researchers has found.
The work shows that Neogloboquadrina pachyderma, a key species in polar climate archives, can grow an outer shell crust with a different chemical make-up from the shell beneath it, even when both are grown in the same conditions.
This discovery could help scientists produce more accurate records of past ocean change.
N. pachyderma is the only polar plankton species in a group of a single-celled organisms called foraminifera. It builds a shell of calcium carbonate from the seawater it grows in. When it dies, that shell can sink to the seafloor and become part of marine sediment.
For climate researchers, these shells are valuable. The shell chemistry can preserve clues about the seawater the organism lived in, for example temperature. By analysing fossil shells from sediment cores, scientists can reconstruct past ocean conditions and learn about the impacts earlier warm periods had on the polar oceans.
But the new study shows that the shell is not always a simple archive. The team, led by iC3 researcher Adele Westgård , grew N. pachyderma under controlled laboratory conditions and analysed the chemistry of different parts of its shell.
The team found that the outer crust had lower levels of magnesium, sodium and boron than the inner shell material, even when both parts formed under the same environmental conditions.
“The surprising part is that the shell does not record the environment in one uniform way”, says Adele. “The same organism can build different parts of its shell with different chemical signatures. That means we need to be careful about what part of the shell we are reading.”
One of the most widely used tools in Arctic paleoclimatology and palaeoceanography is the magnesium-to-calcium ratio in foraminifera shells. In simple terms, more magnesium in the shell often points to warmer water. Less magnesium can point to colder water.
This makes the new finding important. If a shell has a thick outer crust with low magnesium, and that crust is measured together with the inner shell, the final temperature estimate may be too cold.
This matters especially in polar regions, where N. pachyderma is one of the most important species available for reconstructing past ocean conditions. It also matters because the amount of crust can vary from one shell to another.
“This is a biological signal inside a climate signal,” says Adele. “Our task is to separate them, so that we can read the past ocean more accurately.”
The researchers grew living Neogloboquadrina pachyderma in the Foraminiferal Culturing Laboratory at iC3's host university. This allowed them to control the environment and test whether shell chemistry changed because of the surrounding water, or because of the organism’s own shell-building process.
(Co-author Mohamed Ezat leads the Foraminiferal Culturing Laboratory and the PlasmaLab , both located within the university department that hosts iC3. Last year, co-author Freya Sykes published a study showing that varied foraminifera life strategies impact fossil records .)
The researchers used a laser-based analytical method to measure trace elements inside individual shells. This made it possible to measure the chemical signal in both the outer crust and the inner shell, rather than dissolving and measuring the whole shell at once.
The researchers then developed code which identifies and separates the two shell layers based on their distinctly different chemical composition.
The result was clear. The crust and inner shell material were chemically different, even though they formed under equivalent environmental conditions. This points to different shell-building processes, not only to changes in temperature or water chemistry.
“Culturing gives us a rare opportunity,” says Adele. “We can watch the organism build its shell under known conditions. That helps us understand which parts of the chemical signal come from the environment, and which parts come from biology.”
Polar regions are changing fast. To understand what may happen next, scientists need reliable records of what happened during past warm periods.
Marine sediments are one of the best sources of that long-term evidence, but they must be interpreted with care.
This study offers a practical way forward. Future studies can use methods that analyse shell layers separately, or select shells that avoid mixing crust and inner material. That could improve reconstructions of past polar ocean conditions, including temperature.
The wider message is simple: Better climate records need better biology.
At iC3 in Tromsø, researchers will continue to combine laboratory culture, fieldwork, microscopy and geochemistry to sharpen the tools used to understand past, present and future polar change.
The study “Laboratory-grown crust in planktic foraminifera Neogloboquadrina pachyderma; insights into resolving inaccuracies in polar palaeotemperature estimates” is published in Geochimica et Cosmochimica Acta . The supporting dataset is available through DataverseNO.
Lead author Adele Westgård is a postdoc at the iC3 Polar Research Hub and the iC3 collaborative program i2B into the Blue . She works on past climate change, polar ocean conditions, geochemistry and marine ecology. Co-author Mohamed Ezat works on past climate variability, ocean circulation, the carbon cycle and foraminifera. Co-author Freya Sykes works on foraminifera, past ocean change and research coordination.
All three are part of an iC3 research unit that aims to reliably estimate how carbon cycles and ocean ecosystems altered during past ice sheet retreat, so that we can improve our modelling of how the future may unfold. They are based at the Department of Geosciences of UiT The Arctic University of Norway, located in Tromsø.
Geochimica et Cosmochimica Acta
Experimental study
Cells
Laboratory-grown crust in planktic foraminifera Neogloboquadrina pachyderma; insights into resolving inaccuracies in polar palaeotemperature estimates
26-Apr-2026