Novel single calibration allows reconstruction of paleo-ocean temperatures in both polar regions

Reconstructing past ocean temperatures is essential for understanding how Earth's climate has changed over time and how it may change in the future. But in the polar regions, where temperatures hover around the freezing point and biological activity is highly seasonal, finding reliable temperature records has long been a challenge.

A new study, published in Nature Communications Earth & Environment, presents a promising new solution for both the Arctic and the Antarctic that requires only one calibration. 

The international team of researchers, led by Simon Belt and including Jochen Knies from the iC3 Polar Research Hub, has demonstrated that two lipid molecules produced by marine diatoms—microscopic algae that flourish in polar waters—can be used to reconstruct past sea-surface temperatures in both regions.

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A new polar thermometer

Diatoms produce a variety of organic compounds, including highly branched isoprenoid (HBI) lipids. The researchers found that the relative abundance of two of these compounds, known as HBI III and HBI IV, closely reflects sea-surface temperature.

Using samples from across both polar regions, the team developed a single calibration for both the polar regions that works over a temperature range from approximately -1 to 18 °C. The new proxy was then successfully applied to sediment records spanning timescales from the end of the last Ice Age to a 750,000-year record from Fram Strait.

According to Jochen Knies, co-author of the study:

“The strength of the new proxy lies in its ability to provide a consistent way of reconstructing polar ocean temperatures using the same approach in both hemispheres. This gives scientists a valuable new tool for comparing climate records across the Arctic and Antarctic.”

Figure: a SST reconstructions for core 2 (JM99-1200) located in the Andfjorden (northern Norway). The dark blue line corresponds to the continuous EZ₂₅-based SST record, while the orange and red lines represent partial SST records derived from alkenones and a foraminiferal transfer function^, respectively. b SST records from Palmer Deep (West Antarctic Peninsula): EZ₂₅-based SSTs from core 15 (ODP 1098; dark blue line), TEX₈₆ SST (0–150 m) record from core 15 (ODP 1098; grey line) and  SST record from core 14 (JPC-10; black line). In each case, the proxy standard error is indicated with a vertical error bar symbol using the same colour as the corresponding data plot. Credit: Belt et al. (2026), Nature Communication Earth & Environment.

Reading the climate archive

Marine sediments preserve chemical traces left behind by organisms living in the ocean. Layer by layer, these sediments build up a natural archive that allows scientists to reconstruct past environmental conditions long after the organisms themselves have disappeared.

The new temperature proxy expands the range of information that can be extracted from these archives. By applying it to sediments of different ages, the researchers demonstrated that it captures major climate transitions and long-term changes in polar ocean temperatures.

Jochen explains that:

“Understanding how polar oceans responded to past climate shifts is essential for improving predictions of future change. Reliable reconstructions help scientists test climate models against real evidence from Earth's history, strengthening confidence in projections of how the polar regions may evolve in a warming world.”

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Why it matters

The Arctic and Antarctic play a critical role in regulating Earth's climate. Changes in polar ocean temperatures influence sea ice, ocean circulation, marine ecosystems and, ultimately, the global climate system.

Until now, scientists have had relatively few temperature proxies that perform reliably under the extreme conditions of both polar oceans. The new diatom-based method fills an important gap by offering a robust and widely applicable way to reconstruct past sea-surface temperatures from sediment records.

Developing better tools for reading the geological record allows researchers to place today's rapid environmental changes into a much longer climate perspective. The further back scientists can look, the better they can understand the processes shaping the future of the polar regions.

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Find out more

The study “Diatom lipids open window to past ocean temperatures in the polar regions” is published open access in Nature Communications Earth & Environment.

Lead author Simon Belt is a Professor at the University of Plymouth.

Co-author Jochen Knies is a researcher in the Department of Geosciences at UiT The Arctic University of Norway and at The Geological Survey of Norway. He is part of the iC3 Polar Research Hub where he leads a research unit that aims to reliably estimate how carbon cycles and ocean ecosystems altered during past ice sheet retreat. He additionally co-leads the Into the Blue (i2B) project. His research focuses on reconstructing the past of the Arctic Ocean.