Researchers at the Max Planck Institute for Chemistry and Mainz University have developed a method to reconstruct past seawater temperatures from rhodoliths

Understanding how marine ecosystems respond to a warming ocean requires detailed records of past temperatures. An international research team led by Johannes Gutenberg University Mainz (JGU) and the Max Planck Institute for Chemistry (MPIC) in Mainz has now found a new way to reconstruct daily seawater temperatures over a four-month period. The key lies in rhodoliths: nodule-like calcareous structures formed by red algae that occur in oceans worldwide. The new approach turns a single nodule into a high-resolution climate archive that can track daily temperature swings as well as warming trends. Combining three-dimensional X-ray imaging with geochemical analyses and a special algorithm, the team reconstructed daily seawater temperatures from a single rhodolith collected in the central Red Sea. Their findings are published in the journal Communications Earth & Environment.

An archive that was difficult to read

Typically, researchers reconstruct past climate conditions, for example on reefs, using skeletons of corals, bivalves, and microfossils, such as foraminifera. However, these archives are increasingly constrained by shrinking habitats. Rhodoliths, by contrast, occur all over the world from shallow water down to about 200 meters, making them a widely available archive. The challenge has been their complex three-dimensional branching structure and asynchronous growth, which complicate the reconstruction of a clear, linear timeline within the layered calcareous skeleton. The team addressed this by combining several techniques. First, the researchers stained the algae with a dye that specifically accumulates in calcium carbonate and left them to grow in the field for four months, using the stain as a time marker to measure how fast the branches grow. Then, they scanned the nodule using micro-computed tomography, an X-ray technique also used in medical CT scanners, to map its internal growth layers in three dimensions. Next, they fired a laser along each branch to vaporize tiny bits of the algae's skeleton. A mass spectrometer then analyzed these bits to measure changes in the ratios of the chemical elements magnesium to strontium in each layer, allowing the researchers to determine the seawater temperature at the time each layer had formed. Finally, the team applied a data-alignment algorithm known as dynamic time warping to merge the data from multiple branches into a single, continuous timeline. "Each branch captures only part of the record, and determining when each branch grew can be difficult", explained Lena Li, lead author and researcher at JGU and MPIC. "By mathematically aligning the growth of the branches, we can reconstruct a continuous, daily timeline from a single nodule."

Validation in the Red Sea

The researchers applied their method in the Red Sea, an extremely salty, nutrient-poor basin with high evaporation and intense sunlight. On its shallow reef flats, water temperatures range from about 18 to nearly 38 degrees Celsius over the year. These conditions make a precise, high-frequency temperature record particularly valuable. Using a single rhodolith from a central Red Sea reef flat, the team reconstructed 133 days of temperature data from March to July 2024. They then compared the results with measurements from temperature loggers at the site. The reconstruction closely matched the observed values, capturing both the seasonal warming trend and short-term variations.

Beyond rhodoliths

The study is a proof of concept that could be extended to rhodoliths across the globe. "In principle, the alignment technique could also be applied to other fragmentary records, such as several coral cores from a single reef or temporally overlapping records from different organisms, to combine them into a single continuous record spanning longer intervals," Li concludes. In the future, this approach could also be applied to rhodoliths from other locations. This would enable the collection of high-resolution climate data across larger regions and longer time periods. Ultimately, this would help to better reconstruct paleoclimatic development and more accurately predict the impacts of global warming on threatened marine ecosystems.

The study was carried out by an international team from six institutions: Johannes Gutenberg University Mainz (Institute of Geosciences) and the Max Planck Institute for Chemistry in Mainz, together with Kiel University / GEOMAR Helmholtz Centre for Ocean Research Kiel (Germany), the University of Barcelona (Spain), the University of Toronto (Canada) and King Abdullah University of Science and Technology (KAUST, Saudi Arabia), where the rhodoliths were collected and imaged.