Environmental physicists reveal fluctuations over the past 230,000 years based on the isotopic composition of deep-sea sediment samples

Over the past 230,000 years, the growth and retreat of the Icelandic ice sheet have led to major changes in the seawater chemistry of the North Atlantic, caused by the interaction between volcanism and ice in Iceland. Scientists at the Institute of Environmental Physics at Heidelberg University have demonstrated this using sediment cores from the northeastern Atlantic – the Rockall Plateau. The researchers analyzed the isotopic composition of the rare-earth element neodymium in these drill cores. According to their findings, the isotopic signal fluctuated significantly over the course of glacial and interglacial periods, following the “rhythm” of the ice ages, as shown by the studies led by Prof. Dr. Norbert Frank.

The isotopic composition of neodymium (Nd) in seawater is one of the most commonly used tools for reconstructing how ocean circulation has changed in the recent geological past. The method is based on the central assumption that the chemical “fingerprints” characteristic of the composition of large water masses have remained largely constant over time. Research by the Heidelberg scientists now suggests that this fixed isotopic composition no longer applies to the North Atlantic during glacial periods. Their analyses of deep-sea sediments from the Rockall Plateau show that the chemistry of North Atlantic seawater has changed repeatedly and significantly. The researchers attribute these changes to volcanic material from Iceland.

Their investigations are based on the fact that different types of continental rock exhibit different isotopic signatures. The ratio of the specific isotopes ¹⁴³Nd and ¹⁴⁴Nd of neodymium dissolved in seawater reflects the rocks from which the rare-earth element was extracted. As the ice sheet in Iceland advanced during a glacial and carved its way through the Icelandic basaltic bedrock, it produced large quantities of fine, highly reactive rock dust. This material was released into the surrounding North Atlantic Ocean, where it dissolved and released neodymium with a distinct radiogenic signature – enriched in ¹⁴³Nd. Analysis of neodymium isotopes from the ODP Site 982 sediment core on the Rockall Plateau shows that the upper water masses of the North Atlantic were consistently much more radiogenic during glacial maxima than during the warm interglacial periods, and that these fluctuations correlated closely with the volume of the Icelandic ice sheet.

The Heidelberg scientists developed a so-called box model that allowed them to reproduce their analyses of the deep-sea sediment samples. According to the model, the changes in seawater chemistry resulting from volcanic input were not strongly linear but rather responses to the dynamics of the ice in Iceland – they peaked during periods of rapid glaciation. “The isotopic signal followed the rhythm of the ice ages almost step by step,” says Dr. Antao Xu. “This suggests that the Icelandic ice sheets themselves were the driving forces behind the repeated reshaping of the North Atlantic’s seawater chemistry over the past 230,000 years.” The research findings also show that the ice sheet regulated the influx of micronutrients such as iron. This likely had an impact on marine life and the carbon cycle during the ice ages, according to Dr. Xu, who is a member of the “Physics of Environmental Archives” research group led by Prof. Frank.

“The results of our research underscore that ice sheet systems at high latitudes play a crucial role in regulating ocean chemistry,” emphasizes Prof. Frank. According to the scientist, the findings have far-reaching implications. “They urge caution regarding the assumption that there are fixed isotopic fingerprints that can be used to reconstruct past ocean circulation. Furthermore, they reveal a previously underestimated connection between the cryosphere, continental weathering, and ocean chemistry.”

The latest research was part of a project funded by the German Research Foundation that investigated how ocean circulation and geochemical cycles evolved during major climate change phases in the past. It is based on sediment cores from an expedition conducted by the Ocean Drilling Program (ODP) / International Ocean Discovery Program (IODP), which were provided by the IODP Bremen Core Repository at MARUM – Center for Marine Environmental Sciences at the University of Bremen. The results of the Heidelberg study were published in “Science Advances”.