Cretaceous oceans sound the alarm about melting ice and the North Atlantic Current, vital for the European climate

Ocean health 66 million years ago analysed in an international study with the participation of EHU researcher Vicente Gilabert and published in Nature Communications

An international team of researchers, with the participation of micropalaeontologists from the University of Zaragoza and the University of the Basque Country (EHU), has demonstrated in Nature Communications that over the last few million years of the Cretaceous period there was a reorganisation of the continents that altered salinity and temperature patterns and connections between oceans. The scientists warn that this scenario bears certain similarities with the current situation, in which the discharge of fresh water, this time from the accelerated melting of the Greenland and Arctic polar icecaps, is raising doubts about the stability of the North Atlantic Current, which is vital for the European climate.

The study, led by Polish geologist Wiesława Radmacher and involving micropalaeontologists Vicente Gilabert (EHU), José Antonio Arz (University of Zaragoza) and Ignacio Arenillas (University of Zaragoza), shows that at the end of the Cretaceous, the Arctic ocean began to receive large amounts of fresh water from rivers. At the same time, the gradual closure of the Central American seaway weakened the connection between the Atlantic and the Pacific, while the Arctic Ocean was connected with the rest of the oceans exclusively through the Greenland–Norway seaway.

“This geographical cocktail altered the global ocean circulation and caused less saline waters to float above denser bodies of water; that hindered mixing due to the different densities of the layers, a phenomenon known as ocean vertical stratification. The process was particularly intense in the Arctic (the source of fresh water), in the North Atlantic and in the ancient Tethys Ocean, which today corresponds to the Mediterranean,” explained Vicente Gilabert of the Department of Geology at the University of the Basque Country and co-author of the study alongside José Antonio Arz and Ignacio Arenillas of the Department of Earth Sciences-IUCA at the University of Zaragoza.

Salinity and sea temperature are decisive factors for marine life: they determine the abundance of phytoplankton and zooplankton, the basis of the entire food chain, and regulate the exchange of oxygen and carbon dioxide between the ocean and the atmosphere. According to the study, had the Chicxulub asteroid not struck the Yucatan Peninsula (Mexico) 66 million years ago causing extreme climate change and the mass extinction event that wiped out the dinosaurs, this drift towards greater vertical stratification could have led to a serious environmental crisis due to a lack of oxygen in the deep waters of the oceans.

Warning for the future

“Even with their obvious differences, the oceans at the end of the Cretaceous were showing signs of vulnerability similar to those we detect today in our seas and oceans,” warned Vicente Gilabert.

The international team combined advanced climate models, such as the COSMOS system which couples the atmosphere with the ocean, with micropalaeontological and geochemical analyses. So they studied the marine microfossils of planktonic foraminifera (zooplankton protozoa) and dinoflagellates (phytoplankton algae) from stratigraphic sections and boreholes in Greenland, Norway, the Barents Sea, the Atlantic, the Mediterranean and the Caribbean Sea.

These microscopic traces have made it possible to accurately reconstruct variations in salinity, temperature and oxygenation in the seas during the Maastrichtian, the final age of the Cretaceous. It spanned the interval from approximately 72 to 66 million years ago. As José Antonio Arz pointed out: “Comparing micropalaeontological and geochemical results with computer models has helped to rule out unrealistic simulations and select the more reliable scenarios relating to what the ocean dynamics were like during the Maastrichtian.”

“This discovery brings us closer to better understanding what our planet was like at such an exceptional moment as the one preceding the Fifth Great Extinction,” stressed Ignacio Arenillas. In short, the work not only sheds light on a key episode in the Earth's history, but also serves as a warning for the future. “Right now, the accelerated melting of Greenland and the Arctic and the increase in freshwater discharges into the Atlantic are under intense scientific scrutiny, as they could alter the North Atlantic return current, which regulates the European climate and the planet's thermal balance,” they said.

Bibliographical reference:

Ocean freshening near the end of the Mesozoic

Wiesława Radmacher, Igor Niezgodzki, Vicente Gilabert, Gregor Knorr, David M. Buchs, José A. Arz, Ignacio Arenillas, Martin A. Pearce, Jarosław Tyszka, Mateusz ikołajczak, Osmín J. Vásquez, Sarit Ashckenazi-Polivoda, Sigal Abramovich, Mariusz Niechwedowicz & Gunn Mangerud

Nature Communications volume 16, Article number: 7238 (2025)

DOI : 10,1038 945 -025 -62189 -9