The Mediterranean during a severe climate crisis around 13,000 years ago

Around 13,000 years ago, a climate crisis caused a global drop in temperatures in the Northern Hemisphere. This episode of intense cold, known as the Younger Dryas, also caused great aridity throughout the Mediterranean basin, which had a great impact on terrestrial and marine ecosystems. But what do we know about the impact that this climate change had on the circulation of water in the Mediterranean Sea?

During the Younger Dryas, the flow of water masses from the eastern Mediterranean into the Atlantic Ocean through the Strait of Gibraltar doubled, according to a new study.

The study is part of the doctoral thesis being developed by Sergio Trias-Navarro, under the direction of Professor Leopoldo Pena and Professor Isabel Cacho, from the Consolidated Research Group (GRC) in Marine Geosciences of the Faculty of Earth Sciences at the University of Barcelona (UB). The work presents some of the most relevant results of the TIMED project of the European Research Council (ERC-Consolidator Grant), and has an outstanding participation of members of the GRC in Marine Geosciences, and experts from La Sapienza University of Rome, the University of Palermo (Italy), and the Federal Polytechnic School of Zurich (Switzerland).

The Younger Dryas is the most intense climate change in the last 13,000 years and has the greatest impact on a planetary scale. Its completion marked the beginning of the Holocene, the interglacial period in which we currently find ourselves. «During the Holocene there has also been climate variability, as now the episodes known as the Little Ice Age, the medieval climate anomaly or the Roman warm period. But this climatic variability had a lower relative intensity with different regional climatic expressions, without the capacity to generate changes on a global scale”, details Professor Isabel Cacho, from the Department of Earth and Ocean Dynamics.

The work also analyzes the last sapropel, an episode of the Holocene after the Younger Dryas linked to an extraordinary increase in rainfall in the Mediterranean region, especially in North Africa. The study provides the first quantification of the changes that occurred in the deep circulation of the eastern Mediterranean during this episode, and has estimated that they were four times smaller than the changes produced during the Younger Dryas. As for the impact that this event could have on the oceanography of the North Atlantic, experts indicate that it is unknown.

The new work supports the hypothesis that the increase in the contribution of salt from the Mediterranean to the Atlantic waters during the Younger Dryas was key to reactivating the North Atlantic circulation: it generated rapid warming in Europe and the Mediterranean, which marked the beginning of the Holocene.

“The Mediterranean water masses are one of the primary sources of salt for the North Atlantic. The salinity of the waters is an important factor in oceanography, since it determines the density of the masses of water. Therefore, it is a key process in the formation of deep waters in the Atlantic Ocean and is the engine of global ocean circulation”, highlights the researcher Sergio Trias-Navarro.

An innovative technique to study the oceans of the past

As in previous GRC work in Marine Geosciences, the team has applied the innovative technique of radiogenic neodymium isotopes as geochemical tracers to reconstruct past oceanographic conditions. This analytical work has been carried out in the LIRA and PANTHALASSA laboratories, unique research support facilities in Catalonia located in spaces of the Faculty of Earth Sciences and the Scientific and Technological Centers of the UB (CCiTUB) and coordinated by the experts Leopoldo Pena and Isabel Cacho.

‘Compared to other types of geochemical tracers, neodymium isotopes have the great advantage of being conservative. Therefore, they do not interact or are affected by biological processes, for example, the biological productivity or the degradation of organic matter”, highlights Professor Leopoldo Pena, co-author of the work. “This technique allows us to go beyond time and can be applied to both present and past oceanographic reconstructions. Thus, it allows us to know the dynamics of the ocean and reconstruct oceanography long before we could observe or measure it ourselves with other scientific tools,” adds the expert.

The UB team has applied the innovative technique of neodymium isotopes thanks to UB research support equipment, unique in all of Catalonia. (Photo: University of Barcelona. CC BY)

Younger Dryas: a mirror for the Mediterranean of the future?

There are still many unknowns about the potential impact of Mediterranean waters on the North Atlantic circulation. Despite there being more scientific interest, “many of the oceanographic studies focused on the Atlantic Ocean do not consider the Mediterranean, and perhaps the role of Mediterranean waters in the Atlantic circulation has been minimized”, the authors remark.

The latest report from the Intergovernmental Panel on Climate Change (IPCC) presents the Younger Dryas as an example of the foreseeable changes in rainfall that will take place in the Mediterranean as a consequence of a very probable reduction in the North Atlantic circulation. “On the other hand, according to the projections for the end of the 21st century, the circulation of the Mediterranean will weaken and, consequently, its contribution to the Atlantic Ocean will also weaken,” says Isabel Cacho, coordinator of the TIMED project.

“The Younger Dryas is not a perfect analogue for future changes, as we are currently dealing with a much more amplified greenhouse effect. Even so, our work reveals that the change in aridity expected by the end of the century is capable of inducing an intensification of the Mediterranean circulation, although the expected warming could counteract this effect. For this reason, we need to better understand the relative weight that these two variables —temperature and humidity— have had on the evolution of the Mediterranean circulation”, concludes the research team.

The study is titled “Eastern Mediterranean water outflow during the Younger Dryas was twice that of the present day”. And it has been published in the academic journal Communications Earth & Environment, from the Nature group. (Source: UB)