Water outflow from the Mediterranean to the Atlantic Ocean doubled about 13,000 years ago

This large influx of fresh Mediterranean water into the Atlantic could have reactivated the weakened North Atlantic circulation. Previously, it was hypothesized that the cold stadial period reduced the Atlantic Meridional Overturning Circulation (AMOC) in the North Atlantic region.

With the reactivation of ocean currents, parts of Europe and the Mediterranean began to emerge from the cold and experienced gradual rapid warming. According to the new research, this was the start of the Holocene epoch. 

"Mediterranean water masses are one of the primary sources of salt in the North Atlantic. Water salinity is an important factor in oceanography, as it determines the density of water masses. It is, therefore, a key process in the formation of deep waters in the Atlantic Ocean and is the driver of global ocean circulation," explained Sergio Trias-Navarro, one of the authors of this new study, in an official statement. 

The research is based on an innovative technique that uses neodymium (Nd) isotopes to reconstruct the oceanic and atmospheric conditions in the Mediterranean since the last deglaciation about 14,000 years ago.

The study also notes that this reactivation of ocean currents resulted in more rainfall, particularly in North Africa. While it also caused severe aridity episodes across some parts of the Mediterranean basin, wreaking havoc on terrestrial and marine ecosystems. 

The Younger Dryas sheds light on the rainfall changes that could occur in the Mediterranean influenced by cascading climate change. It can slow down or even stop ocean circulation in certain places in this region in the future.

"The Younger Dryas is not a perfect analog for future changes, as we are currently facing a much more amplified greenhouse effect. Still, our study reveals that the change in aridity which is expected by the end of the century is capable of inducing an intensification of the Mediterranean circulation, although the projected 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 results have been published in Communications Earth & Environment.

Study abstract:

Eastern Mediterranean deep-intermediate convection was highly sensitive to varying inputs of fresh water fluxes associated with increased rainfall during the African Humid period (15-6 kyr Before Present). Here we investigate changes in the water-outflow from the Eastern Mediterranean Sea since the last deglaciation using neodymium isotope ratios. Our results indicate enhanced outflow during the Younger Dryas, two times higher than present-day outflow and about three times higher than during the last Sapropel. We propose that the increased outflow into the western Mediterranean over the Younger Dryas was the result of the combined effect of 1) enhanced climate-driven convection in the Aegean Sea and 2) reduced convection of western deep water during this period. Our results provide solid evidence for an enhanced Younger Dryas westward flow of Eastern Mediterranean sourced waters in consonance with an intensification of Mediterranean water-outflow during a weakened state of the Atlantic circulation.