2017-09-26

Did the evaporation of the Mediterranean trigger widespread volcanism?


Artistic interpretation of the proposed lowstand
of the Mediterranean level during the salinity
crisis. Authors: Pibernat and Garcia-Castellanos
We are 130 years ahead of the recognition of a hypersaline, evaporitic stage in the Mediterranean (now known to roughly correspond to the Late Messinian, 6 to 5.3 million years ago);
50 years after documenting widespread submarine and riverine erosional features that suggest a subaerial exposure of parts of the Mediterranean Sea;
40 years after the first abissal drilling reaching the top of a salt layer thicker than 1 kilometer...
And yet, the most intriguing and debated question around the Messinian salinity crisis remains whether there was a large sea level fall during the crisis, more than a few hundreds of meters, perhaps more than a kilometer. Evidence in favor and against is piling up on the desks of scientists. 

We now publish a new piece of evidence that supports a Yes answer to this long-standing question. A fall in the level of the Mediterranean Sea about 6 million years ago may have increased volcanic activity over the entire region (Sternai et al., 2017, Nature Geosc.).

Geoscientists inspecting the Realmonte mine in Sicily,
where Messinian salt is commercialized. 
A layer ranging from 1 to 2 km of salt (halite) spreads below much of the Mediterranean seabed, formed when the Mediterranean Sea became isolated from the Atlantic Ocean about 6.0 to 5.3 million years ago, leading to evaporation and sea-level fall in an event known as the Messinian salinity crisis. The rate and amount of sea-level fall in the Mediterranean during this time is strongly debated. However, if the sea-level drop was dramatic and rapid, it could have unloaded Earth’s surface, decompressing the mantle below. Such mantle decompression can enhance magma production and, in turn, lead to volcanic eruptions at the surface.

Pietro Sternai and the rest of us test this idea using a combination of geological data and numerical modelling. Dated magma intrusions and volcanic eruptions in the region show that there was a pulse of increased volcanic activity towards the end of the Messinian salinity crisis. By calculating changes in the surface load caused by a kilometre-scale drop in sea level, and taking into account the counter weight of the increased density of the remaining highly saline water and accumulating salt deposits we verify that such changes in sea level are sufficient to unload and decompress the mantle, triggering a significant increase in volcanism over the Mediterranean.
Decompression and vertical rebound of the lithosphere
in response to a sudden evaporation of the sea. 

The results provide independent support for the idea that sea-level fall during the Messinian salinity crisis was rapid and occurred on a dramatic scale, and also highlights the sensitivity of Earth’s solid interior to changes at the surface.

Check also the News & Views article by Jean-Arthur Olive: “This proposed link will motivate the collection of high-resolution field data that better constrain the timing of volcanism in the Mediterranean, along with the development of novel approaches for coupled lithosphere–magma dynamics.”

Original paper:
Sternai et al, 2017, Nature Geosc. http://dx.doi.org/10.1038/ngeo3032