Mare Nostrum

The origin of one of the most fascinating processes that have ever affected marine waters in our Mediterranean: the Messinian crisis.

“Pangea is the single land mass that is believed to have given rise to the present continents. Its outline has now been plotted and its further disruption has been projected into the future “

With the theory of continental drift, we are aware that what we now see as separate lands once formed Pangea: a universal continent, then fragmented, to generate today’s world. The original Pangea concept was proposed in 1920 by Alfred Wegener.

The separation of the continents to be precise, is due to tectonic movements and seafloor spreading. In fact, the earth has a thick lithosphere of about 100 km, which probably due to the forces generated in the astenosphere (lower layer than the lithosphere), has been crushed into a given number of separate plates (10 are those of greatest importance). Furthermore, the subduction zones along the Earth’s mantle are formed due to the fact that one plate can be hotter and heavier than another due to the influence of molten rocks. “Thermal activity”, kinetics, entropy, have always been the engine of our planet and will always be: here is another demonstration.

The Atlantic and Indian oceans are rift oceans, formed where the continents have separated. The Pacific, on the other hand, is an ancestral ocean that has gradually become smaller with the growth and formation of the other river basins.

So we can say that Pangea was a continent with irregular boundaries and surrounded by a single large ocean: Panthalassa, which is none other than the ancestral Pacific. To the east of this ocean there was a triangular shaped bay, which separated Eurasia from Africa. This baja is Tethys sea, so what we now call the Mediterranean.

At this point we can avoid continuing towards what is the mere geological aspect of the process of formation of the continents and focus on one concept: today’s Mediterranean is the ancestral Baja of the Pacific ocean called Tethys sea.

I don’t know about you, but when I discovered this, I saw the Mediterranean through different eyes. I saw it bearer of an ancient history, which has had the opportunity to change in all its characteristics for a longer time. In fact, we are talking about 200 million years ago, 200 million years of biodiversity and habitats that have followed one another.

Reconstructing what is the original formation of the Mediterranean can help us, making a time jump of millions of years, to better understand one of the most mysterious periods lived by this particular and ancient basin.

We are speaking, as anticipated, of the Messinian salinity crisis. It is an event dated to 5.96-5.33 million years ago (according to the composition of volcanic rocks, Cenozoic (Miocene)) which has been perpetuated for the next 2 million years. During this period of time the Mediterranean became a large salt lake due to the interruption of water exchanges with the Atlantic. Previously, the relationship between the Mediterranean and the Indian ocean had also been interrupted. In fact, the Mediterranean couldn’t carry out water exchanges with any basin.

The temperatures were much higher than today, and so this variable with the impossibility of water exchange, allowed a progressive evaporation of the Mediterranean basin leading to extinction processes, but at the same time also of speciation. Furthermore, particular habitats were formed with high salinity, and deposits of rock salt in depth that are still present today.

When we talk about the Strait of Gibraltar we actually have to specify that this was not present in the same features of today. In fact, this consisted of 2 corridors that brought water from the Atlantic to the Mediterranean: the Rifean Corridor to the south and the Betic corridor to the north.


Credit: Deep roots of the Messinian salinity crisis
Image of how the Strait of Gibraltar once looked like, where we can distinguish 2 openings. Today’s borders are defined in red
.

However, what I will try to explain to you is how it was possible to lose the connection with the Atlantic. The hypotheses to explain this phenomenon were three:

1) decrease in sea level of 60 meters

2) horizontal shortening associated with crustal tassels (a sheet of rock that has moved sideways over neighboring strata as a result of an overthrust or folding) movement

 3) tectonic uplift

The first hypothesis is not plausible since this decrease in sea level would have been insufficient to prevent the entry of water into the Mediterranean. Furthermore, it has been shown that the beginning of the evaporitic deposits (5.96 ± 0.02 million years) does not correspond to the dating given by the isotopic ratio d18O commonly interpreted to reflect the paleoclimatic changes that have affected these waters. This ratio is illuminating, because if we have a substantial decrease in sea level, we would have also a corresponding glaciation, and instead we can thus effectively demonstrate that this didn’t happen.

Let’s move on to the second hypothesis: it is an improbable mechanism, for two reasons. First, because emplacement of crustal nappes in the early Miocene had already ceased before the late Miocene in the north (Betic corridor), and second, there is no evidence that the location of the crustal nappe blocked the the Rifean corridor (south) in the late Miocene.

The corridors of Betic and Rifean are crossed by a volcanic belt, 500 km long, 200 km wide, which extends from south-eastern Spain through the eastern Alboran Sea in north-west Africa. Since volcanism is the superficial expression of the mantle processes, the age and geochemistry of the volcanic rocks of this belt have been used to trace the geodynamic evolution of the Alboran mantle, in order to clarify the origin of the uplift from the late Miocene to Pliocene. This geological feature can explain the third hypothesis. In fact there is a dramatic change in the composition of the mafic volcanic rocks, derived from the mantle between 6.3 million years ago and 4.8 million years ago indicating that a major change in mantle dynamics occurred slightly earlier, or at the beginning of this time interval. These mantle processes may have produced the necessary uplift to close the marine passages during the Miocene through subduction processes that will surely have generated strong earthquakes (in fact, even seismic data support this hypothesis).

These elevations generated and the period of permanence of these have led to current times, and part of the current biodiversity. Knowing the past, understanding the present and trying to identify the future path is the key to understanding nature in its entirety.  Is enough for today, but we will return to the topic and we will face it from the point of view of biodiversity.

Maria Bruno

“Deep roots of the Messinian salinity crisis” Svend Duggen et al. NATURE, VOL 422 (2003).

“The breakup of pangaea” Robert S. Dietz and John C. Holden. Scientific American, Vol. 223, No. 4 (October 1970).

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