The Messinian Salinity Crisis (2/3) - The open scientific question

[Previous chapter: Salt all over the Med]
[Next chapter: Causal mechanisms for a huge salt pan]
[This is a popular-science article. Details on our research will appear here].

As I wrote in the previous part, the Mediterranean Sea suffers from a very negative budget, also in terms of water: the amount of water delivered by the rivers and by rainfall is not enough to compensate its water losses by evaporation. A surface current entering across the Strait of Gibraltar brings relatively fresh water into the Med. A smaller one flowing underneath flushes out dense, salty water caused by evaporation. 

Did this outflow current not exist, there'd be no mixing with the Atlantic, and all incoming salt would then concentrate in the Mediterranean as it did during the Messinian. In other words, if the outflow is cancelled then the Med would become a giant salt pan; if both inflow and outflow were cancelled, then the Med would just dry up to a level of about 1500-2200 meters below sea level. 

Gravity waves produced by the interplay between the surface
inflow and the submarine outflow across the Strait of Gibraltar.
Date: 06.03.2004 Credit:
NASA Johnson Space Center - Earth Sciences and Image Analysis (NASA-JSC-ES&IA)

By the way, as soon as geologists and oceanographers learned all this, a German architect had the idea of putting it into practice. During the 1920's, Herman Sörgel was pretty good at promoting his project for a huge dam between Algeciras and Tanger (Atlantropa Project) among a large range of wealthy investors. He had the humanitarian aim of bursting economic exchange between Europe and Africa, taking the dubious advantage of a partially desiccated Mediterranean. Anyway, the German rulers at the time had different plans for both Europe and Africa, they were not so much into humanitarism, WWII arrived, and Atlantropa (always look at the bright side) never came to practice. 

Map of the Atlantropa Project (source: Wikimedia Commons)

But going back to science: the cause for the Messinian salinity crisis was (and still is) as controversial as its timing and reach. Was it a major global sea-level fall below the threshold of the seaways? Or a tectonic uplift across the Betic-Rifean corridors? Or a combination? The latter is currently the mainstream opinion among scientists, mostly because the timing of global sea-level drops (related to changes in insolation due to orbital precession of the Earth) does not seem to be timely nor enough in amplitude as to close the marine gateways with the Atlantic. Thus, the uplift of the Gibraltar Arc by tectonic forces must have been the driving mechanism. 

Hydraulic calculations say that the gateway must have been very small (a few tens of meters in depth) during the first stage of the crisis to explain the large salt deposition. A deeper seaway would allow too much mix with the Atlantic and preclude brine formation, whereas a shallower one could not impede the sea level fluctuations (which have a similar amplitude) to repeatedly disconnect and refill the Mediterranean basin. This multiple flooding hypothesis, earlier adopted to explain the large amount of Messinian salt, is now regarded as unlikely, since it would imply repeated up and down tectonic motions for which we don't know of any geological mechanism (Govers, 2009; Garcia-Castellanos and others, 2009).

So, why the combination of rapid, up and down sea level changes, combined with a slow tectonic uplift derived into a long, roughly constant inflow of water through the gateway, instead of repeated drawdown and flooding? 

Before answering that question, we need to dig into the tectonic evolution of the western Mediterranean, which is complex and definitely contentious. Based on polar wander paths and stratigraphy, most geologists agree today that during the Oligocene the western Mediterranean formed as a back-arc basin at the convergence between Africa and Eurasia. Back-arc extension is a process that occurs behind areas where a lithospheric plate subduces under another. In the Mediterranean, the subduction takes place still nowadays, and it accommodates the ongoing approach of several millimeters per year between Africa and Europe. This subduction consists generally of the African plate underthrusting either the European plate or the different Mediterranean oceanic microplates. The Balearic-Betic-Rif mountain chain (aka Gibraltar Arc) would be the western-most end of this back-arc extension. In the case of the Alboran Sea (between Iberia and Morocco) the direction of subduction has not yet been agreed (actually, all possible directions of subduction have been already proposed in scientific papers), but our tomographic studies suggest that it is actually Iberia that may have subduced under Africa and then rolled back until it was left hanging underneath the Betics. 

Tearing and sinking of a lithospheric slab in the Earth's mantle may be occurring underneath the Gibraltar Arc today (after Spakman & Wortel, 2004). This mechanism can produce large vertical motions of the surface that could have closed the seaways during the Messinian. 

Once this lithospheric slab started to roll back towards the west, it detached from the overlying Iberia, tearing apart towards the west. The resulting uplift of the detached areas could have closed the Tethyan marine corridor. For a while the waters of the Atlantic and Mediterranean mixed through the Betic and Rifean corridors (Figure 3), but by ~6 Million years ago evidence of mammal exchange (including camels) between Africa and Iberia indicate that some sort of landbridge existed (Garcés et al., 1998). The sinking of the subducted slab caused uplift of the order of 1 km along the African and Iberian margins, closing the Betic and Rif corridors, isolating the Mediterranean and triggering the Messinian Salinity Crisis. This model is supported by extensive laser ⁴⁰Ar/³⁹Ar age dating combined with O-Sr-Nd-Pb isotope studies of igneous rocks from the Arc of Gibraltar (Duggen, 2003), and by tomographic imaging from our recent study. 

Satellite view of the Betics, Rif and Gibraltar Strait.
Camera looking NE. The snowed Spanish Sierra Nevada
is above the center of the image.
Source: NASA, with permission.

Reconstruction of the paleogepgraphy of the Gibraltar Arc prior to the Messinian salinity crisis. Several corridors connected the two ocean domains. Source: Wikimedia Commons

So, there exist some ideas about what deep tectonic mechanisms may have closed the Mediterranean Sea. But the question remains: How does the slow uplift motion produced by such mechanisms reconcile with the survival of a small connection in spite of rapid global sea level fluctuations? I'll detail the explanation we found in the last part of this post.

Links:

• Why is the ocean salty (USGS).

Scientific papers:

• Spakman, W., M.J.R. Wortel, Tomographic view on Western Mediterranean geodynamics, in: W. Cavazza et al. (eds.), The TRANSMED Atlas, The Mediterranean region from crust to mantle: New York, Springer-Verlag, 31–52 (2004).
• Garcés, M., W. Krijgsman, J. Agustí, Chronology of the late Turolian deposits of the Fortuna basin (SE Spain): implications for the Messinian evolution of the eastern Betics. Earth Planet. Sci. Lett. 163, 69–81 (1998).

• Krijgsman, W., Hilgen, F., Raffi, I., Sierro, F., & Wilson, D. (1999). Chronology, causes and progression of the Messinian salinity crisis Nature, 400 (6745), 652-655 DOI: 10.1038/23231
• Duggen, S., Hoernle, K., van den Bogaard, P., Rüpke, L., & Phipps Morgan, J. (2003). Deep roots of the Messinian salinity crisis Nature, 422 (6932), 602-606 DOI: 10.1038/nature01553
• Garcia-Castellanos, D., Estrada, F., Jiménez-Munt, I., Gorini, C., Fernàndez, M., Vergés, J., & De Vicente, R. (2009). Catastrophic flood of the Mediterranean after the Messinian salinity crisis Nature, 462 (7274), 778-781 DOI: 10.1038/nature08555
• Govers, R. (2009). Choking the Mediterranean to dehydration: The Messinian salinity crisis Geology, 37 (2), 167-170 DOI: 10.1130/G25141A.1