The Zanclean Flood: Refilling the Mediterranean

While the Mediterranean Sea is one of the cradles of ancient human civilisation, the water body is rather young geologically, approximately 5.33 million years old.  Researchers have found that this figure marks the date of the Zanclean flood, an epic breach of the Strait of Gibraltar that turned the desiccated Mediterranean basin into a sprawling arm of the Atlantic Ocean in a matter of a few months to perhaps two years.  To understand how the Mediterranean filled, one must travel back much further than 5.33 million years ago, however.

The Mediterranean Sea is the descendant of the ancient Tethys Ocean, the ocean that separated the Eurasian continent to the north from Africa, Arabia, and India to the south.  Formed in the aftermath of the breakup of Pangaea, the Tethys emerged 150 million years ago as the north/northeasterly movement of Africa, Arabia, and India began pinching off a portion of the great Panthalassic Ocean that covered most of the planet at this time (the rest of Panthalassa essentially became the Pacific Ocean).

Blakey_90moll

The Earth, 90 million years ago.  The Tethys is the large body of water lying in between Europe, Asia, Africa, and India.  Source: Dr. R. Blakey, http://jan.ucc.nau.edu/~rcb7/mollglobe.html.  Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.

Tethys would progressively shrink as Africa, Arabia, and India pushed northward.  By about 20 million years ago, the Tethys had been reduced to a set of channels connecting the Atlantic Ocean with the Indian Ocean (one branch went through the area of the modern Persian Gulf; the other branch, the Paratethys, extended through the areas of the Black, Caspian, and Aral seas, all of which are remnants of the old Tethys basin).  India and Arabia would collide into Asia, closing the north channel around 10-15 million years ago, trapping the Black and Caspian Seas, and closing the Persian Gulf.  This left a single oceanic outlet to the Atlantic via the Strait of Gibraltar separating modern Spain from Morocco.  What was left of Tethys was now a proto-Mediterranean Sea.

Eventually, this body of water would be closed off as well.  Approximately 5.96 million years ago, Spain and Morocco collided, and the connection between the Atlantic and Mediterranean closed, cutting off the proto-Mediterranean’s oceanic water supply, triggering what is known as the Messinian salinity crisis (named for the Messinian age of the Miocene, which lasted from 7.246 to 5.332 million years ago).  The lack of inflow triggered a large evaporative process that dried up the entire basin except at its absolute lowest elevations, where rivers from the mainland would naturally flow toward.

As shown in sea floor core samples, during the crisis the basin evaporated and was reflooded a stunning 69 times, as rising mountains temporarily closed off the sea, but periods of either (or both) rising sea levels or tectonic subsidence caused a refilling of the basin.  The crisis lasted for approximately 300 000 years before the opening closed again a final time 5.6 million years ago, and the Mediterranean Sea evaporated within a millennium.  The small brackish water bodies at the very bottom of the basin had a surface elevation of between at most 1 500 and 2 700 m (4 920 ft-8 855 ft) below sea level.

The last reflooding 5.33 million ago (the border between the Messinian and Zanclean ages, hence the ‘Zanclean flood’ name) appears to have been a result of tectonic subsidence.  Just the smallest shift in elevation in the Spain-Morocco collision zone could tilt the regional water balance toward the far less elevated Mediterranean.  The drop in elevation from the sea-level Atlantic Ocean to the bottom of the dry Mediterranean basin was drastic.  The small river channel that emerged in the rift zone separating Spain from Morocco due to this subsidence eventually turned into a torrent of water as the river quickly eroded away the rock around it once the channel eroded the rock down to a critical depth.

The gash created in the Strait of Gibraltar was a 250 m (820 ft) deep incision that stretched 200 km (320 mi) from west to east, eroding a path through the rock at the rate of approximately 40 cm (16 in) per day at its peak.  The discharge of this channel was immense, as water rushed into the Mediterranean from the Atlantic at a rate 1 000 times greater than the discharge of the modern Amazon, causing daily sea level rises in the basin of up to 10 m (32 ft) per day.  At its height, water flowed through the channel at a rate of up to 300 km/h (185 mph).  While the actual process may have taken many centuries to complete, perhaps 90% of the current water volume of the Mediterranean rushed into the basin in the span of a few months up to two years.

320px-Etapa1 320px-Etapa2 320px-Etapa3 320px-Etapa4

Source: P. Bahí (Llotja Advanced School of Art and Design in association with Amical Viquipèdia), http://commons.wikimedia.org/wiki/Commons:Llotja.  Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.

This four-image sequence shows the filling, draining, and refilling of the Mediterranean Sea during the Messinian salinity crisis.  1) 6.5 million years ago, numerous channels cutting across the modern Iberian Peninsula and northern Morocco kept the Mediterranean level with the Atlantic Ocean.  2) By 6 million years ago, all but one of these channels closed up as the African Plate moves northward, restricting water flow into the Mediterranean and causing the water in that body to become increasingly hypersaline.  3) Eventually, the final channel across Iberia closes and the Mediterranean begins drying.  Rivers flowing into the shrinking bodies of water create massive gorges as they plunge off the now-exposed continental shelves.  Meanwhile, the rapid evaporation of such a massive water bodies leaves layers of rock salt over a kilometre thick in places.  4) The Zanclean flood occurs as a new water channel (the Strait of Gibraltar) is incised to the south of the previous channel, and water rushes into the fill the entire Mediterranean basin via the new channel, and eventually the Strait of Sicily, until the Mediterranean is filled back to global sea level.

At some point a few million years in the future, the continued northeastward movement of Africa will close the Strait of Sicily and the Strait of Gibraltar, and the Mediterranean Sea will dry up once again over the course of a millennium similar to the events of the Messinian sailinity crisis.  Eventually, the entire basin will close as the African Plate continues to collide with Eurasia (the same collision that has already produced the Pyrenees, the Alps, and the myriad ranges of Turkey), and the Mediterranean sea floor will either be pushed up into new mountains or subducted under Europe.

640px-Tectonic_map_Mediterranean_EN.svg

A tectonic map of the Mediterranean Sea.  Source: Woudloper, http://commons.wikimedia.org/wiki/File:Tectonic_map_Mediterranean_EN.svg.  Licensed under the Creative Commons Attribution-Share Alike 1.0 Generic licence.

Further Reading

Agence France-Presse (2009).  Dramatic flood filled Mediterranean Sea.  World News Australia, 10 December 2009.  Available at http://www.sbs.com.au/news/article/1149387/Dramatic-flood-filled-Mediterranean-Sea.  Accessed 13 July 2012.

Dutch, S. (1998).  Closure of the Tethys.  Continental Drift and Plate Tectonics, 22 December 1998.  Available at http://www.uwgb.edu/dutchs/platetec/closteth.htm.  Accessed 13 July 2012.

Garcia-Castellanos, D. (2011).  Isolation of the Mediterranean by competing tectonic uplift and erosion in the Gibraltar Arc.  Available at https://sites.google.com/site/daniggcc/research-interests/messinian.  Accessed 13 July 2012.  Adapted from D. Garcia-Castellanos and A. Villaseñor (2011), Messinian salinity crisis regulated by competing tectonics and erosion at the Gibraltar arc, Nature 480 (359-393).  Available at https://docs.google.com/uc?id=0B_xuyENh5ksFN2MwYWE5YjUtNzNkOS00OGZlLWFkMDYtOWMxYTViYjRiZjc0&export;=download&hl;=en.   Accessed 13 July 2012.

Garcia-Castellanos, D. et al. (2009).  Catastrophic flood of the Mediterranean after the Messinian salinity crisis.  Nature 462: 778-781.

Gill, V. (2009).  Ancient Mediterranean flood mystery solved.  BBC News, 9 December 2009.  Available at http://news.bbc.co.uk/2/hi/science/nature/8404363.stm.  Accessed 13 July 2012.

Nearby Articles