Theoretical refilling of the Mediterranean Sea between the Miocene and Pliocene Epochs
The Zanclean flood or Zanclean deluge is theorized to have refilled the
Mediterranean Sea 5.33 million years ago.[1]
This flooding ended the
Messinian salinity crisis and reconnected the Mediterranean Sea to the Atlantic Ocean, although it is possible that even before the flood there were partial connections to the Atlantic Ocean.[2] The re-connection marks the beginning of the
Zancleanage which is the name given to the earliest age on the
geologic time scale of the
Pliocene.
According to this model, water from the
Atlantic Ocean refilled the
dried up basin through the modern-day
Strait of Gibraltar. Ninety percent of the
Mediterranean Basin flooding occurred abruptly during a period estimated to have been between several months and two years, following low water discharges that could have lasted for several thousand years.[3] Sea level rise in the basin may have reached rates at times greater than ten metres per day (thirty feet per day). Based on the erosion features preserved until modern times under the
Pliocene sediment,
Garcia-Castellanoset al. estimate that water rushed down a drop of more than 1 kilometre (0.6 mi) with a maximum discharge of about 100 million cubic metres per second (3.5 billion cubic feet per second), about 1,000 times that of the present-day
Amazon River. Studies of the underground structures at the Gibraltar Strait show that the flooding channel descended gradually toward the bottom of the basin rather than forming a steep waterfall.[4]
Background
The geologic history of the
Mediterranean is governed by
plate tectonics involving the
African Plate, the
Arabian Plate and the
Eurasian Plate which shrank the previously existing
Tethys Ocean until its western part became the present-day Mediterranean.[5] For reasons not clearly established, during the latest
Miocene the Mediterranean was severed from the
Atlantic Ocean and partly dried up when the
Guadalhorce and
Rifian corridors that had previously connected the Mediterranean to the Atlantic closed,[6] triggering the
Messinian Salinity Crisis with the formation of thick salt deposits on the former seafloor[7] and erosion of the continental slopes.[8] The
Nile and
Rhône carved deep
canyons during this time.[4] Water levels in the Mediterranean during this time dropped by kilometres;[9] the exact magnitude of the drop and whether it was symmetric between the
Western Mediterranean and the
Eastern Mediterranean is unclear;[10] it is possible that interconnected seas remained on the floor of the Mediterranean.[11]
The presence of Atlantic fish in Messinian deposits[11] and the volume of salt deposited during the Messinian Salinity Crisis implies that there was some remnant flow from the Atlantic into the Mediterranean even before the Zanclean flood.[6] Already before the Zanclean flood, increased precipitation and
runoff had lowered the salinity of the remnant sea,[7] leading to the deposition of the so-called "Lago Mare" sediments,[12] with some water putatively originating in the
Paratethys north of the Mediterranean.[13]
Event
The Zanclean flood occurred when the
Strait of Gibraltar opened.[14] Tectonic subsidence of the Gibraltar region may have lowered the
sill until it breached.[7] The exact triggering event is not known with certainty;
faulting or
sea level rise are debatable. The most widely accepted hypothesis is that a stream flowing into the Mediterranean eroded through the Strait of Gibraltar until it
captured the Atlantic Ocean[9] and that the Strait did not exist before this erosion event.[15]
During the flood, a channel formed across the Strait of Gibraltar,[14] which starts at the
Camarinal Sill in the Strait of Gibraltar.[16] The channel is eroded into the seafloor of the
Alboran Sea,[17] splits around the Vizconde de Eza high of the
Alboran Sea[18] and eventually connects with the
Alboran Channel before splitting into several branches that end in the Algero-Balear basin.[16][19] The channel has a U-like shape in its starting region, which is consistent with its formation during a giant flood.[20] The formation of the channel mobilized about 1,000 cubic kilometres (240 cu mi) of rock,[21] which was deposited in the Alboran Sea in the form of giant submarine bars.[22] The sector of the Zanclean channel that passes through the Camarinal Sill may have a different origin, however.[10]
Whether the Zanclean flood occurred gradually or as a catastrophic event is controversial,[23] but it was instantaneous by geological standards.[12] The magnitude of a catastrophic flood has been simulated by modelling. One single-dimensional model assumes a catastrophic flood of more than 10–100
sverdrup.[note 1] Another estimate assumes that after the first breach of the sill, the flowing water eroded the threshold and formed the channel across the Gibraltar strait, increasing the flow of water which in turn increased the erosion until water levels rose enough in the Mediterranean to slow the flood.[20]
Under such a scenario, a peak discharge of over 100,000,000 cubic metres per second (3.5×109 cu ft/s) occurred with water velocities of over 40 metres per second (130 ft/s); such flow rates are about a thousand times larger than the discharge of the
Amazon River and ten times as much as the
Missoula Floods.[26] This flood would have descended a relatively gentle ramp into the Mediterranean basin, not as a giant
waterfall.[27] Later simulations using more explicit geography constrain the flow to about 100 sverdrup, which is about 100,000,000 cubic metres per second (3.5×109 cu ft/s). They further indicate the formation of large
gyres in the
Alboran Sea during the flooding[24] and that the flood eroded the Camarinal Sill at a rate of 0.4–0.7 metres per day (1.3–2.3 ft/d).[28] The exact size of the flood is dependent on the pre-flood water levels in the Mediterranean and higher water levels there would result in a much smaller flood.[29]
The flood affected only the
Western Mediterranean at first, because the Sicily Sill (located at the present
Straits of Sicily) formed a barrier separating its basin from the
Eastern Mediterranean basin[30] that probably overflowed through the Noto Canyon across the
Malta Escarpment;[31] in addition a sill may have existed in the eastern Alboran Sea at this time.[32] During the flooding across the Noto Canyon, vortices and reverse flows occurred,[33] and large amounts of sediments were emplaced in the Ionian Sea.[34] While it was at first assumed that the filling of the eastern Mediterranean would have taken thousands of years, later estimates of the size of the Strait of Gibraltar channel implied that it would have taken much less, potentially less than a year until reconnection.[35]
A large flood is not the only explanation for the reconnection of the Mediterranean with the Atlantic and concomitant environmental changes; more gradual reflooding of the Mediterranean including reflooding through other water sources is also possible.[36][37][38] The absence of a catastrophic flooding event is supported by geological evidence found along the southern margin of the Alboran Sea.[39] On the other hand, deposits found around the
Malta Escarpment imply that one intense flood led to the reconnection across the Straits of Sicily.[40]
Timing
The timing of the Zanclean flood is uncertain, with one possibility being a flood around 5.33 million years ago;[41] the end of the
Messinian/
Miocene and beginning of the
Zanclean/
Pliocene is usually associated with the flood.[42] The main Zanclean flood may have been preceded by an earlier smaller flood event,[10][43] and the presence of deep sea terraces has been used to infer that the refilling of the Mediterranean occurred in several pulses.[44] Complete refilling of the Mediterranean may have taken about a decade.[7]
Consequences
The Zanclean flood created the
Strait of Gibraltar; it is doubtful that tectonic or volcanic events could have created the strait since the main plate boundaries do not run through the strait and there is little seismic activity in its area.[45] The current morphology of the strait is characterized by two
aquatic sills:
Camarinal Sill, which is 284 m (932 ft) at its deepest point; and the deeper
Espartel Sill[46] farther west. The narrowest part of the strait is located east of either sill,[47] and it is considerably deeper than the sills.[46] It is possible that these sills were formed after the flood through gravity-induced movement of neighbouring terrain.[48]
The Zanclean flood caused a major change in the environment of the Mediterranean basin; the continental "Lago Mare" facies was replaced by
Zanclean deep sea deposits.[7] The flood may have affected global climate, considering that the much smaller flood triggered when
Lake Agassiz drained did result in a cold period.[49] The hypothesized remote effects reached as far as the
Loyalty Ridge next to
New Caledonia in the Southern Hemisphere.[50]
Rising sea levels made the deeply
incisedNile river become a
ria as far inland as
Aswan, some 900 km (560 mi) upstream from the modern coast.[51] The Zanclean flood resulted in the final isolation of numerous Mediterranean islands such as
Crete,[52] resulting in
speciation of animals found there.[53] On the other hand, the formation of the Gibraltar Strait prevented land animals from crossing over between Africa and Europe.[54] Further the reconnection allowed sea animals such as
cetaceans and their ancestors and
pinnipeds to colonize the Mediterranean from the Atlantic.[55]
Evidence of the flooding has been obtained on Zanclean-age sediments, both in
boreholes and in sediments that were subsequently uplifted and raised above sea level.[56] A sharp erosional surface separates the pre-Zanclean flood surface from the younger deposits, which are always marine in origin.[57]
The waters flooding into the
Western Mediterranean probably overspilled into the
Ionian Sea through
Sicily and the Noto
submarine canyon[58] offshore
Avola;[59] the spillover flood had a magnitude comparable to the flood in the Strait of Gibraltar.[60] The rates at which the Mediterranean filled during the flood were more than enough to trigger substantial
induced seismicity.[61] Resulting large
landslides would have sufficed to create large
tsunamis with wave heights reaching 100 m (330 ft), evidence of which has been found in the
Algeciras Basin.[62] The infilling of the basin created tectonic stresses, which would have influenced the development of the
Apennine Mountains.[63]
In his book
Historia Naturalis,
Pliny the Elder mentions a legend that
Hercules dug the Straits of Gibraltar between the Mediterranean and the Atlantic Ocean, connecting the two.[65] The actual Zanclean flood theory however only arose during the 1970s, when it became clear that salt deposits and a widespread
erosion surface in the Mediterranean had been emplaced during a prolonged sea level lowstand, and that the subsequent reflooding took place in only a few millennia or less.[66]
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