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Post-Messinian megaflood of the Mediterranean
[details in this publication: Garcia-Castellanos et al., 2009, Nature, 462]
The flood that put an end to the Messinian desiccation of the Mediterranean is the largest known in Earth's history, yet its abruptness and evolution remain poorly constrained. The record in the Mediterranean sediments indicates an abrupt change from Messinian to normal (open marine) environments, but in geosciences this can mean a transition lasting for tens of thousands of years. Geophysical observations in the Alboran Sea and the Gulf of Cadiz and computer modeling techniques are here used to investigate the feedback between water flow and incision during the Zanclean flood.
At the ICTJA-CSIC, we developed a simple formulation that allows calculating the evolution of megafloods produced by overspill of a water basin into another, incorporating feedback between sill incision and water flow velocity. We use this formulation to model the geometry of an erosion channel crossing the Gibraltar Strait from west to east. The 300-650 m-deep erosion seen in borehole and seismic data are found to be consistent with erosion parameters obtained from independent river-incision studies. According to the model predictions, feedback between incision and water flow implied that the flood necessarily ended as a catastrophic event reaching peak discharges of about 1000 times the present Amazon River, producing incision rates above 0.4 m/day and a sea level rise in the Mediterranean faster than 10 m/day. Although an initial stage involving little water discharge may have lasted up to several thousand years, it eventually develops into a flow of cataclysmic proportions, in which 90% of the water is transferred in a short period ranging between months and a few years.
In the initial stage of our model, water starts seeping through an arbitrarily small water gate located in a bathymetric sill separating the Atlantic and the dry Mediterranean basins. As basal shear-stress incises the sill, water flow increases and so does the incision rate, in a feedback that leads to exponential increase in discharge during the early model stages. All models show a long first phase with very little incision due to the reduced amount of water flow allowed by the initial sill depth of 1 m. As the Gibraltar gate grows deeper and wider, water flow and incision rate increase exponentially. This situation persists until the flow reduction due to the rising level of the Western Mediterranean becomes more important than the growth of the water gate. This event is labeled as Stage 1. Later, the effective slope S is progressively reduced and so does the flow velocity V, the water discharge Q and the erosion rate dzs/dt. As the Sicily Sill is reached (Stage 2), the level of the western Mediterranean remains constant while all water crossing the Gibraltar gate is transferred to the eastern basin. After the eastern basin also fills up to the Sicily sill (Stage 3), the whole Mediterranean will rise synchronously. Headloss across Gibraltar reduces gradually to zero, towards an asymptotic equilibrium (Stage 4) where there is no significant level difference between both oceans.
The code has developed for Linux platforms in C language. It's available here. A simplified version (spreadsheet calculator) of outburst flood evolution is available here.
The
figure shows an example run (this one adopting the stream unit power
approach):
The model incorporates feedback between incision and water flow using finite difference techniques and an explicit solving scheme at regular time steps.
In our model, water flow causes rock incision, following either a shear stress or a stream unit power formulation, both tested in mountain rivers (e.g. Whipple & Tucker, 1999; Lavé & Avouac, 2001; Wobus et al., 2005; Attal et al., 2008). The shear stress approach is written as
where
kb
and a
are
positive constants. An analytical solution of this equation coupled
to slope-driven water flow shows that the sill is incised
exponentially along time in the early stages of flooding. The speed
of this growth is dependent on the lithological erodability kb
and the
slope S
in
the Mediterranean side of the sill.
For the velocity of water flow, we adopt the Manning's formulation:
where
V
is the
average velocity (m/s), n=0.05
is the
roughness coefficient, and Rh
is the
hydraulic radius (m) of the strait connecting the Atlantic and the
Mediterranean.