[related publication: Garcia-Castellanos & Villaseñor, 2011, Nature]
[see also Urgeles et al., 2010, Basin Res.]
[related poster at EGU Meeting in Vienna, 2011]
We have constructed a simple mechanistic model for the closure of the last corridor that connected the Atlantic and the Mediterranean before the Messinian Salinity Crisis (MSC). The depth of the corridor is not prescribed as in previous studies, but it results from the competition between a prescribed uplift rate and an erosion rate calculated from the water flow and head loss along the corridor.
The results obtained for previous estimations of the erosional and uplift parameters show that these two processes may have reached a dynamic equilibrium along the last corridor crossing the Betic-Rif orogen, explaining the long initial connection between both seas during the early stage of the MSC. This long connection is needed to explain the large amount of salt deposited at the bottom of the Mediterranean. The uplift rate required for this competition is comparable to the one predicted by geodynamic models of slab detachment, a model that is supported by seismic tomography imaging the mantle underneath the Betics.
For the calculations we wrote a code in C under Linux. The program is called asalted, is open source, and is available for download here.
Our geodynamic interpretation is based on seismic tomography from an updated earthquake database. This tomography confirms the existence of a slab attached to the Iberian lithosphere in the western Betics and the Gulf of Cádiz, but detached from Iberia at the eastern Betics. This suggests a slab tear propagation towards the west underneath the Betics that may explain the uplift of the Betic corridors as well as the subsidence leading to the Zanclean flood.
Garcia-Castellanos et al., 2009, Nature]
[related poster at AGU Fall Meeting in SF, 2009]
[related outreach publications: ScienceNews; Science; La Recherche]
Although the flood that probably put an end to the Messinian Salinity Crisis in the Mediterranean is the largest known in Earth's history, its rapidity and progress have remained poorly constrained. The record in the Mediterranean sediments indicates an abrupt change from Messinian (evaporitic) to normal (open marine) environments at those times (5.33 Myrs ago). But in geoscience "abrupt" could mean many tens of thousands of years. To address this question, we used geophysical observations in the Alboran Sea and the Gulf of Cadiz and computer modeling techniques to investigate the feedback between water flow and erosion during the Zanclean flood.
We have developed, at the ICTJA-CSIC institute, a simple formulation that allows calculating the evolution of floods produced by overspill of a water basin into another, incorporating feedback between sill incision and water flow velocity. We used 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 (previously interpreted as the result of river erosion during the Messinian desiccation) are found to be consistent with erosion parameters obtained from independent river-incision studies.
The figure shows an example run (this one adopting the stream unit power approach):
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.
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.
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