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Messinian Salinity Crisis Regulated by Competing Tectonics and Erosion at the Gibraltar Arc

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Messinian Salinity Crisis Regulated by Competing Tectonics and Erosion at the Gibraltar Arc LETTER doi:10.1038/nature10651 Messinian salinity crisis regulated by competing tectonics and erosion at the Gibraltar arc D. Garcia-Castellanos1 & A. Villasen˜or1 The Messinian salinity crisis1,2 (5.96 to 5.33 million years ago) was Three mechanisms have been proposed as responsible for the closure caused by reduced water inflow from the Atlantic Ocean to the of the seaways. A global sea-level drop can be discarded because the Mediterranean Sea resulting in widespread salt precipitation and open-ocean benthic 18O/16O ratio, a proxy for glacio-eustasy, does not a decrease in Mediterranean sea level of about 1.5 kilometres due to match the onset of evaporite deposition1,15.Similarly,noevidencehas evaporation3. The reduced connectivity between the Atlantic and yet linked local tectonic fault deformation16 along the corridors to the the Mediterranean at the time of the salinity crisis is thought to have onset of the MSC, a period when tectonic activity was relatively low resulted from tectonic uplift of the Gibraltar arc seaway and global both in the Betic17 and the Rifean corridors18. Accordingly, a long- sea-level changes, both of which control the inflow of water required wavelength, mantle-sourced tectonic uplift of the Betic and/or Rif to compensate for the hydrological deficit of the Mediterranean1,4. mountains6,18–21 is seen as the most plausible cause for the isolation However, the different timescales on which tectonic uplift and of the Mediterranean9, and is supported by the presence of uplifted, changes in sea level occur are difficult to reconcile with the long pre-Messinian marine sediments near the seaways in the Rif6,18 and the duration of the shallow connection between the Mediterranean Betics7,19. However, the physical feasibility of this mechanism has not and the Atlantic5 needed to explain the large amount of salt yet been quantitatively tested, because former models of the MSC4,21,22 precipitated. Here we use numerical modelling to show that predefined the connectivity conditions at the seaway. Because sea level seaway erosion caused by the Atlantic inflow could sustain such a varies on much shorter timescales than tectonic uplift, previous shallow connection between the Atlantic and the Mediterranean by attempts at linking both processes5 predict a succession of short desic- counteracting tectonic uplift. The erosion and uplift rates required cations and floods during both stages of the MSC. This may explain the are consistent with previous mountain erosion studies, with the large amount of salt precipitated during the MSC3,22–24, which requires present altitude of marine sediments in the Gibraltar arc6,7 and with the evaporation of about 50 times the volume of the Mediterranean, but geodynamic models suggesting a lithospheric slab tear underneath is in conflict with the notion of a two-stage MSC. Furthermore, the the region8–10. The moderate Mediterranean sea-level drawdown erosion produced by each flood would deepen and widen the inflow during the early stages of the Messinian salinity crisis3,5 can be channel25,26, implying a succession of multiple tectonic uplift and sub- explained by an uplift of a few millimetres per year counteracted sidence episodes20,26 for which no geodynamic mechanism is known. by similar rates of erosion due to Atlantic inflow. Our findings The alternative to multiple flooding is a long period of continuous suggest that the competition between uplift and erosion can result inflow and limited outflow caused by a shallow connection between in harmonic coupling between erosion and the Mediterranean sea the Mediterranean and the Atlantic. This does not imply a large sea- level, providing an alternative mechanism for the cyclicity observed level drawdown in the Mediterranean during stage 1, and allows for the in early salt precipitation deposits and calling into question pre- required amount of salt to be precipitated in a few hundred thousand vious ideas regarding the timing of the events that occurred during years. However, to avoid both interoceanic mixing and complete the Messinian salinity crisis1. disconnection5 this mechanism would require a fortuitous, ad hoc It is broadly accepted that the period of widespread salt precipitation evolution of both sea level and uplift closely following each other with in the Mediterranean known as the Messinian salinity crisis (MSC) a deviation of only a few tens of metres4. Here we postulate that such a spanned from 5.96 to 5.33 Myr ago1,2. Although there is debate on how long-lasting, shallow connection can be naturally sustained by a com- the events recorded in strata correlate between the marginal shallow petition between the tectonic obstruction of the seaway and its ero- basins and the deeper parts of the Mediterranean, biostratigraphic sional deepening by water inflow, and that this competition was analyses2,11 agree on the occurrence of two stages in both environ- responsible for the salt precipitation during the first stage of the MSC. ments: stage 1 encompasses the early salt precipitation (forming the To test this hypothesis, we combine a model of rock erosion with lower evaporites), producing massive gypsum deposits at the basin classical hydrodynamic equations and a climate-based water budget margins and involving minor sea-level drawdown; stage 2 includes (Fig. 1 and Supplementary Methods) to calculate the timing of water the formation of the upper evaporites and Lago Mare deposits, and a and salt flow between the Atlantic and the Mediterranean and the kilometre-scale Mediterranean sea-level drop evidenced by the erosion produced along the connecting corridor. The evolution of widespread presence of Messinian erosion surfaces11. During stage 2, the reference model in Fig. 2 starts with a 60-m-deep seaway that is the largest rivers flowing to the nearly desiccated Mediterranean uplifted at a constant rate. As the seaway becomes shallower, less water excavated canyons about 2,500 m deep in the Nile delta12 and is able to cross the strait, and the Mediterranean sea level decreases 1,000 m deep at the mouth of the Rhone13. Strontium isotope data14 evaporatively. When the seaway becomes shallower than a few tens of indicate that stage 1 took place in a restricted Mediterranean, that is, metres, the Mediterranean sea level decreases at rates of centimetres one with reduced connectivity with the Atlantic, whereas stage 2 per year. As this sea-level difference across the Gibraltar arc increases, occurred in predominantly continental waters with little or no con- so does the shear stress of the inflow and the associated seaway erosion nection to the ocean. The durations of these two phases have been rate, which eventually compensates the imposed uplift rate. This keeps estimated at 360 and 270 kyr, respectively, on the basis of the assump- the inlet neither open enough to allow double circulation nor completely tion that the 14 to 17 cycles observed in the gypsum of stage 1 are due to closed. If the uplift rate exceeds a critical value, Uc, then it overcomes Milankovitch precessional cycles of insolation. erosion and the threshold rises above global sea level, cancelling the 1Instituto de Ciencias de la Tierra Jaume Almera, CSIC, Sole´ i Sabarı´s s/n, 08028 Barcelona, Spain. 15 DECEMBER 2011 | VOL 480 | NATURE | 359 ©2011 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER Gibraltar arc W. Mediterranean E. Mediterranean Atlantic Ocean seaway Evaporation z Inflow, Q 0 Marginal basins 2 1 Gypsum/ Sill depth z Oscillating level sapropel s z z 1 Cyclical 2 1 2 erosion Sicily sill High (presently salinity 430 m deep) Halite Med. high stand Constant tectonic Small Enhanced head loss, z –z uplift rate inflow,Q 1 0 s Gypsum precipitation Low Deepening erosion seaway MSC rate stage-1 High cyclicity Shallowing erosion seaway rate due to uplift Halite Large Reduced precipitation2 ,Q head loss Med. inflow low stand Figure 1 | Competition between uplift and erosion along the last corridor in water level and salinity can result from feedback (harmonic coupling) connecting the Atlantic and the Mediterranean during stage 1 of the MSC. between tectonic uplift, water flow, evaporation and sill erosion. Sketch not Erosion at the sill is controlled by rock erodibility and water inflow, which is drawn to scale. calculated as a function of sill depth, z0 2 zs, and head loss, z0 2 z1. Oscillations connection permanently. The reference model in Fig. 2 corresponds to incision studies (Fig. 3) and found that values greater than 1 mm yr21 the critical uplift rate (4.9 mm yr21) for the reference erodibility and are needed. These values are close to uplift rates expected from mantle climatic parameters. We searched systematically for the critical uplift geodynamic models simulating slab detachment9,10. Critical uplift rates rates for a range of erodibility values consistent with previous river are also dependent on the hydrological deficit. Changing the climatic a 50 150 rate Halite precip. Cycles of gypsum precipitation (10 ) –1 100 12 kg yr kg yr –1 12 Halite 50 ) (10 Gypsum precip rate Gypsum precip 0 0 Channel width b 15 1.0 (km) ) 10 –1 Flow velocity 0.5 5 (m s Channel width Velocity V 0 0.0 rate (m yr Erosion c ) 60 0.03 –1 Water s 3 40 discharge 0.02 m 3 20 Erosion 0.01 (10 Discharge rate Q 0 0.00 –1 d ) Atlantic level, z 0 0 0 Water level (m) Water (m) (m) z s s 0 –200 z z Stage 2: –400 Sill depth, z desiccation West Mediterranean, 1 –600 Ocean, East Mediterranean, z –100 2 Sill depth, –800 0 10 20 30 40 50 60 70 80 90 100 110 120 Time, t (kyr) (Reference model) U U –1 U –1 = c = 4.9 mm yr = 2.0 mm yr Figure 2 | Calculated evolution of the reference model resulting from Atlantic (blue), and level of the Mediterranean (red).
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