insight into the Messinian salinity crisis
In August of 1970 the DSDP ship Challenger was positioned in the western Mediterranean, south of the Balearic Islands, in almost 3000 m of water. The geologists on board were looking for the source of a prominent sub-sea-floor seismic feature called the M-reflector, and, to their great surprise, they drilled into a thick layer of anhydrite - the first evidence of a vast deposit of evaporite rocks extending across the Mediterranean.
It is now
widely accepted that these evaporites, which formed during the Messinian (late
Miocene) – between 5.96 and 5.33 m.y. - resulted from the closure of the
marine passages between the Atlantic and the Mediterranean, and the subsequent
(and repeated) complete (or near-complete) desiccation of the Mediterranean Sea. The important question, which is still in debate, is the
mechanism by which the Mediterranean became isolated.
The dry and hot Mediterranean basin has been an area with a negative precipitation-evaporation budget for millions of years. Without a significant inflow of Atlantic Ocean water, the Mediterranean Sea cannot be sustained.
Three mechanisms have been proposed to explain the isolation of the Mediterranean during the Messinian, including:
1) a 60 m global drop in sea level due to glaciation,
2) horizontal squeezing, and
3) tectonic uplift.
In a recent paper published in Nature, researchers from the GEOMAR group in Kiel, Germany, cast doubt on all three of these mechanisms, and suggest that a significant change in the dynamics of subduction and in volcanism was responsible for the closure of the waterways (Duggen et al., 2003).
Miocene to Pleistocene aged volcanic rocks are present, both above and below water, in the Alboran Basin of the westernmost Mediterranean. Geochemical and isotopic data acquired by Duggen et al. (2003) show two distinctive types of volcanism, including generally felsic rocks typical of subduction, and more mafic rocks typical of a direct source from the asthenosphere.
|The felsic rocks are probably related to east-dipping subduction underneath southern Spain and northern Morocco, a process that may still be active (Gutscher et al., 2002). Duggen et al. suggest that westward migration (roll-back) of the subduction zone could be responsible both for the change in volcanic rock chemistry at close to 6.3 m.y., and also for significant uplift in the area of the Betic and Rifean waterways.|
mechanism involves steepening of the subducting oceanic slab, and the flow of
hot low-density asthenosphere into where the slab had been, up against the base
of the continental crust. Duggen et al. suggest that the presence of this
relatively buoyant material could have produced up to 1000 m of uplift in the
Betic and Rifean corridor areas - more than enough to close the two waterways.
Duggen, S, Hoernie K, van den Bogaard, P, Rupke, L and Phipps Morgan, J, 2003, Deep roots of the Messinian salinity crisis, Nature, V. 422, p. 602-605.
Gutscher, M A, Malod, J, Rehault, J-P, Contrucci, L, Klingelhoefer, F, Mendes-Victor and L, Spakman, W, 2002, Evidence for active subduction beneath Gibraltar, Geology, V. 30; p. 1071–1074.
Steven Earle, Malaspina University College, Nanaimo, Canada, 2003. Return to Earth Science News