Alain Foucault and Frédéric Mélières

Muséum national d'histoire naturelle, Laboratoire de géologie, 43, rue Buffon, 75005 Paris, France. 


     In central Mediterranean, Pliocene marine sediments show cyclic variations of their mineralogical content likely controlled by astronomically induced climatic changes (mainly precession cycle). Four sections, including several succesive lithologic cycles, choosen within various environments from the base to the very top of the Pliocene, have been studied by high resolution X-ray quantitative mineralogy. Light coloured carbonated beds usually rich in palygorskite and kaolinite, alternate with darker carbonate-poor beds rich in smectite. The latter appear as true sapropels in deep depositional conditions (Hole 964 A, Ionian basin), and as grey-coloured sometimes sapropelic marls in shallower conditions (Punta Piccola, Sicily; Monte Singa and Vrica, Calabria).
Concerning the sources of contribution, the most characteristic mineral is palygorskite. In the Mediterranean realm, only the African continent seems able to provide, through erosion processes, notable quantities of this mineral which is found in relative abundance in African Paleogene deposits of Saharan and peri-Saharan areas. Sea transportation of palygorskite from Africa to Sicily does not seem possible because of the hydrodynamic barrier constituted by the strong surface current that flows through the Siculo-Tunisian strait from NW to SE carrying suspensions in this direction. But it has been shown that, in Central Mediterranean, the wind takes an important part in transportation of particles, being sometimes the only responsible of concentrations observed in different minerals. The reality of this wind transportation from South to North in the Mediterranean is confirmed by many direct observations. Palygorskite is not the only mineral extracted by the wind from the African continent and carried to the Mediterranean. It usually comes along with kaolinite, illite, dolomite and quartz. We can therefore consider that the fluxes of these five minerals have to vary in near identical proportions. This explains the good correlation evidenced within the minerals of this group.

     Smectite and chlorite show variations opposite to those of palygorskite and kaolinite and therefore must be different in origin. They may be supplied by fluvial erosion cutting the emerging Sicilian Island or erosion of more or less remote areas of the same alpine domain.

     We suggest that the above-mentioned sedimentary periodicities characterized by abundance opposition of two groups of minerals results from the alternation of periods respectively of weak and strong rainfalls on both north and south Mediterranean sides.

1. Periods of minimum rainfall.
On the south side of the Mediterranean, a weakening of rainfall increased the aridity of the Sahara, resulting in a reduced and scattered vegetation in peri-Saharan areas. Consequently, soils were far more vulnerable to wind erosion wich lead to an increased wind transportation of minerals (palygorskite, kaolinite, dolomite, quartz, potassium feldspar, and dolomite) from these areas to the Mediterranean. On the northern side of the Mediterranean, a decrease of rainfall resulted not only in a decrease in the river discharge, but also in the sediment load, because fluvial erosion lost its efficiency. Consequently, the detrital material supply of rivers to the sea decreased, increasing the concentration of wind-transported material in the sediment. During these periods, sediments show the highest content in palygorskite and related minerals (kaolinite, dolomite, quartz, and feldspar) and the lowest in smectite and chlorite.

2. Periods of maximum rainfall.
On the south side of the Mediterranean, an increase in rainfall on the Sahara, as well as on peri-Saharan areas, allowed the vegetation to spread over, protecting soils and significantly reducing, or even stopping, the eolian erosion. On the north of the Mediterranean, an increase in rainfall did not notably modify the density of pre-existent vegetation but resulted in an increase in river erosion. Consequently, the sediment yield increased with river discharge and provided more terrigenous supply to marine sedimentation. Sediments deposited during these periods, including sapropels or gray beds, show the highest content in smectite and chlorite.

[Reference: 15th Intern. Sedim. Congress, Alicante (Spain), April 1-17 1998. Abstracts, p. 343-344.]