The end of the Messinian salinity crisis: Evidences from the Chelif Basin (Algeria)

How did the Messinian Salinity Crisis end is a matter of intense debate between two opposite concepts i.e., the generalised dilution event, the so-called Lago–Mare, followed by the sudden restoration of the marine conditions at the base of the Zanclean, or the early partial or complete marine refill that would have happened earlier during the upper Messinian. The Chelif Basin of Northwestern Algeria, one of the greatest Messinian marginal basins of the Mediterranean, provides an exceptional opportunity to study in detail how this major paleoenvironmental change occurred through continuous sedimentary records of the Miocene–Pliocene boundary. Five sections representative of both the central and marginal areas of the Chelif basin were analysed in detail for sedimentology, mineralogy, foraminifera and ostracod assemblages, and stable isotope composition of carbonates. The late Messinian deposits exhibit a great lithological variability with predominant clastic deposits (sandstones, siltstones, conglomerates, carbonates) with, in some marginal areas, large slided masses of lower Messinian (Tripoli Unit) that settled just before the base of the Zanclean. Most sediments are either azoic or contain microfossils typical of hyposaline conditions sometimes mixed with benthic foraminifera known to be adapted to stressed environments and even to lacustrine conditions. Fewer types of sediment contain assemblages of planktonic foraminifera, including typical Messinian species, which are thought to be reworked. In contrast, diversified communities of benthic and planktonic foraminifera returned suddenly at the base of the Zanclean; the Messinian–Pliocene transition was marked by an abrupt change of the δ18O values of carbonates from variable but predominantly negative values characteristic of low salinity conditions in the upper Messinian to higher and more stable values in the Zanclean typifying marine conditions. The southern margin continued, however, to be submitted to large inputs of continental waters. The beginning of the marine inundation was a transitional period with poorly diversified assemblages of planktonic foraminifera and benthic communities tolerant to oxygen deficient bottom conditions; more stable marine conditions with better bottom water ventilation were set up after a lag time which would roughly correspond to a precession cycle. Except for the more marginal areas where conglomerates and erosional features are observed, the restoration of marine conditions occurred, without any significant erosion in the more central areas where the contact is well marked by strong burrowing activity by benthic organisms. This indicates the rapid substitution in the basin of brackish waters by marine waters. Both mixing of these two kinds of waters and permanence of freshwater inputs in the initial stage of the Pliocene transgression may explain why the community of marine planktonic foraminifera, introduced by the inflowing Atlantic waters, hardly survived in such unfavourable ecological conditions in the Mediterranean and reached a normal development later when more stable marine conditions settled. This study and the comparison with other sections from both marginal and deep Mediterranean basins confirm that the Lago–Mare was a widespread dilution event that affected the whole Mediterranean at the end of the Messinian which was abruptly interrupted by the sudden marine inundation at the base of the Zanclean.