Palaeogeography, Palaeoclimatology, Palaeoecology
The Messinian salinity crisis: Looking for a new paradigm?
Introduction
The dramatic palaeoenvironmental changes of the Mediterranean area related to the Messinian salinity crisis still represent a largely debated item for the scientific community. The significant enhancement of stratigraphic techniques and approaches achieved in the last few years with the establishment of an astronomical polarity timescale for the last 10 My and the diffusion of physical–stratigraphic concepts have greatly improved our chances for a more detailed knowledge of the geological evolution of the Mediterranean marine and terrestrial domains during the Neogene.
However, more than 30 years after the formulation of the Deep Desiccation Model, the basic palaeoenvironmental questions about the Messinian salinity crisis still remain unanswered. The palaeogeography of the Mediterranean area, as well as the physical and chemical structure of the water column through the different Messinian stages and the role of biota, which are key points for the assessment of the evolution of marine and terrestrial ecosystems, are still far from being fully understood.
The lack of a reliable palaeogeographic framework for the Mediterranean area during the Messinian puts severe limitations to the attempts of many international research groups the quantitative model palaeoclimatic and palaeoceanographic scenarios of the Messinian salinity crisis. Three main factors are responsible for this situation: 1) the still unknown nature of the Messinian deposits buried in the deepest Mediterranean basins, 2) the possibly overlooked amplitude and velocity of Messinian tectonic events and 3) the lack of a high-resolution supra-regional stratigraphic framework for the latest Messinian interval (the Lago Mare stage).
All these problems ultimately arise from the great difficulties in establishing correct stratigraphic relationships between marginal and basinal successions in the different regional contexts. This is mainly due to the peculiar characters of the late Messinian deposits that do not allow use of common biomagnetostratigraphic correlation tools. Moreover, outcropping Messinian successions are often representative of marginal depositional contexts, while deep basinal settings are usually buried below the Mediterranean sea-floor and only rarely accessible through commercial seismic and well data.
In this respect, the Apennine foredeep basin represents an exception; due to its geological setting, the relationships between marginal and basinal successions can be established and a geologic and stratigraphic model for its evolution during the Messinian has been recently reconstructed, based on the integration of surface and subsurface data and the use of a physical stratigraphic approach (Roveri et al., 1998, Roveri et al., 2001). The Apennine record of the Messinian events has been usually overlooked in the past, being considered an anomaly in the larger-scale Mediterranean framework. On the contrary, we believe that, although not necessarily representative of all the different conditions experienced in the Mediterranean area, the Apennine sedimentary record of Messinian events provides important insights on the MSC, whose larger-scale implications are probably more important than usually thought.
We are well aware that the chronology of Messinian events is still a controversial issue; however, we will adopt here the cyclostratigraphic model of Krijgsman et al., 1999a, Krijgsman et al., 1999b; Fig. 1), that supports the classical three-fold subdivision of the Messinian stage: 1) a pre-evaporitic (7.251–5.96 Ma) phase, characterized by the common occurrence of euxinic, organic-rich deposits, which record a reduced circulation of deep Mediterranean waters; 2) Lower Evaporites (5.96–∼ 5.60 Ma), with precipitation of shallow-water evaporites in marginal basins; 3) Upper Evaporites, or post-evaporitic Lago Mare stage (∼ 5.60–5.33 Ma), showing the development of non-marine deposits with mollusc, ostracod and dinoflagellate assemblages of Paratethyan affinity (Lago Mare biofacies; Ruggieri, 1967, Iaccarino and Bossio, 1999). The astronomically calibrated time scale for the Neogene still has at the base of this interval a gap lasting 90 ky (Krijgsman et al., 1999a, Krijgsman et al., 1999b), usually related to the deep desiccation of the Mediterranean and the associated tectonic movements resulting from isostatic rebound; the end of this stage is represented by the sudden, almost synchronous return to fully marine conditions in the Mediterranean Basin (Iaccarino et al., 1999). This reconstruction of Messinian events is in good agreement with the two-step model of the MSC proposed by Clauzon et al. (1996), that pointed out the diachronous deposition of evaporites across marginal and basinal settings.
In this paper, after a brief review of the main characters of the Apennine foredeep Messinian stratigraphy, we will discuss some of its most important implications for the MSC and we will put forward some issues and considerations that in our opinion should be addressed to future research projects.
Section snippets
The Apennine foredeep record of Messinian events
In the last 10 years a complete stratigraphic revision of the Apennine Messinian deposits has been carried out using a physical–stratigraphic approach, based on the recognition and basin-wide correlation of unconformities, key-surfaces and sedimentary cycles (Bassetti et al., 1994, Roveri et al., 1998, Bassetti, 2000, Manzi, 2001, Roveri et al., 2001, Ricci Lucchi et al., 2002). The regional-scale studies were inspired by the pioneering work of Gelati et al. (1987) carried out in the subsurface
Main implications for the Messinian salinity crisis (MSC)
In this section we will discuss some general open problems concerning the different phases or stages of the MSC, arising from the experience derived from the Apennine foredeep record. In the next paragraphs we will particularly concentrate on the problems concerning the second and third stages of the MSC, which were characterized by the most important and enigmatic palaeoenvironmental changes.
Conclusions
The generally poorly defined relationships between marginal and basinal successions, the lack of a high-resolution stratigraphic framework for the Lago Mare stage and the poor consideration for the role of Mediterranean-scale tectonic deformations in controlling the main stratigraphic events, limit our full understanding of the MSC. Many problems arise from the “one section” approach, largely diffused in the past; good data and observations, if isolated or not well framed into their regional
Acknowledgements
Data and observations of this paper partially derive from the PhD thesis of V. Manzi. A.M. Borsetti, A. Negri and W. Krijgsman provided substantial support to his work and are here gratefully acknowledged. We wish to thank also F. Ricci Lucchi and M.A. Bassetti for their helpful suggestions and long discussions about the Messinian problems.
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