Late Tortonian–early Messinian stepwise disruption of the Mediterranean–Atlantic connections: constraints from benthic foraminiferal and geochemical data

doi:10.1016/S0031-0182(03)00472-3

Palaeogeography, Palaeoclimatology, Palaeoecology

Volume 198, Issues 3–4, 1 October 2003, Pages 303-319

https://doi.org/10.1016/S0031-0182(03)00472-3 Get rights and content

Abstract

Late Miocene marine land-based sections in the Mediterranean area contain important information on the prelude to the Messinian salinity crisis (MSC). Integrated (bio-, magneto-, tephra- and cyclo-) stratigraphy allows for bed-to-bed correlations of sections throughout the Mediterranean and its borderlands, and enables correlation with extra-Mediterranean (Atlantic) sites. Benthic foraminiferal and geochemical data from five sections were compared to literature data and indicate that: (a) restriction of the Mediterranean from the Atlantic Ocean started during the late Tortonian, (b) the continuous development towards the MSC started as early as ∼7.2 Ma in the deeper parts of the basin and was enhanced by astronomical forcing and (3) the onset of the MSC must have been to a large extent related to differential vertical movements at the Betic and Rifian corridors, the Miocene connections between the Mediterranean and the Atlantic.

Introduction

It has long been recognised that the development towards the Messinian salinity crisis (MSC) in the Mediterranean started some time before the actual deposition of evaporites. A factor of considerable importance was the continuous restriction of the Mediterranean from the Atlantic Ocean due to differential vertical movements, including thrusting, in the Betic and Rif corridors (through southern Spain and northern Morocco respectively, see Fig. 1). The complex structures of the Betic and Rifian cordilleras, together with dating problems, have long hindered a thorough appraisal of the development of the Mediterranean–Atlantic connections through these corridors. By now, continuous research on land-based sections and cores in the Mediterranean and its borderlands has led to extremely accurate dating based on integrated stratigraphic approaches (e.g. Hilgen et al., 1995, Hilgen et al., 2000, Krijgsman et al., 1995, Krijgsman et al., 1997, Krijgsman et al., 1999a, Krijgsman et al., 1999b, Sierro et al., 1997, Sierro et al., 1999, Sierro et al., 2001). In the Betic cordillera, sedimentological and faunal evidence indicates that movements along major strike-slip faults caused differential vertical uplift, and separated basins connected to the Atlantic from basins connected to the Mediterranean (Gebhardt, 1994, Weijermars, 1998, Soria et al., 1999, Soria et al., 2001). Effective disconnection of the Mediterranean and the Atlantic is now thought to have started during the late Tortonian. Land mammal exchange between Africa and Spain occurred at 6.1 Ma (Garcés et al., 1998), which is well before the onset of the MSC at 5.96 Ma, evidencing that by that time the Betic Corridor as such no longer existed.

In marginal basins connected to the Mediterranean, evaporite deposition is expected to be coeval with the MSC, and this appears to be the case (e.g. in the Sorbas and Nijar basins: Sierro et al., 1997, Sierro et al., 1999, Sierro et al., 2001). Older, Tortonian evaporites have been found in the Lorca and Fortuna basins (Garcés et al., 1998, Dinarès-Turell et al., 1999, Krijgsman et al., 2000). Krijgsman et al. (2000) dated the onset of evaporitisation in these basins, and thus the main phase of restriction of the so-called Tortonian salinity crisis (TSC), at 7.8 Ma. In the Lorca and Fortuna basins the regressive series of sediments, typically consisting of diatomites and evaporites just as in the Mediterranean Messinian series, was followed by continental sedimentation. Although the sequence of deposits may be indistinguishable from the Messinian series in the field, the geochemical characteristics (oxygen, sulphur and strontium isotopes and strontium content) of the Tortonian evaporites differ from those of Messinian ones, due to, among others, genesis from other than marine sources and recycling of Triassic sulphates (Playà et al., 2000).

The Moroccan Rifian Corridor is also intensively studied. In the southeastern part of the Rifian Corridor (the Taza–Guercif area) a sequence of deepening and shallowing was described by Krijgsman et al. (1999a). Deepening, i.e. effective opening of the strait, occurred between ∼8.0 and 7.6 Ma, when a depth of around 400 m was reached. At approximately the same time (7.6 Ma) the TSC of the eastern Betics had more or less ended (Krijgsman et al., 2000).

Benthic foraminifers are excellently suited to constrain the expected effect of restriction at the Mediterranean–Atlantic gateways on the deeper-water environment. We present benthic foraminiferal and geochemical data providing evidence for an early, and repeated, onset of a restrictive environmental development, eventually culminating in the MSC. Though the effective disconnection of the Mediterranean from the Atlantic Ocean needs further evaluation, evidence is accumulating that tectonics in the Betic and Rifian cordilleras played an important role in the progressive isolation of the Mediterranean during the late Miocene, most likely in concert with sea level fluctuations and orbital forcing.

Section snippets

Sections and age model

The benthic foraminiferal data discussed in this paper are derived from four land-based sections in the central and eastern Mediterranean and one at the Atlantic margin of the Miocene Rifian Corridor (Oued Akrech, NW Morocco, Fig. 1, Fig. 2). Our data cover the late Tortonian and the early Messinian. The dominant lithological alternations are expressed on a precession scale (∼19–23 kyr; Fig. 2). The lithology of the intra-Mediterranean sections consists of alternations of homogeneous marls and

Palaeobathymetry

Palaeodepths calculated from P/B ratios (see Van der Zwaan et al., 1990) give a fair indication of the bathymetry of the sections during the late Tortonian and early Messinian (Fig. 3B). For the Faneromeni section, depth is estimated to have been around 300–400 m. The Oued Akrech, Monte Gibliscemi, Metochia and major parts of the Monte del Casino sections were deposited in estimated water depths of 800–1000 m or more. Large-scale sea level changes should certainly be reflected, especially in

Stable isotope data

Detailed and reliable stable isotope records throughout the Messinian stage are scarce until now. This is in part due to increasingly bad preservation of foraminiferal faunas towards the MSC, and also to the disappearance of commonly used taxa such as Cibicides wuellerstorfi. We will compare two records from the Mediterranean: Monte del Casino, northern Italy (Kouwenhoven et al., 1999), and Metochia, Gavdos (Seidenkrantz et al., 2000), with data from the southeastern Rif Corridor and the

Effect of MOW in the Atlantic?

Comparing Mediterranean and Atlantic stable isotope records (Fig. 4) it is possible to separate regional and global trends. The δ¹³C values in the Mediterranean and the Rifian Corridor reflect a global shift to heavy values around 8 Ma, and are in line with the global carbon shift from 7.8 Ma onwards, although much lighter values are eventually reached at intra-Mediterranean sites. No lead or lag is reflected in the data sets. The gradual trend towards heavy δ¹⁸O values occurring in the

Acknowledgements

Cooperating Italian, Spanish and Dutch groups collected samples throughout the past decades. G. van ’t Veld and G. Ittmann are acknowledged for sample processing. H. de Waard and A. van Dijk performed geochemical analyses. Benthic foraminifer samples were counted by D. Tervoort-van den Akker, A. Okx, M. Rauhé and J. de Wit. Part of the stable isotope analyses of Metochia was performed in Cambridge. Discussions at different stages with N.J. Shackleton (Cambridge), W. Krijgsman, and W.J.

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