Evolution of the Late Miocene Mediterranean–Atlantic gateways and their impact on regional and global environmental change
Introduction
During the late Tortonian (~ 11.6 to 7.2 Ma), several marine gateways through southern Spain, northern Morocco and potentially Gibraltar, connected the Mediterranean Sea with the Atlantic Ocean (Fig. 1). Plate tectonic convergence between Africa and Iberia, combined with subduction dynamics in the Alborán region, progressively closed these connections during the Messinian (e.g. Duggen et al., 2003, Gutscher et al., 2002). This tectonic forcing combined with eustatic (e.g. Manzi et al., 2013) and climatic (Hilgen et al., 2007) factors resulted in a complex history of varied Mediterranean–Atlantic exchange and high amplitude environmental fluctuations in the Mediterranean including the formation of the world's most recent saline giant (Warren, 2010).
Like other marginal basins, the Mediterranean's near-landlocked configuration makes it sensitive to subtle changes in climate (e.g. Thunell et al., 1988). Consequently, the first environmental responses to gradual restriction of exchange with the Atlantic recorded in the Mediterranean (e.g. faunal and isotopic changes; Fig. 2), predate any evaporite precipitation there by a million years or more. The most extreme palaeoenvironmental changes took place during the so-called Messinian Salinity Crisis (MSC; 5.97–5.33 Ma; Fig. 2; Table 1) when extensive gypsum deposits precipitated in the Mediterranean's marginal basins and kilometre thick halite units formed in the deep basins (e.g. Hsü et al., 1973, Ryan et al., 1973). This was followed by a period during which the sediments recorded highly fluctuating conditions varying from brackish to hypersaline, before returning, in the Early Pliocene, to open marine conditions (Fig. 2; Hsü et al., 1972). These Late Miocene low salinity intervals, known as the Lago Mare, may be the product of an additional freshwater source supplied to the Mediterranean from Paratethys, the lacustrine precursor to the Black and Caspian seas. Like other major freshwater sources, this is a key component of the Mediterranean's freshwater budget, which combined with the gateway dimensions determine its salinity.
The large volume of salt preserved in the Mediterranean necessitates that one or more marine connections with the open ocean remained, at least until the end of the halite stage (5.55 Ma; Krijgsman and Meijer, 2008). However, the location of the last gateway(s) remains highly ambiguous. Field studies of the sedimentary basins in southern Spain (the Betic Corridor) and northern Morocco (the Rifian Corridor) thought to be part of the corridor network (Fig. 1), typically indicate that these areas were closed to marine exchange well before the MSC (e.g. Betzler et al., 2006, Ivanović et al., 2013a, Krijgsman et al., 1999b, Soria et al., 1999, van Assen et al., 2006), while the Gibraltar Strait is thought to have first opened at the beginning of the Pliocene (5.33 Ma) bringing the MSC to an end (e.g. Blanc, 2002, Garcia-Castellanos et al., 2009, Hsü et al., 1973, Hsü et al., 1977). The key problem is that it is extremely difficult to pinpoint the exact location or timing of closure from field data alone, because the sedimentary successions within the corridors have been uplifted and eroded (e.g. Hüsing et al., 2010). Using other datasets to identify the location of each marine corridor, reconstructing its geometry and reducing uncertainty in the age of closure is therefore critical for constraining the process-response chain linking gateway evolution with the development of the Mediterranean's MSC succession. The Atlantic response to a change in gateway configuration is reliant on changes to the density and volume of Mediterranean outflow and consequently also depends on an ability to reconstruct gateway dimensions and the patterns of exchange.
Several indirect approaches to the study of gateway evolution have been employed in the context of the MSC. These include for example, physics-based mathematical models that quantify gateway configuration (e.g. Meijer, 2006, Meijer and Krijgsman, 2005); and the use of isotopic proxies to elucidate changing connectivity (e.g. Flecker and Ellam, 1999, Ivanović et al., 2013a, Topper et al., 2011). Many other techniques have been applied to successions within the main Mediterranean basin rather than the gateway and the palaeoenvironmental information they provide can be compared with similar data on the Atlantic side of the connection to constrain aspects of exchange. In this paper we review, synthesise and integrate both direct and indirect data relating to the Mediterranean–Atlantic gateways during Late Miocene. Our aim is to use this information to reconstruct exchange before, during and after the MSC, consider the regional and global implications and highlight enduring questions that may be amenable to new research methods.
Section snippets
Gateway control on Mediterranean water properties
At present, the Mediterranean Sea loses more water to the atmosphere by evaporation than it receives from rainfall and river runoff. As a result, its surface waters are subject to an increase in salinity and thus in density. This relatively dense water finds its way to the deeper levels of the Mediterranean. In addition, the Mediterranean is also a major heat sink for the Atlantic (the temperature of Atlantic inflow is ~ 16 °C, while Mediterranean outflow is ~ 12 °C; Rogerson et al., 2012).
Evolution of the Atlantic–Mediterranean gateways: direct approach
The distribution of Late Miocene marine sediments across Northern Morocco and Southern Spain resembles a complex network of channels connecting the Mediterranean and Atlantic (Fig. 1, Fig. 5, Fig. 6) and it is this now uplifted area that is considered to be the Late Miocene gateway region (e.g. Santisteban and Taberner, 1983). It is less clear, however, how much of the bifurcating channel pattern reflects the primary configuration of the Late Miocene marine corridors and how much is a function
Evolution of the Mediterranean–Atlantic gateways: indirect approach
The geological records of the Late Miocene marine connections that are now exposed on land are incomplete as a consequence of the unconformities associated with corridor closure and uplift (Fig. 2) and the uncertainty as to corridor location. As a result, important information about the nature and timing of Mediterranean–Atlantic exchange can only be derived from records outside the corridors that respond to some aspect of the exchange or the lack of it. In the Atlantic a variety of water mass
Tectonic drivers of gateway evolution
There are a variety of different tectonic processes that contributed to the evolution of the Mediterranean–Atlantic connections during the Late Miocene–Pliocene. The main tectonic drivers of the location of the connections are likely to have been the combined influence of African–Iberian convergence with slab rollback and westward motion of the Alborán Domain. Regional uplift will also have played a significant role in the closure of these marine connections and this is likely to have resulted
Evolution of Mediterranean–Atlantic exchange during the Late Miocene
The Mediterranean's hydrologic budget is controlled by both the efficiency of the gateway(s) and net evaporation over the Mediterranean (precipitation + runoff–evaporation, P + R-E; Section 2.1). The combination of these two drivers along with a Mediterranean circulation system where surface water flows east becoming more saline, sinks in the Eastern Mediterranean and flows west at depth, typically results in a density contrast between Mediterranean and Atlantic water at the gateway where
Conclusions
Marine gateways are an important control on both local environmental change and global climate. The Late Miocene Mediterranean gateway system that linked to the Atlantic is a good example of this and much can be learnt about the processes and impacts of gateway closure from the study of the sediments preserved within the ancient marine corridors in southern Spain and northern Morocco. Uplift and erosion resulting from the same tectonic drivers of gateway closure, has led to the preservation of
Acknowledgements
The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007–2013/under REA Grant Agreement No. 290201 MEDGATE. The authors would like to thank Javier Hernández-Molina and Mike Rogerson for their helpful reviews and Dr Carla Sands, MEDGATE's superb Project Manager without whom much of the research would not have happened. CC Martins also thanks CAPES by scholarship support (BEX 5366/12-7).
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