A model analysis of atypical marine sedimentation in mediterranean basins

Abstract

In this research relatively simple computational models are used to gain insight in the processes and conditions that give rise to atypical marine sedimentation, i.e. black shales and evaporites, in mediterranean basins. The geometry of a mediterranean basin -- i.e. landlocked and with connections to the global ocean that are small compared to the basin size -- makes it particularly sensitive to changes in climate. The interactions of basin geometry, climate and marine sedimentation are investigated in the Atlantic and Mediterranean basins of the Cretaceous and Miocene, respectively. The mid-Cretaceous proto-North Atlantic basin in the early stage of its opening is of the \emph{m}editerranean type and one of the most pronounced sites of black shale formation during Oceanic Anoxic Event 2 (OAE2, ~94 Ma). Results from an ocean circulation model show that changes in basinal geometry and sea level would affect the circulation and upwelling pattern in the Atlantic. A series of experiments set up to represent pre-OAE, OAE and post-OAE conditions shows that inflow from the Pacific could have brought nutrients to the Atlantic upwelling zones to fuel enhanced primary productivity during OAE2 and not before and after. More generally, upwelling and circulation during OAE2 is appropriate for extensive black shale formation in the proto-North Atlantic. The most recent and one of the world's largest salt giants formed in the Late Miocene Mediterranean during the Messinian Salinity Crisis (MSC, 5.97 - 5.33 Ma). Although studied for 40 years, knowledge of this remarkable event is largely qualitative. A series of box and ocean circulation models is used to gain detailed insight in the main processes and mechanisms involved in the period running up to and in the first two stages of the MSC, test existing hypotheses quantitatively, examine the conditions with which observational data can be reproduced, and build towards a quantitatively supported scenario. Model results show that a simple scenario fits all available data and observational constraints from marginal and deep water settings. In this scenario the progressive closure of the Atlantic-Mediterranean connections pushes the Mediterranean from pre-MSC open ocean conditions to the synchronous onset of gypsum deposition, followed by, at the acme of the crisis, synchronous halite formation with different deposition rates in the two Mediterranean subbasins. To accumulate the observed volume of gypsum and halite during the MSC, an Atlantic-Mediterranean connection must have accomodated two-layer flow, i.e. a gateway of > 10 m, up to stage 2 of the MSC. A blocked outflow scenario is not viable for the whole MSC but may have existed briefly during stage 2 before the connection with the Atlantic was temporarily disrupted and sea level dropped sharply. During the MSC, Sr isotope ratios and salinity vary on a precessional timescale and can be used as a proxy for the Mediterranean water budget. The Late Miocene fresh water deficit is smaller than at present-day due to a considerably higher river discharge. Hence, Sr ratios and salinity in the Mediterranean are more susceptible to precession-driven climate variations. Noteworthy, and important for determining the exact age of MSC deposits, is that peak salinity and Sr values in the Late Miocene Mediterranean are reached a few kyr after, i.e. lag behind, each precession maximum