A model analysis of atypical marine sedimentation in mediterranean basins

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 maximu

A modeling perspective on spatial and temporal variations in Messinian evaporite deposits

During the Messinian Salinity Crisis (MSC), evaporite-dominated sequences formed in marginal and deep basins of the Mediterranean Sea. In the marginal basins, the onset of the MSC is synchronous and a single depositional framework fits all sequences. In contrast, MSC sequences of the western and eastern deep basins appear to comprise a different number of units and differ greatly in thickness. Even though there exist numerous scenarios for deposition in the marginal and deep basins, the link between the two settings is difficult and scenarios are rarely quantitatively supported. We employ a simple box model for the Messinian Mediterranean to examine the causes of (1) spatial variation in thickness and (2) differences in the time of onset of deposition. Model results are compared with actual observations on the MSC sequences. The results show that a large connection between the western and eastern basin is necessary for, and some degree of water column stratification is conducive to, synchronous onset of the MSC in the marginal basins. Moreover, halite deposits in the deep basins are likely to be coeval and have formed in ≈60 ka after a (further) restriction of the Atlantic–Mediterranean connection during the MSC, but without a significant sea level drop. A difference in the net salt gain per unit volume caused the different halite deposition rates in the two basins. A scenario with only a simple restriction of the Atlantic–Mediterranean connection during the Late Miocene – without significant changes in the Mediterranean sea level, the fresh water budget, or the size of the Strait of Sicily – is able to explain the synchronous onset of the MSC, the synchronous marginal evaporite formation and the differences in the deep basinal sequences

A modeling perspective on spatial and temporal variations in Messinian evaporite deposits

During the Messinian Salinity Crisis (MSC), evaporite-dominated sequences formed in marginal and deep basins of the Mediterranean Sea. In the marginal basins, the onset of the MSC is synchronous and a single depositional framework fits all sequences. In contrast, MSC sequences of the western and eastern deep basins appear to comprise a different number of units and differ greatly in thickness. Even though there exist numerous scenarios for deposition in the marginal and deep basins, the link between the two settings is difficult and scenarios are rarely quantitatively supported. We employ a simple box model for the Messinian Mediterranean to examine the causes of (1) spatial variation in thickness and (2) differences in the time of onset of deposition. Model results are compared with actual observations on the MSC sequences. The results show that a large connection between the western and eastern basin is necessary for, and some degree of water column stratification is conducive to, synchronous onset of the MSC in the marginal basins. Moreover, halite deposits in the deep basins are likely to be coeval and have formed in ≈60 ka after a (further) restriction of the Atlantic–Mediterranean connection during the MSC, but without a significant sea level drop. A difference in the net salt gain per unit volume caused the different halite deposition rates in the two basins. A scenario with only a simple restriction of the Atlantic–Mediterranean connection during the Late Miocene – without significant changes in the Mediterranean sea level, the fresh water budget, or the size of the Strait of Sicily – is able to explain the synchronous onset of the MSC, the synchronous marginal evaporite formation and the differences in the deep basinal sequences

A box model of the Late Miocene Mediterranean Sea: implications from combined 87Sr/86Sr and salinity data

Under certain conditions the strontium isotope ratio in the water of a semi‐enclosed basin is known to be sensitive to the relative size of ocean water inflow and river input. Combining Sr‐isotope ratios measured in Mediterranean Late Miocene successions with data on past salinity, one can derive quantitative information on the Mediterranean hydrological budget at times before and during the Messinian Salinity Crisis (MSC). Previous studies obtained this hydrological budget by inverting the salinity and strontium data with steady state solutions to the conservation equations of salt, strontium and water. Here, we develop a box model with a time‐dependent set of equations to investigate the coeval evolution of salinity and Sr ratios under different water budgets, gateway restrictions and riverine Sr characteristics. Model results are compared with the salinity and strontium ratio data from the Mediterranean. With a present‐day water budget, strontium ratios in the Mediterranean never reach the observed Messinian values regardless of gateway restriction and water budget. However, a model with tripled river input, as inferred for the Late Miocene, is able to reproduce the Sr ratios observed. The onset of the MSC can be explained with a simple restriction of the gateway(s) between the Mediterranean and Atlantic. Lower Evaporite gypsum formed in a basin with less outflow to the Atlantic than modeled in previous studies because of the large Late Miocene river input. Evaporite thicknesses predicted by our model and consistent with the Messinian Sr ratios are on the low end of the thickness range inferred from seismics

A regional ocean circulation model for the mid-Cretaceous North Atlantic Basin: implications for black shale formation

High concentrations of organic matter accumulated in marine sediments during Oceanic Anoxic Events (OAEs) in the Cretaceous. Model studies examining these events invariably make use of global ocean circulation models. In this study, a regional model for the North Atlantic Basin during OAE2 at the Cenomanian-Turonian boundary has been developed. A first order check of the results has been performed by comparison with the results of a recent global Cenomanian CCSM3 run, from which boundary and initial conditions were obtained. The regional model is able to maintain tracer patterns and to produce velocity patterns similar to the global model. The sensitivity of the basin tracer and circulation patterns to changes in the geometry of the connections with the global ocean is examined with three experiments with different bathymetries near the sponges. Different geometries turn out to have little effect on tracer distribution, but do affect circulation and upwelling patterns. The regional model is also used to test the hypothesis that ocean circulation may have been behind the deposition of black shales during OAEs. Three scenarios are tested which are thought to represent pre-OAE, OAE and post-OAE situations. Model results confirm that Pacific intermediate inflow together with coastal upwelling could have enhanced primary production during OAE2. A low sea level in the pre-OAE scenario could have inhibited large scale black shale formation, as could have the opening of the Equatorial Atlantic Seaway in the post-OAE scenario

Precessional control of Sr ratios in marginal basins during the Messinian Salinity Crisis?

Based on 87Sr/86Sr data of the Primary Lower Gypsum (PLG) deposits in the Vena del Gesso basin—a marginal basin of the Mediterranean during the Messinian Salinity Crisis—a correlation between 87Sr/86Sr values and precessional forcing has recently been proposed but not yet confirmed. In this study, a box model is set up to represent the Miocene Mediterranean deep basin and a connected marginal basin. Measurements of 87Sr/86Sr in the Vena del Gesso and estimated salinity extrema are used to constrain model results. In an extensive analysis with this model, we assess whether coeval 87Sr/86Sr and salinity fluctuations could have been forced by precession-driven changes in the fresh water budget. A comprehensive set of the controlling parameters is examined to assess the conditions under which precession-driven 87Sr/86Sr variations occur and to determine the most likely setting for PLG formation. Model results show that precession-driven 87Sr/86Sr and salinity fluctuations in marginal basins are produced in settings within a large range of marginal basin sizes, riverine strontium characteristics, amplitudes of precessional fresh water budget variation, and average fresh water budgets of both the marginal and deep basin. PLG deposition most likely occurred when the Atlantic-Mediterranean connection was restricted, and the average fresh water budget in the Mediterranean was significantly less negative than at present day. Considering the large range of settings in which salinities and 87Sr/86Sr fluctuate on a precessional timescale, 87Sr/86Sr variations are expected to be a common feature in PLG deposits in marginal basins of the Mediterranean

Precessional control of Sr ratios in marginal basins during the Messinian Salinity Crisis?

Based on 87Sr/86Sr data of the Primary Lower Gypsum (PLG) deposits in the Vena del Gesso basin—a marginal basin of the Mediterranean during the Messinian Salinity Crisis—a correlation between 87Sr/86Sr values and precessional forcing has recently been proposed but not yet confirmed. In this study, a box model is set up to represent the Miocene Mediterranean deep basin and a connected marginal basin. Measurements of 87Sr/86Sr in the Vena del Gesso and estimated salinity extrema are used to constrain model results. In an extensive analysis with this model, we assess whether coeval 87Sr/86Sr and salinity fluctuations could have been forced by precession-driven changes in the fresh water budget. A comprehensive set of the controlling parameters is examined to assess the conditions under which precession-driven 87Sr/86Sr variations occur and to determine the most likely setting for PLG formation. Model results show that precession-driven 87Sr/86Sr and salinity fluctuations in marginal basins are produced in settings within a large range of marginal basin sizes, riverine strontium characteristics, amplitudes of precessional fresh water budget variation, and average fresh water budgets of both the marginal and deep basin. PLG deposition most likely occurred when the Atlantic-Mediterranean connection was restricted, and the average fresh water budget in the Mediterranean was significantly less negative than at present day. Considering the large range of settings in which salinities and 87Sr/86Sr fluctuate on a precessional timescale, 87Sr/86Sr variations are expected to be a common feature in PLG deposits in marginal basins of the Mediterranean

Biogeochemistry of the North Atlantic during oceanic anoxic event 2: role of changes in ocean circulation and phosphorus input

The geological record provides evidence for the periodic occurrence of water column anoxia and formation of organic-rich deposits in the North Atlantic Ocean during the mid-Cretaceous (hereafter called the proto-North Atlantic). Both changes in primary productivity and oceanic circulation likely played a role in the development of the low-oxygen conditions. Several studies suggest that an increased input of phosphorus from land initiated oceanic anoxic events (OAEs). Other proposed mechanisms invoke a vigorous up-welling system and an ocean circulation pattern that acted as a trap for nutrients from the Pacific Ocean.Here, we use a detailed biogeochemical box model for the proto-North Atlantic to analyse under what conditions anoxia could have developed during OAE2 (94 Ma). The model explicitly describes the coupled water, carbon, oxygen and phosphorus cycles for the deep basin and continental shelves. In our simulations, we assume the vigorous water circulation from a recent regional ocean model study. Our model results for pre-OAE2 and OAE2 conditions are compared to sediment records of organic carbon and proxies for photic zone euxinia and bottom water redox conditions (e. g. isorenieratane, carbon/phosphorus ratios). Our results show that a strongly elevated input of phosphorus from rivers and the Pacific Ocean relative to pre-OAE2 conditions is a requirement for the widespread development of low oxygen in the proto-North Atlantic during OAE2. Moreover, anoxia in the proto-North Atlantic is shown to be greatly influenced by the oxygen concentration of Pacific bottom waters. In our model, primary productivity increased significantly upon the transition from pre-OAE2 to OAE2 conditions. Our model captures the regional trends in anoxia as deduced from observations, with euxinia spreading to the northern and eastern shelves but with the most intense euxinia occurring along the southern coast. However, anoxia in the central deep basin is difficult to achieve in the model. This suggests that the ocean circulation used in the model may be too vigorous and/or that anoxia in the proto-North Atlantic was less widespread than previously thought