ALBERT

Description: Although the present-day Mediterranean-Atlantic water exchange has been extensively studied, little is certain about the dynamics of the Betic and Rifian corridors that existed before the Messinian Salinity Crisis. Due to the difficulties in studying the paleogeographic evolution of these corridors, physics-based knowledge of their behavior is essential to interpret observational evidence and to relate flow structures to gateway geometries. Here we present the first systematic model study of the water exchange through these gateways. We use a parallel version of the Princeton Ocean Model (sbPOM) and a set of idealized bathymetries based on a late Tortonian paleogeography. This analysis represents a major step forward in the understanding of the behavior of the double-gateway system constituted by the Late Miocene Betic and Rifian corridors. We demonstrate that the “siphon” scenario, involving inflow of cold upwelled Atlantic water through the Rifian corridor and outflow of Mediterranean water only via the Betic corridor, is unlikely from a physics perspective. It is shown that two exchange patterns are possible depending solely on the relative depth of the corridors. The implication of this is that geological evidence for the behavior of one corridor provides information about the dimensions of the other. We show that disappearance of outflow in one corridor does not necessarily imply its closure and we establish a guideline to determine how geological evidence can be interpreted as indicating one- or two-layer flow. Based on the model results, we propose new physics-based scenarios for the time interval defined for the siphon.

Description: 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.