Orbital and eustatic control of basin hydrology during the first stage of the Messinian Salinity Crisis

Gypsum and marls from the fifteen cycles of the Messinian Yesares Member in the Sorbas Basin, SE Spain, were analysed using a multi-isotope approach to reconstruct palaeo-hydrological conditions of the basin and the wider Mediterranean region during the deposition of the Primary Lower Gypsum (PLG) of the Messinian Salinity Crisis (MSC) (~5.97–5.60 Ma). By analysing structurally-bound water in sedimentary gypsum (CaSO4·2H2O), the past oxygen and hydrogen isotopic composition of waters during evaporite formation is determined. These measurements are combined with water salinity inferred from gypsum fluid inclusions and isotopic analysis of strontium (87Sr/86Sr), calcium (δ44/40Ca), sulphur ((δ34S) and oxygen of sulphate ((δ18OSO4) in the same samples, as well as isotopic analysis (44/40Ca, 87Sr/86Sr, 18Ocarb, 13C) of the interbedded carbonate marls. The PLG in the Sorbas Basin did not form solely from the evaporation of seawater but rather precipitated from a hybrid brine consisting of seawater and a significant inputof freshwater. The seawater/freshwater ratio changed through time and was influenced by (1) tectonic uplift and basin restriction, (2) obliquity-driven glacial-interglacial sea level changes and (3) precession-driven freshwater influx. A progressive freshwater increase up-section reflects tectonic restriction of the basin, supported by 87Sr/86Sr trends, which start at seawater values in Cycle 2 and deviate upward. Two minor reversals in 87Sr/86Sr at ~5.84 Ma and ~5.72 Ma reflect the role of obliquity-controlled sea level and eccentricity-modulated precession. Maximum divergence of 87Sr/86Sr from the seawater curve, as well as precipitation of authigenic dolomite with high 18Ocarb, represent a temporary regime change to a freshwater-dominated system coincident with the strongest glacial stages (Marine Isotope Stage TG20) and an eccentricity minimum at ~5.75 Ma. A gradual tectonic restriction of the Atlantic connection, superimposed on orbitally controlled glacio-eustatic (obliquity-dominated) and freshwater input (precession-dominated) changes, led to gypsum-marl deposition in Mediterranean marginal basins during the early MSC.