Decoding the Mediterranean salinity crisis

Abstract

This historical narrative traces the steps to unravel, over a span of 40 years, an extraordinary event in which 5% of the dissolved salt of the oceans of the world was extracted in a fraction of a million years to form a deposit more than 1 million km 3 in volume. A buried abyssal salt layer was identified with reflection profiling and sampled during the Deep Sea Drilling Project, Leg 13. The dolomite, gypsum, anhydrite and halite in the drill cores paint a surprising picture of a Mediterranean desert lying more than 3 km below the Atlantic Ocean with brine pools that shrank and expanded by the evaporative power of the sun. The desert drowned suddenly when the Gibraltar barrier gave way. The explanation of 'deep-basin, shallow-water' desiccation and the notion of a catastrophic Zanclean flood had a mixed reception. However, the hypothesis became broadly accepted following subsequent drilling expeditions. Nevertheless, as experts examined the evaporate facies and sequences of equivalent age in the terrestrial outcrops, weaknesses appeared in the concept of repeated flooding and drying to account for the magnitude of the deposits. The 'Rosetta Stone', used to decipher conflicting interpretations, turns out not to be the deposits but the erosion surfaces that enclose them. These surfaces and their detritus -formed in response to the drop in base level during evaporative drawdown -extend to the basin floor. Evaporative drawdown began halfway through the salinity crisis when influx from the Atlantic no longer kept up with evaporation. Prior to that time, a million years passed as a sea of brine concentrated towards halite precipitation. During the later part of this interval, more than 14 cyclic beds of gypsum accumulated along shallow margins, modulated by orbital forcing. The thick salt on the deep seabed precipitated in just the next few cycles when drawdown commenced and the brine volume shrank. Upon closure of the Atlantic spillway, the remnants of the briny sea transformed into salt pans and endorheic lakes fed from watersheds of Eurasia and Africa. The revised model of evaporative concentration now has shallow-margin, shallow-water and deep-basin, deep-water precursors to desiccation.

The impact of the Messinian salinity crisis on the petroleum system of the Eastern Mediterranean: a critical assessment using 2D petroleum system modelling

“…Messinian times. The SWIT is expected to have been much higher during the drawdown 536 than present due to the overall Messinian arid climate (Hsü, 1972;Ryan, 2009 we therefore elected to use a Messinian SWIT of 40°C in our model. 539…”

“…SU7 is a distinct seismic package identified throughout 414 the basin, composed of a thick evaporitic series of Late Miocene age (Hsü, 1972;Neev, 415 1975;Ryan, 1978Ryan, ,2009 Robertson (1998b) to contain extensive solution porosity. This is attributed to flushing by 422 meteoric waters during the MSC suggesting that the Eratosthenes Seamount was exposed 423 sub-aerially (Robertson, 1998c The lower boundary of SU8 is a composite unconformity.…”

The Messinian salinity crisis: open problems and possible implications for Mediterranean petroleum systems

“…This model has undergone some modifications through time, all implying that, at a certain point, the Mediterranean desiccated almost completely and its slopes underwent a phase of subaerial exposure and vigorous erosion related to the rejuvenation of an entire fluvial drainage system (Lofi et al 2005;Ryan 2009;Bache et al 2012). This phase of generalized exposure would have led to the formation of an erosional surface (the Messinian erosional surface -MES), which is one of the main stratigraphic features in both onshore and offshore records, and a key one for their correlation.…”

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

“…The same effect may be found in other seas and oceans. Low-salinity water derived from NGH may also be a mechanism for enhanced fluid flow that sustains benthic communities (Mayer et al, 1988;Ryan, 2009;Carey et al, 2014), and bubble-mediated transport within plumes that provides a mechanism for deep sea communities to reach and interact with pelagic communities (Schmale et al, 2015). Monitoring and sampling of deep-sea fluid and gas emission sites should resolve our model for a dynamic source for deep-water mixing.…”