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The mystery of Mediterranean low-salt gypsum deposits

Around six million years ago, at the end of the Miocene, the Mediterranean Sea underwent a period of rapid drying, gradually turning into a saline giant. This Messinian salinity crisis, which lasted 630,000 years, was the result of the gradual closure of the Strait of Gibraltar, linking the Mediterranean basin and the Atlantic Ocean, following movements in the earth's crust. However, the geological, climatic and biological conditions that led to the formation of this Mediterranean Saliferous Giant, which contains over one million km3 of evaporites, remain poorly understood.

The mystery of Mediterranean low-salt gypsum deposits

Publication date: 16/06/2022

Press, Research

Related teams :
Stable Isotope Geochemistry

Related themes : Earth System Science

Gypsum, one of the evaporitic minerals formed during this extreme geological episode, is a hydrated salt made up of calcium, sulfate and two water molecules. Studying the isotopic composition of the latter is particularly interesting for understanding the hydrological conditions under which gypsum was formed. What’s more, the tiny water droplets, known as fluid inclusions, that were captured inside this mineral when it precipitated are veritable water archives.

 

 

Very low salinity raises questions

While gypsum is usually formed by evaporation from extremely salty waters (around 110 g/kg of water), the study of the fluid inclusions and isotopic composition of Messinian gypsum reveals that it was formed from waters with salinities similar to those of present-day seawater (on average 35 g/kg). The formation of this low-salt gypsum is thus one of the many enigmas posed by the Mediterranean Saliferous Giant.

In a new study, a team led by IPGP researchers combined isotopic measurements of gypsum water and fluid inclusion salinity with those of strontium and sulfate ion, elements that can be used to trace the origin of the water masses involved in gypsum formation. The scientists then modeled the evolution of these hydrological tracers during the evaporation of a mixture of seawater and river water in different proportions.

Microfossils of sulfo-oxidizing bacteria in Messinian gypsum crystals (a), (b) Transmitted-light thin sections of twinned selenite gypsum crystals (Banengo section, Piedmont basin (a) and Monte Tondo section, Vena del Gesso basin (b)). An internal stratification, given by alternating turbid and limpid laminae, is recognizable in the re-entrant angle of the two crystals. (c) Detail of (a) showing stratification in the reentrant angle of the twin. Turbid laminae (T) rich in filamentous microfossils alternate with clearer laminae (L) in which the latter are rare. (d) Thin UV-light lamina of filaments (white arrows) from the Vena del Gesso basin (Monte Tondo section). The strong autofluorescence of the filament and the tiny opaque grains of iron sulfide correspond to the product of the diagenetic transformation of the original sulfur globules produced by sulfo-oxidizing bacteria. (e) UV-light thin section of an organic matter flake trapped in selenite gypsum crystals from the Vena del Gesso basin (Monte Tondo section). (© Aloisi et al., 2022)
Succession of biogeochemical processes responsible for the formation of "low-salt gypsum". The key bacterial process, the formation of sulfate (SO42-) by oxidation of elemental sulfur (S°), is catalyzed by sulfo-oxidizing bacteria in the maximum precession, arid season, during vertical mixing events in the water column (Aloisi et al., 2022).

As the measured data did not verify any of the hypotheses put forward, the team proposed a new scenario: the precipitation of low-salt Messinian gypsum may have been catalyzed by the activity of sulfur-oxidizing bacteria. Indeed, bacterial fossils of this type have been observed in all gypsum deposits characterized by these low salinities. This biological catalysis would have led to the production of sulfate, a process capable of promoting the formation of gypsum without increasing the concentration of ions – notably chloride, sodium, magnesium and potassium – which are the main contributors to salinity.

If confirmed, this biological scenario would extend the range of environments that favor the deposition of marine gypsum, and would also imply the existence of an additional biological coupling between the calcium, sulfur and carbon cycles.

 

Ref : Aloisi G., Guibourdenche L., Natalicchio M., Caruso A., Haffert L., El Kilany A., Dela Pierre F., The geochemical riddle of “low-salinity gypsum” deposits, Geochimica et Cosmochimica Acta, Vol. 327, 2022, P. 247-275, DOI: 10.1016/j.gca.2022.03.033

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