Author: Laetitia Guibourdenche (IPGP, Paris)
« I was engaged a few years ago in a course of experiments on hydrogen gas, which was procured in the usual method, by the solution of iron turnings in diluted sulphuric acid. The sulphate of iron hence resulting, (…) remained undisturbed, and unnoticed for about a twelve month. At the end of this time, the vessel being wanted, I was about to throw away the liquor, when my attention was excited by an oily appearance on its surface, together with a yellowish powder, and a quantity of small hairs (…) there was discovered at the bottom of the vessel a sediment consisting of the bones of several mice, of small grains of pyrites, of sulphur, of crystallized green sulphate of iron, and of black muddy oxyd of iron. »
Pepys, W.H., 1811. XVIII. Notice respecting the decomposition of sulphate of iron by animal matter. Trans. Geol. Soc. Lond. 1, 399–400. Series 1.
What was the feeling of William H. Pepys, when he discovered a pyritized mouse in a jar of ferrous sulfate ? Did he already know that pyrite and « deoxygenated sulfate » formation through « animal matter » reaction with sulfate naturally occurs as one of the main regulators of Earth’s surface chemistry ? Little is known about what he did of this exciting discovery but nowadays we understand better how this can happen.
In the modern oceans, organic carbon is mineralized to CO2 by oxygen consumption : this phenomenon is known as aerobic respiration.
CH3COO– + 2O2 → H2O + 2CO2 + OH–
But as you go deeper and closer to the to the sediment-water interface, oxygen concentration decreases progressively thanks to aerobic respiration, then, other compounds will start to be implied in the oxidation of organic matter. Such alternate metabolic pathways as nitrate reduction, manganese reduction, iron reduction, sulfate reduction and methanogenesis depend on oxygen concentration, from oxic to suboxic and, ultimately, anoxic.
As can be seen from the scheme proposed by Tostevin et al 2019 , sulfate reduction is -energetically speaking- not the most favorable reaction to oxidize organic matter. However, sulfate is the second most abundant anion in seawater, and therefore is highly available to react with organic matter. In fact, about 50% of the organic matter flux to the seafloor is oxidized through sulfate reduction. This widespread occurrence of hydrogen sulfide (H2S) production close to the sea floor is possible thanks to a huge and still poorly known family of microbes : microbial sulfate reducers .
CH3COO– + SO42- → 2HCO3– + HS–
These microbes reduce sulfate (SO42-) to hydrogen sulfide (H2S), a toxic gas for human health and most of the organisms living near the ocean surface. The permanent degassing of hydrogen sulfide diffusing upward into the water column would dangerously threaten the aerobic life.
Hopefully, sulfate reduction occurs most of the time in the sub-seafloor and most of the hydrogen sulfide produced by sulfate reducing microbes is neutralized. In our modern oceans, about 97% of hydrogen sulfide is reoxidised to sulfate (mostly thanks to another family of sulfur loving bacteria : sulfide oxidizer) and the rest reacts with ferric iron to form solid pyrite, like in Pepys’s lab.
This fragile balance of the sulfur and carbon cycles intertwining guarantees oxic surface oceans and the life we know in them nowadays. This tenuous equilibrium ensured mostly by microbial life still hides a lot of mysteries : metagenomic investigation of sulfidic sediments revealed that probably 99% of the microorganisms involved in theses cycles are still unknown…
Anyways, the understanding of those mechanisms are high stakes when studying the evolution of Earth surface environment, because euxinic conditions (high level of H2S and no O2) prevailed in ancient global oceans for long periods. During Precambrian times, but also during several shorter periods through the Phanerozoic, euxinia probably had a very strong influence on the evolution of life. Still, nowadays, dead zones, where oxygen is absent and where algae blooms cause emissions of hydrogen sulfide, flourish due to human activity, and cause the death of large swaths of sea life .
These desequilibria in the sulfur cycle induced, and still provoke, large accumulation of hydrogen sulfide caused by massive reduction of oceanic sulfate.
But during the period that interests us, the Messinian Salinity crisis, it is the opposite, it is the accumulation of huge amount of sulfate -and not hydrogen sulfide- that caused the massive precipitation of gypsum (Ca.SO4.2H2O). Evaporites like gypsum are generally described as rocks that precipitate from saturated solutions thanks to evaporation. So, was it the drying of the Mediterranean that allowed the precipitation of so much gypsum ?
One could consider gypsum as a « fossil » of the water from which it precipitates, and, in fact, the study of its chemistry provides some insights on the composition of the parent brine. Some recent studies [5,6] highlighted that in marginal basins, messinian gypsum have precipitated from water masses with lower salinities than modern sea-water. In that case, what brought so much sulfate to allow the precipitation of messinian gypsum ? Rivers ? Parathethys ? Microbes ? What are the mechanisms that allow precipitation of low-salinity gypsum ? What kind of sulfur cycling was ruling the Mediterranean during the Messinian ?
We need to improve our understanding of the controls on the sulfur cycle to answer these questions since no modern analogs are known…
 Tostevin R., Poulton S.W. (2019) Suboxic Sediments. In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg
 B.B. Jørgensen Mineralization of organic matter in the sea bed – the role of sulphate reduction Nature, 296 (1982), pp. 643-645, 10.1038/296643a0
 Rickard, D. (2012). Sulfidic sediments and sedimentary rocks. Newnes.
James Owen. (2010) World’s largest dead zone suffocating sea. National Geographic
 Evans, N., Turchyn, A.V., Gazquez, F., Bontognali, T.R.R., Chapman, H.J. and Hodell, D.A., 2015, Coupled measurements of d18O and dD of hydration water and salinity of fluid inclusions in gypsum from the Messinian Yesares Member, Sorbas Basin (SE Spain), Earth and Planetary Science Letters, 430, 499-510
 Natalicchio, M., Pierre, F.D., Lugli, S., Lowenstein, T.K., Feiner, S.J., Ferrando, S., Manzi, V., Roveri, M., Clari, P., 2014. Did the Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin (Italy). Geology, 42(3), 179-183.