Crystal traps: the late Miocene Mediterranean evaporites and their hidden diatom content

by Luca Pellegrino |

When looking around for ancient sedimentary archives containing diatoms and associated microfossils, we immediately think about diatomaceous earth and related rocks (Zahajská et al., 2020). Hardly our attention is captured by the so-called ‘evaporites’, i.e. sedimentary rocks containing minerals that nowadays precipitate in salterns and sabkhas, like gypsum (CaSO4·2H2O) and halite (NaCl). If we are searching for traces of life, why studying rocks formed in hypersaline environments, inhospitable for the most majority of life forms? For a long time, this (wrong) assumption limited the efforts aimed at revealing the potential paleobiological content of the evaporites (Gibson & Benison, 2023). Actually, a noteworthy property of the evaporitic minerals is their very fast growth rate (up to tens of µm/days), which makes them ideal for the search of biosignatures. In other words, they can rapidly entrap the biogenic material possibly present in the sedimentary environment, sealing it in a sort of sarcophagus for a very long time (Schreder-Gomes et al., 2022). This explains why evaporites are primary targets for the search of life on Mars (Benison & Karmanocky III, 2014; McMahon et al., 2018) and should be investigated in much more detail on Earth by diatomists!

Several evaporitic events occurred during Earth’s history, leading to the formation of the so-called Salt Giants (Warren, 2016). Around 6 Ma (Messinian, late Miocene) the Mediterranean basin underwent a severe restriction due to the tectonic uplift of the area that nowadays approximately corresponds to the Gibraltar Arc. Strong reduction of the Atlantic inflow compromised the hydrologic balance (water input vs water evaporation), rapidly transforming the Mediterranean in a huge saltern: the youngest Salt Giant on Earth was born. In less than 650.000 years, more than 1 million km3 of salts deposited. According to the initial interpretations, the Mediterranean desiccated, with dramatic consequences on the Mediterranean marine biota, that recovered from this environmental perturbation only with the re-establishment of the Atlantic connection around 5.3 Ma (Hsu et al., 1973). Here you can find a virtual reconstruction of these catastrophic events: https://www.youtube.com/watch?v=IezMBKwZvHY. However, due to the lack of modern analogues, the scientific debate on the causes and consequences of the so-called Messinian salinity crisis is still open (Carnevale et al., 2019; Agiadi et al., 2024; Krijgsman et al., 2024; Roveri et al., 2025).

Impressive witnesses of the so-called Messinian salinity crisis are preserved offshore (below the seafloor) as well as onshore, in the Mediterranean landscape. Locally, entire reliefs are made up of thick gypsum beds, like in the case of the Vena del Gesso ridge (Fig. 1).

Figure 1. A) Distribution of Messinian evaporites in the Western Mediterranean. Star indicates the outcropping area of the Vena del Gesso ridge. B) The Vena del Gesso ridge (Northern Apennines, Italy). From: Pellegrino et al. (2024) – Geobiology.

Even a simple lens allows to appreciate a very peculiar component of the microscopic world ‘freezed’ in Messinian gypsum: an intricate network of spaghetti-like, mm-long hollow filamentous structures (Fig. 2) that have been recently interpreted as fossilized remains of giant sulfide-oxidizing bacteria (Dela Pierre et al., 2015; Natalicchio et al., 2021).

Figure 2. Bacterial filaments preserved in gypsum. From: Pellegrino et al. (2024) – Geobiology.

Increasing the magnification, diatoms pop up (Fig. 3): with a few exceptions, they mostly consist of very small (<20 µm) specimens, occurring clustered or isolated. They comprise centric and pennate, planktonic and benthic taxa: the most common are Chaetoceros vegetative cells, rhizosolenids, thalassiosiroids, naviculoids and nitzschioids, but also extraordinarily well-preserved specimens of the genus Lampriscus occur, as well as others (e.g., Rhabdonema, Surirella). Far from being exhaustive, this list comprises diatom genera (e.g., Chaetoceros, Thalassiosira, Rhizosolenia) which are mostly marine and do not thrive in modern hypersaline environments like salterns (Pellegrino et al., 2021). Along with other evidences, this raises important questions about the salinity of waters from which gypsum precipitated during the Messinian (Natalicchio et al., 2014; Aloisi et al., 2022).

Figure 3. Dense cluster of nano-sized thalassiosiroids.

In some cases, the degree of preservation is surprising: even photosynthetic pigments and putative cell ultrastructures can still be found associated with diatom frustules (Fig.4). On the other hand, examining the residue deriving from the dissolution of gypsum crystals with a scanning electron microscope, also strongly altered diatom remains can be recorded, along with minerals like dolomite (CaMg(CO3)2) and pyrite (FeS2) deriving from the anaerobic degradation of organic matter by sulfate reducing bacteria. Such contrasting preservation degrees indicate that gypsum growth rate was not uniform, much likely in response to seasonal hydrologic changes: it ranged from extremely rapid during arid phases to very slow or even interrupted during more humid periods, in this case exposing diatom organic matter and biogenic silica to degradative processes, much likely bacterially-mediated (Pellegrino et al., 2024). Intriguingly, independent geochemical evidences demonstrated an active sulfur biogeochemical cycle during the Messinian salinity crisis, fueled by a significant benthic flux of organic carbon (Guibourdenche et al., 2022). What if our favorite primary producers played a pivotal role in this?

Figure 4. Extraordinary preservation of diatoms in Messinian gypsum: (A) Naviculoid diatom bearing autofluorescent globules (lipid droplets? shrunken chloroplasts?), observed with confocal laser scanning microscope. (B) Spectral analysis highlights the preservation of photosynthetic pigments. From: Pellegrino et al. (2024) – Geobiology.

Further interdisciplinary efforts are needed in order to properly address the paleoenvironmental meaning of diatoms preserved in these crystal traps, as well as for understanding their biogeochemical role during the most dramatic environmental change experienced by the Mediterranean basin.

 

Luca is a Research Fellow at the Department of Earth Sciences of the University of Torino (Italy). His main research interests focus on sedimentology, diagenesis, paleobiological content and paleoceanographic meaning of siliceous and evaporitic deposits, especially those formed during the Neogene in the Mediterranean region.

 

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