How I got into diatoms: Luca Pellegrino

Luca Pellegrino – Dipartimento di Scienze della Terra (Università degli Studi di Torino)

I am an Italian PhD student in the Earth Sciences Department of the University of Torino (Italy). My researches concern the Upper Miocene diatomaceous deposits of the Mediterranean region, with special emphasis on the Italian sedimentary successions deposited during the late Tortonian-early Messinian, i.e. around 7-6 Ma. My aim is to better understand the paleoceanographic processes responsible for their deposition, mostly through SEM-based sedimentological and micropaleontological investigations. 

My interest in this topic arose some years ago, after a discussion with my supervisor, who hinted about the impressive accumulation of diatom-rich sediments globally characterizing the latest Miocene and attributable to the so called ‘latest Miocene biogenic bloom’ (e.g., Farrell et al., 1995; Cortese et al., 2004). The circum-Mediterranean Upper Miocene diatomites are special if compared to many other coeval diatom-bearing deposits, because their sedimentation preceded one of the most studied and debated paleoenvironmental changes affecting the Mediterranean basin during its geodynamic evolution, the well-known Messinian Salinity Crisis (hereafter MSC; e.g., Krijgsman et al., 1999). During this event, lasting from 5.97 to 5.33 Ma, the Mediterranean became a sort of giant salina, and more than 1,000,000 km³ of evaporites (gypsum and halite) were deposited. The MSC was originally considered to be the result of a complete desiccation of the Mediterranean due to the tectonically-driven restriction of the Atlantic gateways and the consequent establishment of a negative hydrologic budget, since according to the models, without the Atlantic contribution, river runoff and precipitation could not compensate for the evaporation of the Mediterranean waters. 

However, an increasing amount of paleontological and geochemical evidences seem to suggest a more complex scenario, where the marine biosphere probably played a fundamental role (for example, in controlling the biogeochemical sulfur cycle), and was not just a passive witness or rather a ‘victim’ of the catastrophic evaporation of the Mediterranean water column (e.g., Carnevale et al., 2019). Therefore, since many Upper Miocene diatomaceous successions outcropping in the Mediterranean region stratigraphically underlie the evaporitic units, a closer look at the diatomites may help to decipher the paleoceanographic dynamics that occurred immediately before the onset of the MSC. In other words, diatomites represent an excellent opportunity to better address the possible triggers heralding this dramatic environmental crisis. Moreover, the diatomites are full of macro- (e.g., excellently preserved fishes) and microfossils (not only diatoms!), representing a sort of coffer of the Mediterranean paleobiodiversity, that is far from being completely explored!

On the left, a simplified stratigraphic log describing the typical architecture of the pre-evaporitic Messinian successions in Sicily. Black color corresponds to organic-rich shales or sapropels; white to diatomites; gray to marls. Overall, the succession is 30-m thick. This cyclic lithological alternation reflects the high sensitivity of a semi-enclosed basin to the periodical variation of the insolation patterns governing the precipitation rates on a millennial-scale and ultimately controlled by the Earth’s orbital parameters (see Hilgen and Krijgsman, 1999). On the right, the spectacular alternation of pale and dark, intensively folded and faulted layers, corresponding to diatomites and organic-rich shales, respectively, belonging to the Upper Miocene Beida Stage of Algeria (Anderson, 1933). References ·         Anderson, R.v.V., 1933. The diatomaceous and fish-bearing Beida stage of Algeria. The Journal of Geology 41 (7), 673–698.  ·         Carnevale, G., Gennari, R., Lozar, F., Natalicchio, M., Pellegrino, L., Dela Pierre, F., 2019. Living in a deep desiccated Mediterranean Sea: An overview of the Italian fossil record of the Messinian salinity crisis. Bollettino della Società Paleontologica Italiana 58 (1), 109-140. ·         Cortese, G., Gersonde, R., Hillenbrand, C.D., Kuhn, G., 2004. Opal sedimentation shifts in the World Ocean over the last 15 Myr. Earth and Planetary Science Letters 224, 509-527. ·         Farrell, J.W., Raffi, I., Janecek, T.R., Murray, D.W., Levitan, M., Dadey, K.A., Emeis, K.-C., Lyle, M., Flores, J.-A., Hovan, S., 1995a. Late Neogene sedimentation patterns in the eastern equatorial Pacific Ocean. Proc. ODP, Sci. Results 138, 717– 756. ·    Krijgsman, W., Hilgen, F.J., Raffi, I., Sierro, F.J., Wilson, D.S., 1999. Chronology, causes and progression of the Messinian salinity crisis. Nature 400, 652-655.