IDS Early Career Researcher, best oral presentation
by Luca Pellegrino, University of Torino, Department of Earth Sciences |
Diagenesis of biogenic silica during the Messinian salinity crisis: an overlooked process with significant implications
The late Miocene (11.63-5.33 Ma) was globally characterized by remarkable geodynamic events associated with significant changes in climate and ecosystems: Himalayas and Andean chain uplifted, monsoonal precipitations intensified, carbon dioxide concentration and temperature dropped, grasslands and deserts expanded, humans split from chimpanzees, oceanic circulation strengthened, some marine mammals attained giant body sizes and, last but not least, there was a significant increase in the diatomaceous deposition.
While these events were occurring around the world, the Mediterranean basin experienced a dramatic transformation, from semi-enclosed basin connected to the ocean to giant salina: due to the tectonic restriction of the Atlantic gateways during the Messinian (~7.24-5.33 Ma), the Mediterranean became increasingly sensitive to the climatic forcing of the hydrologic cycle, with the periodical establishment of bottom water anoxic conditions. The severe reduction of the Atlantic inflow led to the accumulation of a huge volume (>1×106 km3) of evaporitic rocks, mostly containing gypsum (CaSO4·2H2O) and halite (NaCl), in a very narrow stratigraphic interval (~5.97-5.33 Ma), during the so-called Messinian salinity crisis. Since the discovery of the Mediterranean Salt Giant in the 1970s, the scientific community was involved in an exciting debate, concerning especially the magnitude of water column fluctuations (did the Mediterranean completely desiccate or not?) and the possible impact of the salinity crisis on marine eukaryotes (were they completely annihilated or not?).
But what do diatoms have to do with this story? Intriguingly, before the onset of the Messinian salinity crisis, a significant increase of diatom-bearing sediment deposition occurred in the Mediterranean. This “opal burst” lasted from ~7 to 6 Ma, and involved all the main peripheral sectors of the Mediterranean basin. After 6 Ma, with the onset of the salinity crisis, the occurrence of diatomaceous sediments in the Mediterranean drastically dropped. Instead of diatomaceous and other biogenic sediments, the large majority of the Messinian sedimentary successions deposited after 6 Ma are characterized by gypsum-rich beds alternated with shales enriched in organic matter, dolomite and clays. Such a decrease in diatomaceous accumulation may be interpreted in two ways: i) a diminished productivity of diatoms in the Mediterranean waters, possibly related to the establishment of hypersaline conditions; ii) the scarce preservation of biogenic silica in the sedimentary record. Up to now, the first hypothesis prevailed: in this perspective, the Messinian salinity crisis was a “point of no return” that, once reached, hampered the persistence of the vast majority of marine eukaryotic life, diatoms included. Nevertheless, few efforts have been made in order to test the alternative hypothesis, i.e. the possible effect of diagenesis (i.e. all those physical, chemical and biological processes affecting sediments after their deposition) in altering the former biogenic signal preserved in the Messinian deposits.
The aim of the research that I carried out with an international team of researchers from Italy (Francesco Dela Pierre, Marcello Natalicchio, Giorgio Carnevale, Linda Pastero and Nicolò Zanellato – University of Torino), Germany (Jörn Peckman and Daniel Birgel – University of Hamburg) and Japan (Richard Jordan – Yamagata University) was to deepen the possible role of early diagenetic processes in modifying the preservation of biogenic silica in sediments formed during the first stage of the Messinian salinity crisis, between 5.97 and 5.60 Ma, at the northernmost offshoot of the Mediterranean region.
By means of a combined approach comprising petrographic observations of polished thin sections and freshly broken sediment chips using light and electron microscopes, and conducting elemental, mineralogical and geochemical analyses, we compared the preservation of biogenic silica in different lithologies (diatomaceous shales, diatom-bearing mudstones and dolomitic mudstones). The results indicate the fundamental role of sulfate reducing bacteria in consuming the organic matter associated with sediments originally enriched in diatomaceous remains and deposited under anoxic conditions. Degrading organic matter, these bacteria increased the alkalinity and pH of sediment pore waters, favoring at the same time the precipitation of dolomite (CaMg(CO3)2) and the alteration of biogenic silica. The consequent build-up of dissolved silica, in combination with pore water cations, led to the formation of the authigenic clays (i.e., clays formed in-situ and not transported by runoff into the basin) abundantly recorded in the dolomitic mudstones. Such clays represent therefore the overlooked by-product of the interaction between the organic matter continuously supplied by diatoms during the Messinian salinity crisis and the bacterial communities that thrived on the Mediterranean anoxic seafloor. If one considers the high abundance of clay-rich dolomitic sediments in the sedimentary successions deposited during the Messinian salinity crisis, this might suggest that the reduction of the diatomaceous accumulation in the Mediterranean after 6 Ma was a diagenetic artifact. If so, when thinking about the Mediterranean during the Messinian salinity crisis, we must imagine a more complex scenario than a “desert” where marine eukaryotic life was completely annihilated. In this scenario, the close interaction between the deep biosphere and diatoms (as well as other primary producers), may have played a fundamental role in forming rocks that are only apparently devoid of the remains of life…
For more details
Dr. Luca Pellegrino is a sedimentologist, you can read more about him on the Young ISDR site (published in October). For more on this subject, please read:
Pellegrino et al. (2023) – From biogenic silica and organic matter to authigenic clays and dolomite: insights from Messinian (upper Miocene) sediments of the Northern Mediterranean. Sedimentology 70, 505-537.