Athina Tzevahirtzian – Universitá degli Studi di Palermo (Italy)

Scala dei Turchi or “Stairs of the Turks” is one of the most popular Mediterranean destinations located on the southern coast of Sicily, in the area of Capo Rossello. Its attraction grew through the legend of these unusual staircases, also revealed by its name, referring to the Moorish and Turk pirates who used to attack the island and found shelter in this bay during the XVI century; but also due to the enormous popularity gained with the Inspector Montalbano novels of Andrea Camilleri, whose stories are set in these places. Scala dei Turchi’s turquoise waters and its peculiar white rock formation have drawn the attention of millions of travellers. However, these cliffs are more than one simple formation and have kept the interest of many geologists. They hide a long and ancient story that permits us to trace our planet’s paleoclimate and paleoenvironment.

So how can we read between the lines of a cliff?

Rocks are not soulless and rock layers may not be chaotic as they seem…“Stratigraphy” is an important field in geology that studies the nature, distribution, and relations of the stratified sedimentary or igneous rocks called “strata” (in other words of the different rock layers) of the Earth’s crust. The “father of stratigraphy” is a Danish geologist, Nicholas Steno, who in 1669 pioneered the stratigraphic Steno’s laws.

Through stratigraphic analyses, geologists are provided with valuable clues about the age of the Earth and its chronological history, deciphered by careful study of the strata (“lithostratigraphy”) and also fossils found within the strata (“biostratigraphy”). Over the past 25 years, the science of stratigraphy has evolved to include time-correlative data from vastly disparate components of the Earth system. The Earth’s stratigraphic record of astronomically forced climate change originates from variations in the Earth’s orientation with respect to the Sun imposed by gravitational attraction from the Moon, Sun and other planets. The coupling of these variations with the incoming solar radiation, known as insolation, is embodied in the Milankovitch theory of climate. In the study of Milankovitch cycles, stratigraphy is used to test whether sedimentary cycles are related to astronomical climate forcing by precession (21 kyr), obliquity (41 kyr) and eccentricity (100-400 kyr). Thus, astronomically forced paleoclimatic variations influence climate-sensitive sedimentation and can be exploited as a geochronometer, it is what we call “cyclostratigraphy”. Thus, stratigraphy enables us to understand and explain various environmental effects and how they shaped our present, by taking into account internal and external forcings.


Milankovitch cycles: a) Eccentricity (100-400 kyr)  b) Obliquity (41 kyr) c) Precession (21 kyr).  Source: NASA


What can these rocks tell us? What is the story hidden behind the strata of these cliffs?

The Scala dei Turchi outcrop, geologically known as “Punta di Maiata”, forms a cliff that goes back to the time of the Messinian and the Zanclean ages; that is to say around 5.3 Ma (million years ago). More exactly, this cliff is witnessing the latest stage of the Messinian Salinity Crisis, represented by the Arenazzolo (continental silty-clays) at the contact with the Trubi Formation (limestones deposited in deep marine environments). The basic sedimentary cycles of the Trubi marls are precession controlled and quadripartite, showing a grey-white-beige-white colour alternation and two carbonate minima per cycle (grey and beige).  


The lithological cycles of the Trubi Formation in Punta di Maiata, controlled by the precession of the equinoxes (21 kyr).  Legend: be: beige; b: white; g: grey. ©Antonio Caruso


The whole succession of Punta di Maiata is characterized by this quadruplet of calcareous and marly limestones of the Zanclean age, which reflect a different content of calcium carbonate and clay minerals, and testifies the return to normal marine condition after the Messinian Salinity Crisis. Calcium carbonate is essentially linked to the abundance of shells of unicellular microfossils (foraminifers and coccolithophores; for more information read “Foraminifera: What are they and why are we studying them” from Francesca Bulian). The grey levels are rich in microfossils typical of sub-tropical warm waters and clay minerals, among them some illite originating mainly from Northern Europe. Finally, the beige levels are richer in organisms of more temperate waters and palygorskite, a clayey mineral from the Saharan region.

The Trubi lithological variation is a cyclic sequence repeating 96 times throughout a thickness of 120 m in Punta Maiata, and is due to mineralogical changes triggered by precessional cycles (21 kyr) that included paleoclimatic and paleoceanographic variations in the Mediterranean Sea. During the insolation maxima of the precessional cycles, the temperatures were higher and winds prevailed from Northern Europe, while during insolation minima, the temperatures were cooler and winds prevailed from the south (monsoons) transporting clay minerals from the Saharan region. Besides, variations in the thickness of the beige layers were induced by obliquity Earth axis (41 kyr). Finally, larger-scale eccentricity (100 kyr) related cycles are clearly visible in the weathering-profile of the cape. In fact, the layers richer in carbonate content were linked to phases of eccentricity minima.

The Trubi formation along the beach rests in discordance on the silty and sandy clays of the Arenazzolo (Messinian age), still well visible a few years ago. Finally, in the upper part of the cliffs, the succession covers also a part of the younger Piacenzian age, that is to say 3.60-2.588 Ma.


Visible Messinian/Zanclean boundary in Punta di Maiata: discordance of the Trubi formation (Zanclean age) on the silty and sandy clays of the Arenazzolo (Messinian age).  ©Antonio Caruso.


So if you decide to visit Scala dei Turchi, don’t forget that these impressive stairs are more than a simple summer “insolation” destination…they allow you to travel back in time some 4 million years, and every step you make permits you to traverse 21 thousand years!

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References:

Caruso A. 2016. A field trip along the Caltanissetta basin: The evolution from open marine conditions to the Messinian Salinity Crisis and the Zanclean reflooding, Program – AMGG Field Trip.

Hansen J.M. 2005. Steno in: Famous Geologists, Danish Research Agency, Copenhagen, Denmark

Hilgen, F. J. 1991. Extension of the astronomically calibrated (polarity) time scale to the Miocene/Pliocene boundary, Earth Planet. Sci. Lett., 107, 349–368.

Hilgen F.J. & Langereis C.G. 1988. The age of the Miocene-Pliocene boundary in the Capo Rossello area (Sicily), Earth and Planetary Science Letters, 91,214-222.

Hilgen, F. J., and C. G. Langereis. 1989. Periodicities of CaCO3 cycles in the Pliocene of Sicily: Discrepancies with the quasi-periods of the Earth’s orbital cycles?, Terra Nova, 1, 409 – 415.

Hinnov L.A. 2013. Cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences, Geological Society of America Bulletin 2013, 125, no. 11-12; 1703-1734, doi:10.1130/B30934.1

Hinnov L.A. 2018. Cyclostratigraphy and Astrochronology in 2018, Chapter 1 in: Stratigraphy & Timescales, Volume 3, https://doi.org/10.1016/bs.sats.2018.08.004

Lourens, L. J., A. Antonarakou, F. J. Hilgen, A. A. M. Van Hoof, C. Vergnaud-Grazzini, and W. J. Zachariasse. 1996. Evaluation of the Plio-Pleistocene astronomical timescale, Paleoceanography, 11, 391–413.

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