"(...) the inhabitants have called them the Columns of Hercules; they believe that they were dug through by him; upon which the sea, which was before excluded, gained admission, and so changed the face of nature."Pliny knew very little about the formation of the Earth: he simply lacked the knowledge we have accumulated since the 18th century to understand the processes shaping landscape. However, he understood that the Earth is not static but it changes as a result of dynamic processes. And he did know the widespread salt outcrops present along most of the Mediterranean coast: The Romans used profusely the gypsum salt crystals as window glasses, for example. Today, we can only imagine what the origin of Pliny's account was, but science is giving us strong evidence that the Mediterranean was once indeed largely desiccated and that it was later refilled in a catastrophic event, just as those native Iberian peoples believed.
In 1867, paleontologist Karl Mayer-Eymar realized that those gypsum outcrops had all a similar geological age, and named that period Messinian after the impressive salt mines near the Sicilian town of Messina. In 1877, Carl Ochsenius published an influential work on the formation of the salt giants, those large deposits of salt recognized in the geological record of many regions. The period of Mediterranean-wide salt deposition was recognized in 1954 as a dramatic environmental crisis, and accordingly became known as the Messinian Salinity Crisis, after the italian geologist Raimondo Selli. Only in the last decades has been the crisis properly dated as 5.3 to 6 million years old, around the time when the first hominids walked in Africa.
|Present inflow and outflow from the Atlantic across the Gibraltar|
Strait. The outgoing current is denser and runs underneath the inflow.
Both were used by the submarines during World War II to sneak
in and out of the Med silently (Prenhall/Pearson).
|Ochsenius theory of barriers |
or thresholds (1877) to
explain the presence
of large salt deposits in the
Present water salinity at the surface of the ocean, showing a
saltier Mediterranean caused by the higher evaporation
But what happened then offshore? If the salts were related to a pan-Mediterranean event, salt should also have accumulated in the deeper parts of the sea. Only in the 1960s, studies of the reflection of seismic waves (echoes of vibrations sent from a boat) started to show ubiquitous evidence of a layer a few hundred meters below the sea floor. It was named the 'M reflector', and extended laterally up to approximately the 1500 m depth contour of the present sea, suggesting that one single process was responsible for it. Today we know that below the M reflector about 10% of the salt contained in the entire global ocean is trapped since the Messinian times.
In the 70's, 3000-m-deep drillings carried out on board the Glomar Challenger proved the presence of salt deposits offshore Mallorca, demonstrating that the classical onshore outcrops had an equivalent in the open sea. It also found anhydrites and pebbles, suggesting that the Mediterranean consisted of a series of brackish lakes, possible remnants of a desiccated Med. But these were just weak circumstantial evidence, and both could be (and were indeed) argued.
Distribution of Messinian salts in the Mediterranean
All the research published was supporting the existence of a great evaporitic basin affecting both the shallow marine basins at the margins of the Mediterranean and the deeper inner parts of the sea. But still this does not imply a desiccation or a large drawdown of the Med, as discussed above.
Section across the Nile in Aswan (Egypt) by Chumakov (1967),
based on wells. It shows a valley excavated by the
river during the MSC, now filled with later sediments.
|Valley excavated during the Messinian salinity crisis at the|
mouth of the Ebro River, as derived from recent seismic
reflection images. Scientific paper here.
[Next chapter here]
- Pliny's Historia Naturalis full text.
- More science diffusion on the MSC in this article and in David Bressan's blog.
- The scientific paper will be linked here upon publication.
- Loget, N., Van Den Driessche, J. On the origin of the Strait of Gibraltar. Sedim. Geol. 188–189, 341–356 (2006).
- Chumakov, I. S. (1973), Pliocene and Pleistocene deposits of the Nile valley in Nubia and upper Egypt, Initial Rep. Deep Sea Drill. Proj., 13, 1242-1243.
- Ryan, W. B. F., Decoding the Mediterranean salinity crisis. Sedimentology 56, 95-136 (2008). doi: 10.1111/j.1365-3091.2008.01031.x