Geo-Joint: The Zanclean Flood

Perhaps you have stood on a beach at a lagoon mouth and watched what happens when a stormy sea breaks through the sand berm that had sealed off the lagoon. It starts as a small stream, but soon turns into a rapid torrent as the sand wears away, and the higher lagoon water heads downhill to the sea. Something like that happened in reverse about 5.33 million years ago, but I’m getting ahead of myself.

Many millions of years before that flow, the continents of the Earth were breaking out of the singular landmass of Pangaea and crawling all over the place. The also once-singular ocean, Panthalassa, got broken up into various basins and channels, including the Tethys Sea, roughly between the future Africa and Eurasia. As the landmasses moved to resemble the configurations we recognize today, they trapped an arm of the ocean north of Africa that became the deep basin of the Mediterranean Sea. As you know, the Mediterranean has a rather tight pinch point where it transitions into the Atlantic Ocean at the Strait of Gibraltar between Spain and Morocco. If you’ve been reading the Joint regularly, you’ll recall that Gibraltar is a generally prosperous and quiet place these days, but it wasn’t always so.

Thanks to the uplift caused by the northward movement of Africa, that narrow strait was fairly easily closed off about 5.6 million years ago, slowly cutting the Mediterranean Basin off from the outside world of the Atlantic. As the connection narrowed, the ocean feed and inflowing rivers couldn’t keep up with the powerful process of evaporation in that warm climate, and salt began to precipitate on the seafloor. Over the course of about a thousand years after complete cutoff, the Mediterranean shrank dramatically to perhaps 25% of its former extent and to where its surface was somewhere between 5,000 and 8,800 feet below sea level. Salinity from this long process of ever-dimininshing input was so high that salt deposits beneath the floor of the Mediterranean are in some places well over a half-mile thick.

The Earth never stops moving, however, and either tectonic subsidence or a rise in sea level caused the Atlantic to overtop the barrier that had held it back. Known as the Zanclean flood, the incoming Atlantic Ocean waters entered not as a waterfall into a giant basin, but more as flood down a huge ramp, similar to those lagoon waters mentioned earlier. While it might have started slowly, and some have estimated it took centuries to totally re-fill, newer research says that as much as 90% of the process took place in as little as a few months to a couple of years. At times, the volume of flowing water is thought to have reached that of a thousand Amazon rivers. Hydrologists estimate this by the evidence left in the deep and lengthy erosional channel left below the strait and into the Alboran Sea, a portion of the Mediterranean, south of Spain. Drilling cores show that this gully has been filled by later sediment deposit, but while the flow was raging, it cut nearly 16 inches a day into solid bedrock. The water doing this work flowed at up to 185 miles per hour at its maximum, raising the level of the Mediterranean over 30 feet a day.

As catastrophic as was the erosion done to soil and rock torn by the rushing sea, the ecological upending must also have been fantastic. The thousand years of of dessication that shrank the Mediterranean established entirely new environments that were suddenly underwater again, and eventually home once more to marine species long absent. Weather patterns no doubt changed as the dry areas quickly changed to a vast sea. It is even thought that the very speed of Earth’s rotation may have been affected by the huge and rapid shift in weight. The added weight itself caused the crust to depress into the mantle, altering the tectonic balance. It was no small thing. Nature adjusts, given enough time, to almost anything. And while the Zanclean flood obviously wrought massive upheaval, drill coring in the Mediterranean tells us it was only the latest in almost 70 cycles of plugging, breaching, and re-filling of the basin. It may well happen again long after we’re gone.

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