Climate: A Very Short Introduction (Very Short Introductions) by Mark Maslin

Author:Mark Maslin [Maslin, Mark]
Language: eng
Format: epub, azw3, mobi
Publisher: Oxford University Press, USA
Published: 2013-06-26T16:00:00+00:00

The great salt crisis

About 6 million years ago the gradual tectonic changes resulted in the closure of the Strait of Gibraltar. This led to the transient isolation of the Mediterranean Sea from the Atlantic Ocean. During this isolation the Mediterranean Sea dried out several times, creating vast evaporite (salt) deposits. Just image a huge version of the Dead Sea where a few metres of seawater cover a vast area. This event is called the Messinian Salinity Crisis and it was a global climate event because nearly 6 per cent of all dissolved salts in the world’s oceans were removed. By 5.5 million years ago the Mediterranean Sea was completely isolated and was a salt desert (Figure 31). This was roughly the same time as palaeoclimate records indicate that the Northern Hemisphere was starting to glaciate. But at about 5.3 million years ago the Strait of Gibraltar reopened, causing the Terminal Messinian Flood, also known as the Zanclean Flood or Zanclean Deluge. Scientists have envisaged an immense waterfall higher than today’s Angel Falls in Venezuela (979 m), and far more powerful than either the Iguazu Falls on the boundary between Argentina and Brazil or the Niagara Falls on the boundary between Canada and the USA. More recent studies of the underground structures at the Gibraltar Strait show that the flooding channel may have descended in a rather more gradual way to the dry Mediterranean. The flood could have occurred over months or a couple of years, but it meant that large quantities of dissolved salt were pumped back into the world’s oceans via the Mediterranean–Atlantic gateway. This stopped the Great Ice Age in its tracks, and was entirely due to how oceans circulate. As we saw in Chapter 2 the Gulf Stream not only keeps Europe warm but also drives the deep-ocean circulation and keeps the whole planet relatively warm. Five million years ago the deep-ocean circulation was not as strong as it is today. This is because fresher Pacific Ocean water was still able to leak through the Panama ocean gateway which is discussed below. So the sudden massive increase in salt due to the Terminal Messinian Flood increased the salt in the North Atlantic Ocean ensuring a very vigorous Gulf Stream and sinking water in the Nordic Seas. With all this tropical heat being efficiently pumped northwards the slide into any further great ice ages was halted about 5 million years ago. We had to wait another 2.5 million years before the global climate was ready to try again.

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