We’re getting really pretty close to the present, geologically speaking, and most of the rest of the episodes will focus on the most recent 10 million years of earth history. If our calendar was at a proper scale, these last 10 million years would have to be squeezed into the last 18 hours of December 31. We’ll spread it out some.
Today I have two different water-related topics to discuss, both from near the end of the Miocene epoch, about 5 or 6 million years ago. The Messinian age is the last subdivision of the Miocene Epoch, from 7.2 to 5.3 million years ago. Near the end of this time, the Mediterranean Sea dried up.
|Mediterranean Sea bathymetry map from NASA (public domain) annotated by Gibson. Heavy black lines outline deepest parts which might have remained lakes or brackish areas when the rest of the Mediterranean was dry land.|
With the Mediterranean cut off from oceanic circulation, evaporation could have become significant. The climate generally was not especially hot – in fact the planet was approaching the glacial period. But it’s been suggested that the evaporation took the lead because precipitation decreased, mostly due to orbital cycles that affected rainfall. The argument is a little bit circular, as it calls on reduced solar energy as a cause of lower evaporation in the Atlantic, and therefore less rainfall in Europe, and therefore more evaporation in the Mediterranean. There is no clear consensus on the role of climate changes in this event.
But it definitely did happen. Vast deposits of salt and gypsum dating to the Messinian age – named for Messina, Sicily, where such deposits are found – show it clearly. Drilling in the modern sea has also revealed evaporites in the deep-sea sediments as well. In places, the salts are interbedded with typical marine sediments including foraminiferal oozes, indicating that there were repeated times of drying and return of oceanic conditions.
A completely different line of evidence for the Mediterranean being dry at this time is submarine canyons, which must have been cut by rivers flowing across dry land, but which are now far below sea level. At the end of the Messinian, the end of the Miocene Epoch, the Mediterranean basin filled for the last time. The dam at Gibraltar was breached. There is debate as to whether the refilling was a gentle process or catastrophic – it might have taken place over many hundreds or thousands of years, although the image of a gigantic waterfall half a mile high with the water of 1,000 Amazon Rivers gets your attention. The only evidence regarding the nature of the refilling is in structures cut into the sediments around Gibraltar, beneath younger rocks – and the information there has been interpreted both ways.
But fast or slow, the Mediterranean did refill.
|Saturated thickness of Ogallala aquifer, by Kbh3rd,|
used under Creative Commons license.
Deposition of the rocks of the Ogallala Formation, which contain the aquifer, began in very late Miocene time, about 6 million years ago, and continued through the Pliocene, until about 2 million years ago when gentle uplift changed the setting from more depositional to more erosional again. The sediments that became the Ogallala were largely the sediments that were being stripped off the Rocky Mountains to the west, in the exhumation phase of their development, which we talked about December 14.
The entire package of Ogallala Formation rocks is up to 1,500 feet or so in thickness, and the aquifer itself, in nice porous sands, has a saturated thickness as much as 1,000 feet in places, but it’s mostly 50 to 400 feet of saturation. Most of the Ogallala formation was deposited by rivers, similar to the rivers of the High Plains today, but some parts of it are wind-blown silts and sands as well.
I was involved in a study of part of the Ogallala in the Texas Panhandle about 20 years ago, for an environmental project. The unit is really highly varied, and contains things like impermeable clay beds that can produce little perched aquifers, like bowls of water sitting above the main unconfined aquifer further down.
Given its importance to the region, it’s no surprise that a lot of attention is being paid to the drawdown caused by intense use of the groundwater in the Ogallala. In places, the drawdown is more than 300 feet – definitely a problem, because recharge to the aquifer, replenishing the water that is removed, comes almost entirely from precipitation, and this region is pretty much arid in terms of climate.
—Richard I. Gibson
Saturated thickness of Ogallala aquifer, by Kbh3rd, used under Creative Commons license.
Mediterranean Sea bathymetry map from NASA (public domain) annotated by Gibson.