The Tethys
Ocean separated Africa and Europe during the Jurassic and Cretaceous periods but
was mostly eliminated as a result of the collision of the continents. Elements
of this ocean survive today as the Mediterranean, Black, Caspian, and Aral Seas.
The
Mediterranean Sea maintained its connection to the Atlantic and Indo-Pacific
Oceans until early in the Miocene when it was reduced with the joining of the
two continents along the Middle East front around 14 million years ago (mya).
This joining of the two continents began a gradual change to a more arid
Mediterranean climate.The Mediterranean Sea connection to the Atlantic Ocean was maintained through various avenues (see figure below) until the closure of the Rifean Corridor in the early Messinian.
The closure of the Atlantic access precipitated a rapid environmental and climatic change driven by high evaporation rates in the Mediterranean Sea and the inability of riverine sources to replenish the water loss. This event is generally referred to as the Messinian Salinity Crisis and a rough timeline is as follows:
· 5.96 mya – Closure of the Rifean
Corridor and partial dessication of the Red Sea
· 5.8 mya – Mediterranean almost dries
out. Massive dessication leaves a deep, dry basin 3 to 5 km below sea level
with a few hyper-saline pockets
· 5.5 mya – Less dry climatic
conditions ensue resulting in more fresh water from the rivers. This fresh
water progressively fills the basins and dilutes hyper-saline pockets into larger
pockets of brackish water
· 5.33 mya – Zanclean flood. Strait of
Gibraltar opens up, quickly filling the Mediterranean with water from the
Atlantic Ocean. The end of the crisis.
As described
in Progeo Piemonte (Climate variability and past environmental changes: lessons
from the Messinian record of the Tertiary Piemonte Basin), “In less than a
million years, deep sea sediments are replaced by shallow sea deposits,
continental deposits, lacustrine sediments and eventually deep sea sediments.”
According to
Nesteroff (The Sedimentary History of the Mediterranean during the Neocene),
all of the Messinian deposits they encountered during their drilling explorations
in the region “proved to be evaporitic species comprised of dolomitic marls
interbedded with massive gypsum, anhydrite and halite.” On land, they found
that, in the same period, The Tortonian blue marls were suddenly replaced by
either evaporitic series or by lacustrine and continental deposits. The evaporitic
deposits are primarily found in the deepest part of the sea but some fragments
are found on margins that have been tectonically uplifted.
According to
Progeo Piemonte, Messinian age rocks present in the Langhe describe a
chronological sequence of the events associated with the Messinian Salinity
Crisis:
· Marls and Mudstones – rocks derived from deep sea sediments. These rocks record the
alternation between a warm and humid climate and a cooler, less-humid one.
Microfossils in the rock point to the exact moment when the Mediterranean weas
cut off from the Atlantic.
· Gypsum selenite and laminate – These minerals were formed in water with high salinity and
point to an increased evaporation linked to the isolation of the Mediterranean
Sea.
· Sandstones and mudstones – Sediments deposited on the continent in low-salinity
waters, rich in fossil vertebrate remains and shells of lacustrine molluscs.
These remains testify to a savannah environment with temporary pools of fresh
water.
· Calcareous marl – These rocks tell the story of a re-established full connection between
the Atlantic and the Mediterranean. These rocks are rich in marine planktonic microfossils
recording a deep (around 800 m) marine basin.
· Erosion surface – This surface describes an event of rapid dismantling of sediments
caused by compressive tectonic forces.
©Wine -- Mise en abyme
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