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Australia: The Land Where Time Began |
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Submarine gateways and waterfalls
Stow 1
described the abyss as a
place of turmoil, though it goes unnoticed from above, and without
sound, where massive rivers meander, though they are not within banks,
currents flow continuously, and storms can continue for weeks without
being noticed from above.
As on land the topography of the seafloor is varied
and just as dramatic as that on land. The large oceans are divided into
basins, their floors often being at quite different depths, and the
basins are separated by submarine mountain ranges. At high latitudes
cold-kitchen area bottom waters pile up behind barriers such as the
mountain ranges until they reach the spill point. This usually occurs at
narrow passes that cut across the barrier as occurs on land where
mountain passes cut through ranges. The huge mass of dense water being
funneled through a narrow gateway has its width greatly restricted
causing it to accelerate. As the bottom currents pass through such deep
ocean passageways they can be very erosive scouring loose sediment and
cutting into bare rock. The dense water of a narrow bottom current flowing
at high velocity cascades down slopes and spreads out below when it
enters an adjoining basin, referred to as submarine waterfalls as they
fall from immense heights with a tremendous amount of power, though as
the slopes are gentle Stow1
compares them to cataracts
or rapids in rivers flowing across land. He describes their scale as
awesome, whatever the slope. Beneath the Denmark Strait is the most
impressive submarine waterfall known, falling over the
Greenland-Iceland
Ridge, the water flowing down-slope into the North Atlantic Basin at the
rate of 5,000,000 m3/second and dropping 3.5 km, forming
giant eddies and turbulent whirlpools in the deep ocean, being neither
heard nor seen from the surface. It dwarfs the Angel Falls, Venezuela,
the tallest waterfall in the world on land that drops 1 km. The Cuaira
Falls on the border of Paraguay and Brazil at13,000 m3/second
has the largest average flow rate on land is more than that of Niagara
Falls that is 400 times less than its submarine counterpart. It is only
in recent years that the tremendous power of these bottom currents has
been discovered. The flows
of these currents are constrained against the western coasts of the
continents by the rotation of the Earth and the Coriolis force, where
they become narrow bottom currents with high energy after they enter the
ocean basins, then being known as western boundary undercurrents,
currents that comprise most of the deep water transport of water,
chemicals, heat and nutrients through the global conveyor belt. Huge
amounts of dissolved chemicals as well as the finest of the sediments
are carried for thousands of miles in these strong currents along the
beds of the oceans. When this fine sediment finally settles out of the
currents it forms contourite drifts, giant mounds that have an elongate
shape, sometimes with symmetrical waves of sediment formed on their
surface. It has been found that these contourites sometimes grow in the
same place for as much as 20 My, reaching hundreds of metres thick and
covering an area of seafloor the size of Cuba.
It has been found that complex mechanisms cause the
velocity of these currents to respond to small climatic changes as when
under intermediate climatic conditions as at the present when slightly
more cold water is generated at the high northern latitudes. As so much
water was locked up in glaciers during the last glacial maximum when the
climate was extremely cold, less cold water was produced at high
latitudes. Stow1 suggests there is likely to be a
progressive shut-down of deep water circulation at the much warmer
temperatures that are approaching due to climate change (global
warming). The sediments being deposited are influenced by these changes,
with slightly less of the coarser size at high velocities and slightly
finer at the slower velocities being deposited. The author1
reports a large research program that involved drilling through the
Eirik Drift off the southern tip of Greenland, the preliminary results
indicate that the velocity of the bottom current tracked the major
climatic changes over the last 20,000 years, tough on a shorter
timescale there was a considerable amount of variation, but these had
still to be decoded at the time the book was written. When considering the Tethys Ocean, there was an
equatorial circulation pattern, in contrast to the interpolar deep-water
circulation pattern of the present that is dominated by cold, dense
water from high latitudes. The equatorial circulation pattern of the
Tethys Ocean contributed to a global climate that was more equable, with
warm water surrounding the poles and palm trees growing along the
northern shores of Siberia. At that time it was warm-water currents that
formed along the ocean margins that dominated the deep water
circulation. Dense waters along barren shorelines of the continents were
formed by the intense evaporation that made the water more saline,
therefore denser, as only pure water is removed by evaporation. As this
dense, saline water sank it formed the warm deep-water currents of the
basins. These warm waters spread across the global ocean floor. The
author1 suggests there are 2 regions at the present where
this phenomenon is occurring, the Mediterranean Sea and the Red Sea,
both semi-enclosed basins. It has been suggested by ocean modelers that the
bottom currents would have tracked the surface currents flowing to the
west. The Tethys Ocean became increasingly compartmentalised during the
Cainozoic into at least 3 sectors, the western central and the eastern
sectors as the ocean narrowed. Between these separate basins there were
topographic barriers deep beneath the surface over which the powerful
bottom currents found the lowest path to flow over waterfalls to the
seafloor. The
Stow1 suggests, though he admits it is as yet
unproven, that a similar gateway-waterfall couplet lies beneath the
seafloor of the present near the Straits of Gibraltar separating the
central and western Tethys Basin through a large part of the Cainozoic.
In this region there is sediment that forms a broad cone-shaped swathe
of sediment below the present seafloor of this region that slopes
upwards towards Gibraltar … but it requires further research. The
ancient equivalents of the marine contourites that were drilled and
cored through at sea have been difficult to find in the rock record on
land. Part of the problem is the controversial state of the appearance
of contourites, and also because they are piles of mud that are rather
indistinctive. The author1
claims that as he has observed many examples of contourites on the floor
of the deep sea he is confident he can recognise their subtle,
distinctive features in rocks on southern Cyprus that were once part of
the floor of the Tethys Ocean that have been dated to 30-40 Ma. He
suggests they are the first well-documented examples of contourites that
have been described. They were discovered by the Stow1 and Dr Gisela Kahler, and Dr. Costas
Xenophonotos of the Cyprus Geological Survey. They indicate a period of
intensified bottom currents that was possibly the last gasp of the
equatorial circulation prior to its replacement by the interpolar
circulation that has dominated ever since the Tethys Closed.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |