132 Peter Stille and Graham Shields
In the case of Tethys seawater we observe an even stronger increase in the Nd
isotopic ratio reaching values at 25 Ma (Late Oligocene/Early Miocene) even
higher than those observed for contemporaneous Atlantic seawater. This indicates
that at this period larger masses of Pacific surface waters entered again directly
through the Indian-Tethys seaway into the Tethys basin. Stille et al. (1996)
suggest that this increased inflow of Pacific seawater into the Tethys was probably
directly related to a first order sea level highstand some 80 to 50 Ma ago (Late
Cretaceous/Early Tertiary). This highstand could apparently completely
compensate for the plate tectonic induced narrowing of the Tethys seaway which
also started in the Late Cretaceous.
In the early Miocene (25 Ma) the Nd isotopic composition of Tethys seawater
was intermediate between Atlantic and Pacific seawater. This supports the
hypothesis, that in spite of the narrow seaway it was still essentially Pacific and
not Atlantic seawater which fed the Tethys basin.
The significant decrease of Nd isotopic compositions observed in South
Atlantic, North Atlantic and Tethys seawater indicates that around 25 to 20 Ma
much less Pacific seawater entered these oceans. The strongest decrease is
observable for Tethys seawater. This change of Nd isotopic composition is a hint
that with the collision of the Arabian and Eurasian plates the Indian-Tethys
seaway started to close some 25 Ma ago leading to the breakdown of the circumequatorial circulation patterns of the world's oceans some 20 Ma ago.
A strong decrease of the Nd isotopic composition around 25 to 20 Ma is also
observable for the North and South Atlantic. It is, however, not probable that it
was the closing of the small Indian Tethys seaway alone which caused the strong
shift. Stille et al. (1996) suggest that it was also due to less radiogenic seawater
which entered the Atlantic from the South. A possible explanation for this could
be the separation of Australia from Antarctica about 45 Ma ago and its northward
drift and collision with Indonesia during Middle Miocene times. This plate
tectonic event led to the evolution of today's circulation pattern in the Pacific
realm with two individual current systems for the Indian and Pacific oceans.
Consequently, it was no longer simply Pacific, but Indian ocean water that entered
the Atlantic from the South around Africa 23 to 20 Ma,
At about 20 to 18 Ma the Nd isotopic compositions of the Atlantic and Tethys
oceans evolved differently. Whereas the Nd isotopic composition of Tethys
seawater continued to drop, the isotopic composition of North Atlantic seawater
started to increase again. This decoupling of the Nd isotopic systems of the Tethys
and the Atlantic oceans indicates a narrowing of the seaway between both oceans
at Gibraltar. This led to an almost complete isolation of the Tethys which was now
becoming the present-day Mediterranean sea.
The strong increase of the Nd isotopic composition in the Atlantic might have
been caused by major changes in circulation patterns within the Atlantic ocean
itself some 25 to 20 Ma. They might have been mainly triggered by the opening of
the Drake passage to the South of South America which led to the establishment
of the circum-Antarctic current system. These currents enabled Pacific seawater to
In the case of Tethys seawater we observe an even stronger increase in the Nd
isotopic ratio reaching values at 25 Ma (Late Oligocene/Early Miocene) even
higher than those observed for contemporaneous Atlantic seawater. This indicates
that at this period larger masses of Pacific surface waters entered again directly
through the Indian-Tethys seaway into the Tethys basin. Stille et al. (1996)
suggest that this increased inflow of Pacific seawater into the Tethys was probably
directly related to a first order sea level highstand some 80 to 50 Ma ago (Late
Cretaceous/Early Tertiary). This highstand could apparently completely
compensate for the plate tectonic induced narrowing of the Tethys seaway which
also started in the Late Cretaceous.
In the early Miocene (25 Ma) the Nd isotopic composition of Tethys seawater
was intermediate between Atlantic and Pacific seawater. This supports the
hypothesis, that in spite of the narrow seaway it was still essentially Pacific and
not Atlantic seawater which fed the Tethys basin.
The significant decrease of Nd isotopic compositions observed in South
Atlantic, North Atlantic and Tethys seawater indicates that around 25 to 20 Ma
much less Pacific seawater entered these oceans. The strongest decrease is
observable for Tethys seawater. This change of Nd isotopic composition is a hint
that with the collision of the Arabian and Eurasian plates the Indian-Tethys
seaway started to close some 25 Ma ago leading to the breakdown of the circumequatorial circulation patterns of the world's oceans some 20 Ma ago.
A strong decrease of the Nd isotopic composition around 25 to 20 Ma is also
observable for the North and South Atlantic. It is, however, not probable that it
was the closing of the small Indian Tethys seaway alone which caused the strong
shift. Stille et al. (1996) suggest that it was also due to less radiogenic seawater
which entered the Atlantic from the South. A possible explanation for this could
be the separation of Australia from Antarctica about 45 Ma ago and its northward
drift and collision with Indonesia during Middle Miocene times. This plate
tectonic event led to the evolution of today's circulation pattern in the Pacific
realm with two individual current systems for the Indian and Pacific oceans.
Consequently, it was no longer simply Pacific, but Indian ocean water that entered
the Atlantic from the South around Africa 23 to 20 Ma,
At about 20 to 18 Ma the Nd isotopic compositions of the Atlantic and Tethys
oceans evolved differently. Whereas the Nd isotopic composition of Tethys
seawater continued to drop, the isotopic composition of North Atlantic seawater
started to increase again. This decoupling of the Nd isotopic systems of the Tethys
and the Atlantic oceans indicates a narrowing of the seaway between both oceans
at Gibraltar. This led to an almost complete isolation of the Tethys which was now
becoming the present-day Mediterranean sea.
The strong increase of the Nd isotopic composition in the Atlantic might have
been caused by major changes in circulation patterns within the Atlantic ocean
itself some 25 to 20 Ma. They might have been mainly triggered by the opening of
the Drake passage to the South of South America which led to the establishment
of the circum-Antarctic current system. These currents enabled Pacific seawater to