128 Peter Sttlle and Graham Shields
5.2.4 The Closing of the Tethys and the Opening of the Atlantic
Mesozoic and Cenozoic
The present day oceans formed during this period. It is thus of great
paleogeographic interest to find out how the isotopic compositions of the Pacific,
North and South Atlantic, Indian and Tethys Oceans and also the Mediterranean
developed through the Mesozoic and Cenozoic. Variations in the Nd isotopic ratio
should reflect the changing interlinked relationships between these oceans (Fig.
5 26). In order to derive this Nd isotope evolution, not only phosphate but also
carbonate was used whose strati~aphic age is known and whose initial Sr isotope
ratios are identical to seawater at the time of formation (see data summary in Stille
1992; Stille etal. 1996).
The Nd isotopic composition in the Pacific has remained almost constant over
the last 200 million years and shows eNd values that vary between -5 und - 1. The
early Tethys seawater shows identical isotopic ratios to those of the Pacific which
implies that Pacific seawater found its way into the newly forming Tethys basin
between 200 and 180 Ma. The influence of continental Nd on the Nd isotopic
composition of the Tethys. by way of rivers and resedimentation along the rift
zones became important very soon after the splitting up of the Tethys basin and
caused a lowering of the Nd isotope ratio. To what extent this was brought about
by the development of a physical barrier is not clear. The Nd isotopic composition
of the Tethys became ever more decoupled from that of the Pacific. This
decoupting was first observed by Shaw and Wasserburg (1985). Measurements on
phosphatic concretions from the Vocontian Trough, which was located in the
northern part of the Tethys indicate that Tethys seawater reached lowest, almost
continental crust-like ENd isotope values during the late Early Cretaceous some
100 to 120 Ma ago (Stifle etal. 1996).
No data exist for the early Atlantic. However it can be assumed that its isotopic
composition was the same as for the Tethys before 100 Ma because there was a
wide seaway between both oceans at that time and ocean currents were directed
East-West.
Let us take a look at the evolution of Nd isotopic composition in the Atlantic
and the Tethys over the last 80 million years. This was a time interval during
which significant tectonic events took place (e.g. the rifting open of the South
Atlantic, Himalayan mountain building). Fig. 5.27 illustrates their two very
different behaviors of the Sr and Nd isotopic systems in seawater. Between 60 and
25 Ma, Nd and Sr isotopic ratios rose in the Atlantic and in the Tethys. From 25 to
20 Ma, both isotopic systems measured on carbonates and phosphates from North
and South Atlantic, and Tethys oceans provide the same information with Sr
isotope ratios rising and Nd isotope ratios falling, impling an increase in crustal
influence. The most striking aspect of this diagram is the sudden and rapid rise in
Nd isotopic composition around 20 Ma in the Atlantic ocean. This rise does not
5.2.4 The Closing of the Tethys and the Opening of the Atlantic
Mesozoic and Cenozoic
The present day oceans formed during this period. It is thus of great
paleogeographic interest to find out how the isotopic compositions of the Pacific,
North and South Atlantic, Indian and Tethys Oceans and also the Mediterranean
developed through the Mesozoic and Cenozoic. Variations in the Nd isotopic ratio
should reflect the changing interlinked relationships between these oceans (Fig.
5 26). In order to derive this Nd isotope evolution, not only phosphate but also
carbonate was used whose strati~aphic age is known and whose initial Sr isotope
ratios are identical to seawater at the time of formation (see data summary in Stille
1992; Stille etal. 1996).
The Nd isotopic composition in the Pacific has remained almost constant over
the last 200 million years and shows eNd values that vary between -5 und - 1. The
early Tethys seawater shows identical isotopic ratios to those of the Pacific which
implies that Pacific seawater found its way into the newly forming Tethys basin
between 200 and 180 Ma. The influence of continental Nd on the Nd isotopic
composition of the Tethys. by way of rivers and resedimentation along the rift
zones became important very soon after the splitting up of the Tethys basin and
caused a lowering of the Nd isotope ratio. To what extent this was brought about
by the development of a physical barrier is not clear. The Nd isotopic composition
of the Tethys became ever more decoupled from that of the Pacific. This
decoupting was first observed by Shaw and Wasserburg (1985). Measurements on
phosphatic concretions from the Vocontian Trough, which was located in the
northern part of the Tethys indicate that Tethys seawater reached lowest, almost
continental crust-like ENd isotope values during the late Early Cretaceous some
100 to 120 Ma ago (Stifle etal. 1996).
No data exist for the early Atlantic. However it can be assumed that its isotopic
composition was the same as for the Tethys before 100 Ma because there was a
wide seaway between both oceans at that time and ocean currents were directed
East-West.
Let us take a look at the evolution of Nd isotopic composition in the Atlantic
and the Tethys over the last 80 million years. This was a time interval during
which significant tectonic events took place (e.g. the rifting open of the South
Atlantic, Himalayan mountain building). Fig. 5.27 illustrates their two very
different behaviors of the Sr and Nd isotopic systems in seawater. Between 60 and
25 Ma, Nd and Sr isotopic ratios rose in the Atlantic and in the Tethys. From 25 to
20 Ma, both isotopic systems measured on carbonates and phosphates from North
and South Atlantic, and Tethys oceans provide the same information with Sr
isotope ratios rising and Nd isotope ratios falling, impling an increase in crustal
influence. The most striking aspect of this diagram is the sudden and rapid rise in
Nd isotopic composition around 20 Ma in the Atlantic ocean. This rise does not