5 Isotopic Composition of Seawater 133
flow around Antarctica and enter the South Atlantic through the Drake passage
and to flow northward to join the Gulf Stream in the Gulf of Mexico. This new
admixture of more radiogenic Pacific seawater could explain the increase of the
Nd isotopic composition in the Atlantic some 20 to 18 Ma ago (Stille 1992; Stiile
et al. 1996).
18 to 15 Ma the Nd isotopic composition of the Atlantic ocean dropped again
and evolved to present-day seawater values (Fig. 5.26). This might indicate that
the influx of Pacific seawater into the Atlantic diminished very soon after the
opening of the Drake Passage. However, Stille (1992) and Stille et al. (1996)
suggest that this change in Nd isotopic composition is directly related to the
development of North Atlantic Bottom Water some 15 Ma ago. No data exist for
the isotopic composition of the paleo-North Atlantic bottom water. However,
investigations of present-day North Atlantic Bottom Water demonstrate that they
are less radiogenic in Nd, and have the lowest Nd isotopic compositions ever
measured in the open oceans (Piepgras and Wasserburg 1983). Because upwelling
of cold. nutrient-enriched bottom waters became a widespread phenomenon
during the Miocene leading to the deposition of primary phosphate sediments
(Riggs, 1984), it has been suggested that exchange of these non-radiogenic bottom
waters with surface waters caused the decreasing isotopic composition values in
the Atlantic ocean (Stille 1992; Stille et al. 1996).
The short residence time of Nd clearly provides great potential for identifying
changes in ocean circulation and can be of help in reconstructing paleogeography.
However. equally rewarding aspects of this isotopic system may rest in its
coupling with other geochemical parameters.
5.3 T r a c i n g O c e a n C u r r e n t s U s i n g Nd a n d P b I s o t o p e s
5.3.1 Ocean Currents and Nd Isotopes
In the study of Piepgras and Wasserburg (1983) seawater exchange was
investigated between the Mediterranean and the Atlantic near the Strait of
Gibraltar with the help of Nd isotopes. The outflow from the Mediterranean into
the Atlantic has a significant influence on the temperature and the salinity of the
water in the Atlantic. The high rate of evaporation in the Mediterranean Sea
produces water with salinities greater than 38 per mil. This dense, salty water
flows past Gibraltar at 106 m3/s into the Atlantic. At the same time, less saline
water flows from the surface waters of the Atlantic at the nearly equivalent rate of
1.11 x 106 m3/s into the Mediterranean. The higher density of Mediterranean
water sinks and flows at depths of more than 1000 meters along the continental
shelf of the Atlantic.
flow around Antarctica and enter the South Atlantic through the Drake passage
and to flow northward to join the Gulf Stream in the Gulf of Mexico. This new
admixture of more radiogenic Pacific seawater could explain the increase of the
Nd isotopic composition in the Atlantic some 20 to 18 Ma ago (Stille 1992; Stiile
et al. 1996).
18 to 15 Ma the Nd isotopic composition of the Atlantic ocean dropped again
and evolved to present-day seawater values (Fig. 5.26). This might indicate that
the influx of Pacific seawater into the Atlantic diminished very soon after the
opening of the Drake Passage. However, Stille (1992) and Stille et al. (1996)
suggest that this change in Nd isotopic composition is directly related to the
development of North Atlantic Bottom Water some 15 Ma ago. No data exist for
the isotopic composition of the paleo-North Atlantic bottom water. However,
investigations of present-day North Atlantic Bottom Water demonstrate that they
are less radiogenic in Nd, and have the lowest Nd isotopic compositions ever
measured in the open oceans (Piepgras and Wasserburg 1983). Because upwelling
of cold. nutrient-enriched bottom waters became a widespread phenomenon
during the Miocene leading to the deposition of primary phosphate sediments
(Riggs, 1984), it has been suggested that exchange of these non-radiogenic bottom
waters with surface waters caused the decreasing isotopic composition values in
the Atlantic ocean (Stille 1992; Stille et al. 1996).
The short residence time of Nd clearly provides great potential for identifying
changes in ocean circulation and can be of help in reconstructing paleogeography.
However. equally rewarding aspects of this isotopic system may rest in its
coupling with other geochemical parameters.
5.3 T r a c i n g O c e a n C u r r e n t s U s i n g Nd a n d P b I s o t o p e s
5.3.1 Ocean Currents and Nd Isotopes
In the study of Piepgras and Wasserburg (1983) seawater exchange was
investigated between the Mediterranean and the Atlantic near the Strait of
Gibraltar with the help of Nd isotopes. The outflow from the Mediterranean into
the Atlantic has a significant influence on the temperature and the salinity of the
water in the Atlantic. The high rate of evaporation in the Mediterranean Sea
produces water with salinities greater than 38 per mil. This dense, salty water
flows past Gibraltar at 106 m3/s into the Atlantic. At the same time, less saline
water flows from the surface waters of the Atlantic at the nearly equivalent rate of
1.11 x 106 m3/s into the Mediterranean. The higher density of Mediterranean
water sinks and flows at depths of more than 1000 meters along the continental
shelf of the Atlantic.