112 Peter Stille and Graham Shields
also accompanied by a general rise in 87Sr/86Sr ratio, mirroring the post-Triassic
story in the Phanerozoic. This rise was between 0.7057 and 0.7091, and was
completed by the Middle Cambrian.
This discussion shows that 87Sr/S6Sr minima are not always necessarily the
direct result of enhanced hydrothermal activity. Very low 87Sr/86Sr ratios may
also be the result of a low groundwater, and/or river flux, caused possibly by the
formation of a supercontinent.
5.2 Nd Isotopic Composition of Seawater: Tracer of Water
Circulation Patterns
Neodymium (Nd) is also present as a trace element in seawater. However, Nd
concentrations are far lower than tbr Sr varying between 0.41 * 10 .6 and 4.21 *
10 -6 mg/L. The mean Nd concentration lies around 2.58 +/- 0.23 (' 10 "6 m~_St..
making it 3 million times less abundant in the oceans than St. Despite, the tow
concentrations direct isotopic measurement of Nd in seawater has proved possible.
As a resuit of these analyses we now know that the 143Nd/144Nd isotopic ratio in
seawater is not homogeneous as is the case with Sr. The Nd isotopic ratio varies
not only from ocean to ocean but also within a single ocean basin (Fig. 5.16).
On the basis of investigations by Piepgras et al. (1979) and Piepgras and
Wasserburg (1980, 1982) ENd values (for definition see Sect. 1.2) of -I2 +/- 2
could be determined for the present day Atlantic Ocean, -3 +/- 2 for the Pacific
Ocean and -8 +1- 2 for the Indian Ocean. The different Nd isotopic compositions
between oceans are the result of the isotopically different sources of Nd for these
oceans and the short residence time of Nd in seawater, which is far shorter than
that for Sr and is also shorter than the time of around 3000 years required for the
oceans to become thoroughly mixed (see also Sect. 5.2.1 ).
The relatively high ~Nd value for the Pacific ocean can be attributed to the
juvenile crust, which surrounds the Pacific and also the high production rate of
volcanic rocks within the Pacific itself. The relatively low values for the Atlantic
result from the existence of old continental blocks (cratons) on either side of that
ocean (Brazil. Canada, Fennoscandia, Greenland, West Africa); the crust is
marked by low Sm/Nd ratios, Sm being preferentially incorporated into the
mantle. The ~Nd values show that more than 90% of the Nd in the Atlantic is of
continental origin, while this source represents only 60% of the whole for the
Pacific. As the Nd concentrations in seawater are far lower than those for St, the
direct determination of seawater Nd isotope composition is extremely difficult. In
order to get around this problem, it is necessary to analyze rocks and mineral
components enriched in Nd that are likely to have incorporated and preserved a
seav,ater isotopic signature.
also accompanied by a general rise in 87Sr/86Sr ratio, mirroring the post-Triassic
story in the Phanerozoic. This rise was between 0.7057 and 0.7091, and was
completed by the Middle Cambrian.
This discussion shows that 87Sr/S6Sr minima are not always necessarily the
direct result of enhanced hydrothermal activity. Very low 87Sr/86Sr ratios may
also be the result of a low groundwater, and/or river flux, caused possibly by the
formation of a supercontinent.
5.2 Nd Isotopic Composition of Seawater: Tracer of Water
Circulation Patterns
Neodymium (Nd) is also present as a trace element in seawater. However, Nd
concentrations are far lower than tbr Sr varying between 0.41 * 10 .6 and 4.21 *
10 -6 mg/L. The mean Nd concentration lies around 2.58 +/- 0.23 (' 10 "6 m~_St..
making it 3 million times less abundant in the oceans than St. Despite, the tow
concentrations direct isotopic measurement of Nd in seawater has proved possible.
As a resuit of these analyses we now know that the 143Nd/144Nd isotopic ratio in
seawater is not homogeneous as is the case with Sr. The Nd isotopic ratio varies
not only from ocean to ocean but also within a single ocean basin (Fig. 5.16).
On the basis of investigations by Piepgras et al. (1979) and Piepgras and
Wasserburg (1980, 1982) ENd values (for definition see Sect. 1.2) of -I2 +/- 2
could be determined for the present day Atlantic Ocean, -3 +/- 2 for the Pacific
Ocean and -8 +1- 2 for the Indian Ocean. The different Nd isotopic compositions
between oceans are the result of the isotopically different sources of Nd for these
oceans and the short residence time of Nd in seawater, which is far shorter than
that for Sr and is also shorter than the time of around 3000 years required for the
oceans to become thoroughly mixed (see also Sect. 5.2.1 ).
The relatively high ~Nd value for the Pacific ocean can be attributed to the
juvenile crust, which surrounds the Pacific and also the high production rate of
volcanic rocks within the Pacific itself. The relatively low values for the Atlantic
result from the existence of old continental blocks (cratons) on either side of that
ocean (Brazil. Canada, Fennoscandia, Greenland, West Africa); the crust is
marked by low Sm/Nd ratios, Sm being preferentially incorporated into the
mantle. The ~Nd values show that more than 90% of the Nd in the Atlantic is of
continental origin, while this source represents only 60% of the whole for the
Pacific. As the Nd concentrations in seawater are far lower than those for St, the
direct determination of seawater Nd isotope composition is extremely difficult. In
order to get around this problem, it is necessary to analyze rocks and mineral
components enriched in Nd that are likely to have incorporated and preserved a
seav,ater isotopic signature.