5 Isotopic Composition of Seawater 109
Vail et al. (1977) were able to demonstrate that sea level fluctuations are
correlatable with spreading rates at the mid-ocean ridge. Increased rates of
spreading correspond to periods of transgression, while decreased rates of
spreading correspond to times of regression. As high spreading rates are likely to
be accompanied by enhanced hydrothermal activity, Chaudhuri and Clauer (1986)
assumed that the 87Sr/S6Sr ratio of seawater is controlled by the relative
importance of transgression and regression. During times of high sea level for
example, seawater 87Sr/86Sr ratio should be low.
However, sea level fluctuations and seawater 87Sr/86Sr ratios appear to be only
partly correlated with each other (Fig. 5.15). In this respect, the lowering of the
87Sr186Sr ratio and the beginning of transgression around 180-200 million years
ago may conceivably be connected with the onset of crustal rifting in Pangaea as
discussed in the last section (see Schaltegger et al., 1994; Fiechtner et al., 1992).
From this one can assume that hydrothermal activity at this time was not only
enhanced on the continent but also in the Panthalassa Ocean. The lowest 87Sr/86Sr
ratios were reached during steadily rising sea level just before 120 Ma ago. The
timing of this low point matches particularly well to the time of spreading,
especially that associated with the formation of the first oceanic crust in the
Tethys and the Atlantic. Using this model, however, we cannot explain very well
the decoupled behavior of sea level fluctuations and 87Sr/86Sr ratios in the
Cretaceous period. Here STSr/86Sr ratios increase as the transgression continued.
In Permian -Triassic time, sea level felt as seawater 87Sr/g6Sr reached a minimum.
Both instances can be explained by the compensating input of strontium from the
predominantly "sialic" continents. It can be assumed that there was relatively
minor continental input of strontium during Permian / Triassic time. Lack of
continental weathering would be enough to cause such a low 87Sr/86Sr ratio
v, ithout the necessity for any marked increase in hydrothermal input. Similar
arguments can be brought to bare to explain the anomaly in the Cretaceous, with
the onset of rifting leading eventually to further mountain building and erosion,
however such a solution is not entirety satisfactory.
Chaudurl and Clauer propose another solution. They postulate the presence of
an additional continental source, which would supply radiogenic strontium to the
oceans. As we shall see, groundwater provides a finishing touch to explain some
of the variations in Sr isotope composition during the Phanerozoic.
On the basis of an unfortunately limited number of measurements and
calculations, it can be assumed that the volume of groundwater, which finds its
way into the oceans makes up about 1.7 * 103 km 3. 2 * 1012g of strontium are
transported into the oceans by this route every year. If we assume steady state
conditions in seawater, then the following mass balance may be applied:
Sr,~,R,,,. + Sr,~,,R.~,, + Srd~.Rd,. + Sr,,bR,,h
= (Sr~,, + Sr~,, + Srdr + Sr,,o)R.r
Srn~= Contribution of Sr by river transport (2.21 x 1012gal )
Vail et al. (1977) were able to demonstrate that sea level fluctuations are
correlatable with spreading rates at the mid-ocean ridge. Increased rates of
spreading correspond to periods of transgression, while decreased rates of
spreading correspond to times of regression. As high spreading rates are likely to
be accompanied by enhanced hydrothermal activity, Chaudhuri and Clauer (1986)
assumed that the 87Sr/S6Sr ratio of seawater is controlled by the relative
importance of transgression and regression. During times of high sea level for
example, seawater 87Sr/86Sr ratio should be low.
However, sea level fluctuations and seawater 87Sr/86Sr ratios appear to be only
partly correlated with each other (Fig. 5.15). In this respect, the lowering of the
87Sr186Sr ratio and the beginning of transgression around 180-200 million years
ago may conceivably be connected with the onset of crustal rifting in Pangaea as
discussed in the last section (see Schaltegger et al., 1994; Fiechtner et al., 1992).
From this one can assume that hydrothermal activity at this time was not only
enhanced on the continent but also in the Panthalassa Ocean. The lowest 87Sr/86Sr
ratios were reached during steadily rising sea level just before 120 Ma ago. The
timing of this low point matches particularly well to the time of spreading,
especially that associated with the formation of the first oceanic crust in the
Tethys and the Atlantic. Using this model, however, we cannot explain very well
the decoupled behavior of sea level fluctuations and 87Sr/86Sr ratios in the
Cretaceous period. Here STSr/86Sr ratios increase as the transgression continued.
In Permian -Triassic time, sea level felt as seawater 87Sr/g6Sr reached a minimum.
Both instances can be explained by the compensating input of strontium from the
predominantly "sialic" continents. It can be assumed that there was relatively
minor continental input of strontium during Permian / Triassic time. Lack of
continental weathering would be enough to cause such a low 87Sr/86Sr ratio
v, ithout the necessity for any marked increase in hydrothermal input. Similar
arguments can be brought to bare to explain the anomaly in the Cretaceous, with
the onset of rifting leading eventually to further mountain building and erosion,
however such a solution is not entirety satisfactory.
Chaudurl and Clauer propose another solution. They postulate the presence of
an additional continental source, which would supply radiogenic strontium to the
oceans. As we shall see, groundwater provides a finishing touch to explain some
of the variations in Sr isotope composition during the Phanerozoic.
On the basis of an unfortunately limited number of measurements and
calculations, it can be assumed that the volume of groundwater, which finds its
way into the oceans makes up about 1.7 * 103 km 3. 2 * 1012g of strontium are
transported into the oceans by this route every year. If we assume steady state
conditions in seawater, then the following mass balance may be applied:
Sr,~,R,,,. + Sr,~,,R.~,, + Srd~.Rd,. + Sr,,bR,,h
= (Sr~,, + Sr~,, + Srdr + Sr,,o)R.r
Srn~= Contribution of Sr by river transport (2.21 x 1012gal )