5 Isotopic Composition of Seawater 97
5.1.2 The Role of Diagenesis in Sr Isotope Stratigraphy
Some mention has already been made of the problems encountered in Sr isotope
studies due to diagenetic alteration. As one moves back through geologic time,
analyzing ever more ancient and possibly more altered rocks, it becomes
necessary to constrain or quantify this diagenetic disturbance. Such an approach
has proved successful during several recent large-scale studies, most notably those
of the Bochum/Ottowa research groups. Concentrating on what are clearly the
most likely fossils to have retained seawater isotopic compositions, i.e.
microscopically pristine, low-Mg calcite belemnites and brachiopods (other
workers have concentrated on foraminifera for the Cenozoic), they have embarked
upon an immense exercise to constrain precise trends in seawater St, C and O
isotope ratios through the Phanerozoic era (<545 Ma), by systematically removing
the ugly spectre of diagenetic alteration. Few studies have enlightened us more on
how to constrain seawater Sr isotope ratio in past oceans than the early Sr isotope
studies of Precambrian rocks, pioneered by Veizer (e.g. Veizer, 1983) and so we
will concentrate on this period and the reconstruction of this part of the 'Sr isotope
curve' in the following section.
The Sr isotope composition of seawater through Precambrian time (>545 Ma)
is much less well known than for the Phanerozoic era. This has several reasons.
Precambrian rocks are less common, more likely to be metamorphosed or to be
heavily recrystallized (the result of often deep burial) or are dolomitic, in which
case the carbonate components are likely to have lost more than 90% of their
original strontium Veizer (1983). Our knowledge of Precambrian stratigraphy is
a~so much less sure and the estimated ages of sedimentary suites liable to large
errors. Correlation is hampered to such an extent that Sr isotope stratigraphy
coupled with C isotope stratigraphy is often called upon to arbitrate in
stratigraphic disputes. The absence of shelly fossils in the Precambrian, whose
extent of diagenetic alteration can sometimes be assessed more easily (both
petrographically and chemically), forces us to analyze bulk carbonate sediments.
Constraining the original seawater Sr isotope ratio using bulk carbonate-rich rocks
is an unfortunately tricky and time consuming option requiring the complimentary
application of a battery of petrographic and geochemical approaches.
If we take another look at Burke et al.'s curve (Fig. 5.2), we can see that there
is a great deal of scatter in the data although the Sr isotopic ratio of seawater ought
to have been the same in all the oceans at any one time due to the long residence
time of Sr and by comparison with today. Veizer (1983) discusses the problem of
selecting the most suitable carbonate material for reconstructing real trends in
seawater Sr isotopic ratio. This author was able to find important parameters
which permit the identification of 'most altered' samples in a sample set. He and
others have demonstrated that samples dissolved in HCI for analysis displayed
positive correlation between the amount of detritus and the 87Sr/86Sr ratio. This
implies that radiogenic Sr can be leached from detritus by a strong acid such as
HCl. This Sr most probably derives from the clay fraction which contains
5.1.2 The Role of Diagenesis in Sr Isotope Stratigraphy
Some mention has already been made of the problems encountered in Sr isotope
studies due to diagenetic alteration. As one moves back through geologic time,
analyzing ever more ancient and possibly more altered rocks, it becomes
necessary to constrain or quantify this diagenetic disturbance. Such an approach
has proved successful during several recent large-scale studies, most notably those
of the Bochum/Ottowa research groups. Concentrating on what are clearly the
most likely fossils to have retained seawater isotopic compositions, i.e.
microscopically pristine, low-Mg calcite belemnites and brachiopods (other
workers have concentrated on foraminifera for the Cenozoic), they have embarked
upon an immense exercise to constrain precise trends in seawater St, C and O
isotope ratios through the Phanerozoic era (<545 Ma), by systematically removing
the ugly spectre of diagenetic alteration. Few studies have enlightened us more on
how to constrain seawater Sr isotope ratio in past oceans than the early Sr isotope
studies of Precambrian rocks, pioneered by Veizer (e.g. Veizer, 1983) and so we
will concentrate on this period and the reconstruction of this part of the 'Sr isotope
curve' in the following section.
The Sr isotope composition of seawater through Precambrian time (>545 Ma)
is much less well known than for the Phanerozoic era. This has several reasons.
Precambrian rocks are less common, more likely to be metamorphosed or to be
heavily recrystallized (the result of often deep burial) or are dolomitic, in which
case the carbonate components are likely to have lost more than 90% of their
original strontium Veizer (1983). Our knowledge of Precambrian stratigraphy is
a~so much less sure and the estimated ages of sedimentary suites liable to large
errors. Correlation is hampered to such an extent that Sr isotope stratigraphy
coupled with C isotope stratigraphy is often called upon to arbitrate in
stratigraphic disputes. The absence of shelly fossils in the Precambrian, whose
extent of diagenetic alteration can sometimes be assessed more easily (both
petrographically and chemically), forces us to analyze bulk carbonate sediments.
Constraining the original seawater Sr isotope ratio using bulk carbonate-rich rocks
is an unfortunately tricky and time consuming option requiring the complimentary
application of a battery of petrographic and geochemical approaches.
If we take another look at Burke et al.'s curve (Fig. 5.2), we can see that there
is a great deal of scatter in the data although the Sr isotopic ratio of seawater ought
to have been the same in all the oceans at any one time due to the long residence
time of Sr and by comparison with today. Veizer (1983) discusses the problem of
selecting the most suitable carbonate material for reconstructing real trends in
seawater Sr isotopic ratio. This author was able to find important parameters
which permit the identification of 'most altered' samples in a sample set. He and
others have demonstrated that samples dissolved in HCI for analysis displayed
positive correlation between the amount of detritus and the 87Sr/86Sr ratio. This
implies that radiogenic Sr can be leached from detritus by a strong acid such as
HCl. This Sr most probably derives from the clay fraction which contains