280
L. Ainsaar, T. Martma, T. Meidla, M. Rubel and N. SidaraviCiene
The stable carbon isotope composition of the carbonate rocks of the Haljala
to Nabala stages in South Estonia has been examined altogether in 113 samples
from the Ristikiila and Tartu core sections. The samples are taken from a 34 m
interval in the Ristikiila core and a 28 m interval in the Tartu core section (Fig.
3). Bulk-rock samples were crushed and material for isotope analysis was selected, avoiding obvious veins or burrows. The dolomitization rate of the carbonate
has been estimated with XRD method, whereas dolomite formed mostly 1-5%
(occasionally up to 45%) of the carbonate in limestones and marls, and 20-90%
of carbonate in the intercalations of dolomitized quartzose siltstones. Recrystallization in meteoric or burial environments is much more likely to preserve the
carbon isotopic signatures than the oxygen values, and carbon values may be
preserved even in bulk dolomites (Marshall 1992). The ol3C value in the studied
samples has low positive correlation with dolomite/calcite ratio (r=0.45), referring to a lack or minimum depletion effect of dolomitization. In our interpretation, we consider the ol3C values to reflect the original composition of ancient
seawater.
The carbonate samples were powdered to the grain size of < 10 mm, reacted
with 100% phosphoric acid at 100°C for 15 min, and CO 2 analyzed using the Finnigan-Mat Delta E mass spectrometer. Carbonate ol3C was detected. In Fig. 3,
the l3C/ i2 C ratio is given as per mil deviation from the PDB standard; the reproducibility of the results is higher than 0.1 %0.
The curves of ol3C in Ristikiila and Tartu sections are remarkably similar (Fig. 3).
In both sections a positive shift (-2%0) occurs in the upper part of the Keila Stage. Upsection of the maximum, the ol3C values gradually decrease upward, reaching the initial values in the Oandu and Rakvere stages. The dating of the excursions in the studied sections, being based on the traditional estimates of the
ranges of taxa in stage level resolution, could not provide a necessary precision
level in the boundary interval of the Keila and Oandu stages.
6
Construction of the Scale
The local ranges of ostracode species in each section are given according to their
observed occurrences. Beside the sampling effects, such ranges depend on several factors: organic evolution, environmental changes and sedimentological
phenomena. The first and last occurrences of species in any pair of sections are
practically never exactly in the same order and none of the individual range
charts can be taken as a reliable time scale. Obtaining a time scale means constructing an uncontradictory, composite succession from all observed local
ranges. For this purpose, the total ranges of species and their mutual position in
relation to a universal time axis must be estimated. This can be achieved by different algorithms in quantitative stratigraphy and results in creation of ordinal
or even regular time scales for the correlation and dating. The same scale can be
used also for tracing radiation or extinction events. This approach has the advantage of not being influenced by subjective stratigraphic correlations.
L. Ainsaar, T. Martma, T. Meidla, M. Rubel and N. SidaraviCiene
The stable carbon isotope composition of the carbonate rocks of the Haljala
to Nabala stages in South Estonia has been examined altogether in 113 samples
from the Ristikiila and Tartu core sections. The samples are taken from a 34 m
interval in the Ristikiila core and a 28 m interval in the Tartu core section (Fig.
3). Bulk-rock samples were crushed and material for isotope analysis was selected, avoiding obvious veins or burrows. The dolomitization rate of the carbonate
has been estimated with XRD method, whereas dolomite formed mostly 1-5%
(occasionally up to 45%) of the carbonate in limestones and marls, and 20-90%
of carbonate in the intercalations of dolomitized quartzose siltstones. Recrystallization in meteoric or burial environments is much more likely to preserve the
carbon isotopic signatures than the oxygen values, and carbon values may be
preserved even in bulk dolomites (Marshall 1992). The ol3C value in the studied
samples has low positive correlation with dolomite/calcite ratio (r=0.45), referring to a lack or minimum depletion effect of dolomitization. In our interpretation, we consider the ol3C values to reflect the original composition of ancient
seawater.
The carbonate samples were powdered to the grain size of < 10 mm, reacted
with 100% phosphoric acid at 100°C for 15 min, and CO 2 analyzed using the Finnigan-Mat Delta E mass spectrometer. Carbonate ol3C was detected. In Fig. 3,
the l3C/ i2 C ratio is given as per mil deviation from the PDB standard; the reproducibility of the results is higher than 0.1 %0.
The curves of ol3C in Ristikiila and Tartu sections are remarkably similar (Fig. 3).
In both sections a positive shift (-2%0) occurs in the upper part of the Keila Stage. Upsection of the maximum, the ol3C values gradually decrease upward, reaching the initial values in the Oandu and Rakvere stages. The dating of the excursions in the studied sections, being based on the traditional estimates of the
ranges of taxa in stage level resolution, could not provide a necessary precision
level in the boundary interval of the Keila and Oandu stages.
6
Construction of the Scale
The local ranges of ostracode species in each section are given according to their
observed occurrences. Beside the sampling effects, such ranges depend on several factors: organic evolution, environmental changes and sedimentological
phenomena. The first and last occurrences of species in any pair of sections are
practically never exactly in the same order and none of the individual range
charts can be taken as a reliable time scale. Obtaining a time scale means constructing an uncontradictory, composite succession from all observed local
ranges. For this purpose, the total ranges of species and their mutual position in
relation to a universal time axis must be estimated. This can be achieved by different algorithms in quantitative stratigraphy and results in creation of ordinal
or even regular time scales for the correlation and dating. The same scale can be
used also for tracing radiation or extinction events. This approach has the advantage of not being influenced by subjective stratigraphic correlations.