94
Peter Stilte and Graham Shields
The consistency shown in samples from any one stratigraphic level show that it is
a normal situation for the marine environment to be isotopically homogeneous
with respect to Sr even over geological time-scales confirming that Sr has a long
residence time in seawater. The scatter in the Burke et al. curve is due to sample
alteration and errors in stratigraphic correlation; we will return to the significance
of this scatter and how much of it can be removed successfully later.
Where samples are well preserved, data scatter can be minimalized. Very
precise 87Sr/S6Sr ratios of stratigraphically well constrained marine carbonates
(bivalve pieces) and planktonic foraminifera were published by DePaolo and
Ingrain (1985) and Hodell et al. (1991). Such studies have made detailed
reconstructions possible of of seawater 87Sr/S6Sr evolution in the Neogene oceans
(Fig. 5.3, 5.4). Marine carbonates could be dated and correlated precisely with the
help of the Sr isotope system. Since 1991, further studies have yielded similar
results. Paytan et al. (1993) also reproduced the same trend through the Neogene
using biogenic barite particles. Interestingly enough, their results were, if
anything, slightly lower than those from foraminifera and marine carbonate. Such
deviations from the standard curve commonly lie within the error provided by
analytic precision and reproducibility and so remain controversial ~'e.g. see Dia et
al., 1992 and follow-up in Henderson et al., 1994).
Stille et al. (1994) discuss the stratigraphic potential of Sr isotopes in their
study of the world's largest Neogene phosphate deposits: North Carolina and
Florida. The stratigraphy of these deposits has been constrained very precisely
using seismic stratigraphy, biostratigraphy, sedimentology and sequence
strati~aphy. The deposits are early to mid-Miocene in age. Three important
phosphate-rich sedimentary sequences may be distinguished (FPS, OBS, BBS;
Fig. 5.5). These are further divisable into 18 phosphate-rich horizons that reflect
sea-level fluctuations of the fourth order. Each sequence lasted between 100,000
and 1,000,000 years.
Various phosphate components (authigenic phosphatized brachiopods, fish
bones and teeth) were isotopically studied from sediments of the same age and
from various bore holes. The Sr based, model ages showed variations of up to 2
million years for the same stratigraphic level with the amount of variation
depending on the type of material used for analysis. The most homogeneous Sr
ages were obtained from the phosphatic peloids; their ages differed by no more
than 200.000 years. Investigations of McArthur et al. (1990) showed that this type
of phosphate forms in isotopic equilibrium with seawater. However, this may also
be the case for some of the other phosphatic components investigated. As these
other components can also be of detrital origin, their model ages are less relevant
then those of the phosphatic peloids which formed "in-situ".
Sr and Nd isotopic compositions determined or, phosphatic peloids of the 'FPS1' horizons are displayed in Fig, 5.5. The 87Sr/86Sr ratios decrease rather
constantly with increasing depth and age and so reflect the seawater STSrlS6Sr
curve for this time interval. Comparing these ratios with the seawater 87Sr/86Sr
evolution curve for this time yields an age of formation of 18 to 19 Ma for these
Peter Stilte and Graham Shields
The consistency shown in samples from any one stratigraphic level show that it is
a normal situation for the marine environment to be isotopically homogeneous
with respect to Sr even over geological time-scales confirming that Sr has a long
residence time in seawater. The scatter in the Burke et al. curve is due to sample
alteration and errors in stratigraphic correlation; we will return to the significance
of this scatter and how much of it can be removed successfully later.
Where samples are well preserved, data scatter can be minimalized. Very
precise 87Sr/S6Sr ratios of stratigraphically well constrained marine carbonates
(bivalve pieces) and planktonic foraminifera were published by DePaolo and
Ingrain (1985) and Hodell et al. (1991). Such studies have made detailed
reconstructions possible of of seawater 87Sr/S6Sr evolution in the Neogene oceans
(Fig. 5.3, 5.4). Marine carbonates could be dated and correlated precisely with the
help of the Sr isotope system. Since 1991, further studies have yielded similar
results. Paytan et al. (1993) also reproduced the same trend through the Neogene
using biogenic barite particles. Interestingly enough, their results were, if
anything, slightly lower than those from foraminifera and marine carbonate. Such
deviations from the standard curve commonly lie within the error provided by
analytic precision and reproducibility and so remain controversial ~'e.g. see Dia et
al., 1992 and follow-up in Henderson et al., 1994).
Stille et al. (1994) discuss the stratigraphic potential of Sr isotopes in their
study of the world's largest Neogene phosphate deposits: North Carolina and
Florida. The stratigraphy of these deposits has been constrained very precisely
using seismic stratigraphy, biostratigraphy, sedimentology and sequence
strati~aphy. The deposits are early to mid-Miocene in age. Three important
phosphate-rich sedimentary sequences may be distinguished (FPS, OBS, BBS;
Fig. 5.5). These are further divisable into 18 phosphate-rich horizons that reflect
sea-level fluctuations of the fourth order. Each sequence lasted between 100,000
and 1,000,000 years.
Various phosphate components (authigenic phosphatized brachiopods, fish
bones and teeth) were isotopically studied from sediments of the same age and
from various bore holes. The Sr based, model ages showed variations of up to 2
million years for the same stratigraphic level with the amount of variation
depending on the type of material used for analysis. The most homogeneous Sr
ages were obtained from the phosphatic peloids; their ages differed by no more
than 200.000 years. Investigations of McArthur et al. (1990) showed that this type
of phosphate forms in isotopic equilibrium with seawater. However, this may also
be the case for some of the other phosphatic components investigated. As these
other components can also be of detrital origin, their model ages are less relevant
then those of the phosphatic peloids which formed "in-situ".
Sr and Nd isotopic compositions determined or, phosphatic peloids of the 'FPS1' horizons are displayed in Fig, 5.5. The 87Sr/86Sr ratios decrease rather
constantly with increasing depth and age and so reflect the seawater STSrlS6Sr
curve for this time interval. Comparing these ratios with the seawater 87Sr/86Sr
evolution curve for this time yields an age of formation of 18 to 19 Ma for these