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in a Proterozoic ramp. However, the reported
ranges in
carbonate settings such as platforms, barrier
reefs, atolls and ramps is high, i.e., there are
absolute ranges of 6.9‰ for
18
O from the large majority of
fossil ramp and platform sections fall well within
the fi eld of isotopic variability as seen in this
study and in previous studies of
13
C and
13
C and 5.5‰ for
18
O, respectively.
Clear spatial trends in isotopic composition of
18
O
of modern shallow-water carbonate sediments
(Weber, 1967; Weber & Woodhead, 1969; Land,
1989). As a consequence the isotopic variation
along ancient rock sections that is used for strati-
graphic correlation could easily be explained by
changes in taxonomic composition within indi-
vidual carbonate-producing organism groups, or
simply by facies shifts.
Swart & Eberli (2005) showed that basin to bank
correlations in the Miocene to Holocene of the
Bahamas using
13
C and
2
sediments were found only in the ramp setting,
where carbonate facies also have characteristic
isotopic compositions.
The strongest isotopic excursions along
3
Holocene sections are caused by diagenesis
near a subaerial exposure horizon.
The much lower range of carbon and oxygen iso-
4
topes found in fossil shallow-water carbonates,
e.g., in isotope stratigraphy, are presumably a
consequence of taphonomic and diagenetic
processes, which decrease isotopic variability.
13
C is possible. However, they also
13
C trends through their sediment cores
did not correlate with the global signal of
found that
13
C.
Compositional variation in periplatform settings
was reported to be rather the expression of varying
mixtures of carbonate grains produced on the plat-
form top, which eventually are exported and mixed
with pelagic carbonate. Similar processes were
probably effective in the pre-Neogene, however, the
abundance and isotopic compositions of ancient
platform-dwelling, carbonate-producing organisms
such as corals, calcareous sponges, molluscs, fora-
minifera and calcareous algae were not investigated
in the above-mentioned bulk rock isotope studies.
In summary, there are a number of infl uences
ACKNOWLEDGEMENTS
We thank R. Gless, C. Schroeder, A. Saied and
D. Harazim for help during sample preparation
for stable isotope and X-ray-diffraction analyses.
J. Fiebig ran the Frankfurt mass spectrometer and
R. Petschick the Frankfurt diffractometer. The
thoughtful reviews by W. Patterson (Saskatoon)
and S. Hesselbo (Oxford) improved the paper and
are gratefully acknowledged.
18
O values in shallow-water carbonate
sediments. These include taxonomic variation,
diagenesis and the fact that the majority of grains
are composed of metastable aragonite and HMC.
Taphonomic effects such as bioturbation are of
importance as seen in the investigated Holocene
cores. Therefore, results of bulk analyses of
13
C and
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13
C
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CONCLUSIONS
18
O values of sedi-
ments in modern and Holocene shallow-water
13
C and
1
The variability of
